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+The Project Gutenberg EBook of Tunnel Engineering. A Museum Treatment, by 
+Robert M. Vogel
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Tunnel Engineering. A Museum Treatment
+
+Author: Robert M. Vogel
+
+Release Date: May 24, 2012 [EBook #39785]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TUNNEL ENGINEERING ***
+
+
+
+
+Produced by Chris Curnow, Tom Cosmas, Joseph Cooper and
+the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+
+Transcriber's note:
+
+This is Paper 41 from the _Smithsonian Institution United States
+National Museum Bulletin 240_, comprising Papers 34-44, which will
+also be available as a complete e-book.
+
+The front material, introduction and relevant index entries from the
+_Bulletin_ are included in each single-paper e-book.
+
+Italic emphasis denoted as _Text_.
+
+Whole numbers and fractions: shown as 1-1/2, 3-1/4, etc.
+
+Please see the end of the book for corrections and changes made.
+
+
+
+
+Smithsonian Institution
+
+United States National Museum
+
+Bulletin 240
+
+
+
+
+[Illustration: Smithsonian Press]
+
+
+
+
+Museum of History and Technology
+
+
+Contributions from the Museum of History and Technology
+
+
+_Papers 34-44_
+
+_On Science and Technology_
+
+
+Smithsonian Institution · Washington, D.C. 1966
+
+
+       *       *       *       *       *
+
+
+_Publications of the United States National Museum_
+
+The scholarly and scientific publications of the United States National
+Museum include two series, _Proceedings of the United States National
+Museum_ and _United States National Museum Bulletin_.
+
+In these series, the Museum publishes original articles and monographs
+dealing with the collections and work of its constituent museums--The
+Museum of Natural History and the Museum of History and
+Technology--setting forth newly acquired facts in the fields of
+anthropology, biology, history, geology, and technology. Copies of
+each publication are distributed to libraries, to cultural and
+scientific organizations, and to specialists and others interested in
+the different subjects.
+
+The _Proceedings_, begun in 1878, are intended for the publication,
+in separate form, of shorter papers from the Museum of Natural History.
+These are gathered in volumes, octavo in size, with the publication
+date of each paper recorded in the table of contents of the volume.
+
+In the _Bulletin_ series, the first of which was issued in 1875, appear
+longer, separate publications consisting of monographs (occasionally in
+several parts) and volumes in which are collected works on related
+subjects. _Bulletins_ are either octavo or quarto in size, depending
+on the needs of the presentation. Since 1902 papers relating to the
+botanical collections of the Museum of Natural History have been
+published in the _Bulletin_ series under the heading _Contributions from
+the United States National Herbarium_, and since 1959, in _Bulletins_
+titled "Contributions from the Museum of History and Technology," have
+been gathered shorter papers relating to the collections and research of
+that Museum.
+
+The present collection of Contributions, Papers 34-44, comprises
+Bulletin 240. Each of these papers has been previously published in
+separate form. The year of publication is shown on the last page of
+each paper.
+
+  FRANK A. TAYLOR
+  _Director, United States National Museum_
+
+
+       *       *       *       *       *
+
+
+
+
+CONTRIBUTIONS FROM
+
+THE MUSEUM OF HISTORY AND TECHNOLOGY.
+
+PAPER 41
+
+
+
+
+TUNNEL ENGINEERING--A MUSEUM TREATMENT
+
+
+_Robert M. Vogel_
+
+
+
+
+                                       INTRODUCTION      203
+
+                                     ROCK TUNNELING      206
+
+                              SOFT-GROUND TUNNELING      215
+
+                                       BIBLIOGRAPHY      239
+
+                                          FOOTNOTES
+
+                                              INDEX
+
+
+
+
+[Illustration: Figure 1.--MINING BY EARLY EUROPEAN CIVILIZATIONS,
+using fire setting and hand chiseling to break out ore and rock.
+MHT model--3/4" scale. (Smithsonian photo 49260-H.)]
+
+
+
+
+_Robert M. Vogel_
+
+TUNNEL ENGINEERING--A MUSEUM TREATMENT
+
+
+     _During the years from 1830 to 1900, extensive developments took
+     place in the field of tunneling, which today is an important,
+     firmly established branch of civil engineering. This paper offers
+     a picture of its growth from the historical standpoint, based on
+     a series of models constructed for the Hall of Civil Engineering
+     in the new Museum of History and Technology. The eight models
+     described highlight the fundamental advances which have occurred
+     between primitive man's first systematic use of fire for excavating
+     rock in mining, and the use in combination of compressed air, an
+     iron lining, and a movable shield in a subaqueous tunnel at the end
+     of the 19th century._
+
+     THE AUTHOR: _Robert M. Vogel is curator of heavy machinery and
+     civil engineering, in the Smithsonian Institution's Museum of
+     History and Technology._
+
+
+
+
+Introduction
+
+
+With few exceptions, civil engineering is a field in which the ultimate
+goal is the assemblage of materials into a useful structural form
+according to a scientifically derived plan which is based on various
+natural and man-imposed conditions. This is true whether the result be,
+for example, a dam, a building, a bridge, or even the fixed plant of a
+railroad. However, one principal branch of the field is based upon an
+entirely different concept. In the engineering of tunnels the utility of
+the "structure" is derived not from the bringing together of elements
+but from the separation of one portion of naturally existing material
+from another to permit passage through a former barrier.
+
+In tunneling hard, firm rock, this is practically the entire compass
+of the work: breaking away the rock from the mother mass, and,
+coincidently, removing it from the workings. The opposite extreme in
+conditions is met in the soft-ground tunnel, driven through material
+incapable of supporting itself above the tunnel opening. Here, the
+excavation of the tunneled substance is of relatively small concern,
+eclipsed by the problem of preventing the surrounding material from
+collapsing into the bore.
+
+[Illustration: Figure 2.--HOOSAC TUNNEL. METHOD OF WORKING EARLY
+SECTIONS of the project; blast holes drilled by hand jacking.
+MHT model--1/2" scale. (Smithsonian photo 49260-L.)]
+
+In one other principal respect does tunnel engineering differ widely
+from its collateral branches of civil engineering. Few other physical
+undertakings are approached with anything like the uncertainty
+attending a tunnel work. This is even more true in mountain tunnels,
+for which test borings frequently cannot be made to determine the
+nature of the material and the geologic conditions which will be
+encountered.
+
+The course of tunnel work is not subject to an overall preliminary
+survey; the engineer is faced with not only the inability to
+anticipate general contingencies common to all engineering work, but
+with the peculiar and often overwhelming unpredictability of the very
+basis of his work.
+
+Subaqueous and soft-ground work on the other hand, while still subject
+to many indeterminates, is now far more predictable than during its
+early history, simply because the nature of the adverse condition
+prevailing eventually was understood to be quite predictable. The
+steady pressures of earth and water to refill the excavated area are
+today overcome with relative ease and consistency by the tunneler.
+
+In tunneling as in no other branch of civil engineering did empiricism
+so long resist the advance of scientific theory; in no other did the
+"practical engineer" remain to such an extent the key figure in
+establishing the success or failure of a project. The Hoosac Tunnel,
+after 25 years of legislative, financial, and technical difficulties,
+in 1875 was finally driven to successful completion only by the
+efforts of a group who, while in the majority were trained civil
+engineers, were to an even greater extent men of vast practical
+ability, more at home in field than office.
+
+DeWitt C. Haskin (see p. 234), during the inquest that followed the
+death of a number of men in a blowout of his pneumatically driven
+Hudson River Tunnel in 1880, stated in his own defense: "I am not a
+scientific engineer, but a practical one ... I know nothing of
+mathematics; in my experience I have grasped such matters as a whole;
+I believe that the study of mathematics in that kind of work
+[tunneling] has a tendency to dwarf the mind rather than enlighten
+it...." An extreme attitude perhaps, and one which by no means adds to
+Haskin's stature, but a not unusual one in tunnel work at the time. It
+would not of course be fair to imply that such men as Herman Haupt,
+Brunel the elder, and Greathead were not accomplished theoretical
+engineers. But it was their innate ability to evaluate and control the
+overlying physical conditions of the site and work that made possible
+their significant contributions to the development of tunnel
+engineering.
+
+Tunneling remained largely independent of the realm of mathematical
+analysis long after the time when all but the most insignificant
+engineering works were designed by that means. Thus, as structural
+engineering has advanced as the result of a flow of new theoretical
+concepts, new, improved, and strengthened materials, and new methods
+of fastening, the progress of tunnel engineering has been due more to
+the continual refinement of constructional techniques.
+
+
+A NEW HALL OF CIVIL ENGINEERING
+
+In the Museum of History and Technology has recently been established
+a Hall of Civil Engineering in which the engineering of tunnels is
+comprehensively treated from the historical standpoint--something not
+previously done in an American museum. The guiding precept of the
+exhibit has not been to outline exhaustively the entire history of
+tunneling, but rather to show the fundamental advances which have
+occurred between primitive man's first systematic use of fire for
+excavating rock in mining, and the use in combination of compressed
+air, iron lining, and a movable shield in a subaqueous tunnel at the
+end of the 19th century. This termination date was selected because it
+was during the period from about 1830 to 1900 that the most
+concentrated development took place, and during which tunneling became
+a firmly established and important branch of civil engineering and
+indeed, of modern civilization. The techniques of present-day
+tunneling are so fully related in current writing that it was deemed
+far more useful to devote the exhibit entirely to a segment of the
+field's history which is less commonly treated.
+
+[Illustration: Figure 3.--HOOSAC TUNNEL. WORKING OF LATER STAGES with
+Burleigh pneumatic drills mounted on carriages. The bottom heading is
+being drilled in preparation for blasting out with nitroglycerine.
+MHT model--1/2" scale. (Smithsonian photo 49260-M.)]
+
+The major advances, which have already been spoken of as being ones of
+technique rather than theory, devolve quite naturally into two basic
+classifications: the one of supporting a mass of loose, unstable,
+pressure-exerting material--soft-ground tunneling; and the
+diametrically opposite problem of separating rock from the basic mass
+when it is so firm and solid that it can support its own overbearing
+weight as an opening is forced through it--rock, or hard-ground
+tunneling.
+
+To exhibit the sequence in a thorough manner, inviting and capable of
+easy and correct interpretation by the nonprofessional viewer, models
+offered the only logical means of presentation. Six tunnels were
+selected, all driven in the 19th century. Each represents either a
+fundamental, new concept of tunneling technique, or an important,
+early application of one. Models of these works form the basis of the
+exhibit. No effort was made to restrict the work to projects on
+American soil. This would, in fact, have been quite impossible if an
+accurate picture of tunnel technology was to be drawn; for as in
+virtually all other areas of technology, the overall development
+in this field has been international. The art of mining was first
+developed highly in the Middle Ages in the Germanic states; the tunnel
+shield was invented by a Frenchman residing in England, and the use of
+compressed air to exclude the water from subaqueous tunnels was first
+introduced on a major work by an American. In addition, the two main
+subdivisions, rock and soft-ground tunneling, are each introduced by
+a model not of an actual working, but of one typifying early classical
+methods which were in use for centuries until the comparatively recent
+development of more efficient systems of earth support and rock
+breaking. Particular attention is given to accuracy of detail
+throughout the series of eight models; original sources of descriptive
+and graphic information were used in their construction wherever
+possible. In all cases except the introductory model in the
+rock-tunneling series, representing copper mining by early
+civilizations, these sources were contemporary accounts.
+
+The plan to use a uniform scale of reduction throughout, in order
+to facilitate the viewers' interpretation, unfortunately proved
+impractical, due to the great difference in the amount of area to be
+encompassed in different models, and the necessity that the cases
+holding them be of uniform height. The related models of the Broadway
+and Tower Subways represent short sections of tunnels only 8 feet or
+so in diameter enabling a relatively large scale, 1-1/2 inches to the
+foot, to be used. Conversely, in order that the model of Brunel's
+Thames Tunnel be most effective, it was necessary to include one of
+the vertical terminal shafts used in its construction. These were
+about 60 feet in depth, and thus the much smaller scale of 1/4 inch
+to the foot was used. This variation is not as confusing as might be
+thought, for the human figures in each model provide an immediate and
+positive sense of proportion and scale.
+
+Careful thought was devoted to the internal lighting of the models, as
+this was one of the critical factors in establishing, so far as is
+possible in a model, an atmosphere convincingly representative of work
+conducted solely by artificial light. Remarkable realism was achieved
+by use of plastic rods to conduct light to the tiny sources of tunnel
+illumination, such as the candles on the miners' hats in the Hoosac
+Tunnel, and the gas lights in the Thames Tunnel. No overscaled
+miniature bulbs, generally applied in such cases, were used. At
+several points where the general lighting within the tunnel proper has
+been kept at a low level to simulate the natural atmosphere of the
+work, hidden lamps can be operated by push-button in order to bring
+out detail which otherwise would be unseen.
+
+The remainder of the material in the Museum's tunneling section
+further extends the two major aspects of tunneling. Space limitations
+did not permit treatment of the many interesting ancillary matters
+vital to tunnel engineering, such as the unique problems of
+subterranean surveying, and the extreme accuracy required in the
+triangulation and subsequent guidance of the boring in long mountain
+tunnels; nor the difficult problems of ventilating long workings, both
+during driving and in service; nor the several major methods developed
+through the years for driving or constructing tunnels in other than
+the conventional manner.[1]
+
+
+
+
+Rock Tunneling
+
+
+While the art of tunneling soft ground is of relatively recent origin,
+that of rock tunneling is deeply rooted in antiquity. However,
+the line of its development is not absolutely direct, but is
+more logically followed through a closely related branch of
+technology--mining. The development of mining techniques is a
+practically unbroken one, whereas there appears little continuity or
+relationship between the few works undertaken before about the 18th
+century for passage through the earth.
+
+The Egyptians were the first people in recorded history to have driven
+openings, often of considerable magnitude, through solid rock. As is
+true of all major works of that nation, the capability of such grand
+proportion was due solely to the inexhaustible supply of human power
+and the casual evaluation of life. The tombs and temples won from the
+rock masses of the Nile Valley are monuments of perseverance rather
+than technical skill. Neither the Egyptians nor any other peoples
+before the Middle Ages have left any consistent evidence that they
+were able to pierce ground that would not support itself above the
+opening as would firm rock. In Egypt were established the methods of
+rock breaking that were to remain classical until the first use of
+gun-powder blasting in the 17th century which formed the basis of the
+ensuing technology of mining.
+
+Notwithstanding the religious motives which inspired the earliest rock
+excavations, more constant and universal throughout history has been
+the incentive to obtain the useful and decorative minerals hidden
+beneath the earth's surface. It was the miner who developed the
+methods introduced by the early civilizations to break rock away from
+the primary mass, and who added the refinements of subterranean
+surveying and ventilating, all of which were later to be assimilated
+into the new art of driving tunnels of large diameter. The connection
+is the more evident from the fact that tunnelmen are still known as
+miners.
+
+
+COPPER MINING, B.C.
+
+Therefore, the first model of the sequence, reflecting elemental
+rock-breaking techniques, depicts a hard-rock copper mine (fig. 1).
+Due to the absence of specific information about such works during the
+pre-Christian eras, this model is based on no particular period or
+locale, but represents in a general way, a mine in the Rio Tinto area
+of Spain where copper has been extracted since at least 1000 B.C.
+Similar workings existed in the Tirol as early as about 1600 B.C. Two
+means of breaking away the rock are shown: to the left is the most
+primitive of all methods, the hammer and chisel, which require no
+further description. At the right side, the two figures are shown
+utilizing the first rock-breaking method in which a force beyond that
+of human muscles was employed, the age-old "fire-setting" method. The
+rock was thoroughly heated by a fierce fire built against its face and
+then suddenly cooled by dashing water against it. The thermal shock
+disintegrated the rock or ore into bits easily removable by hand.
+
+[Illustration: Figure 4.--HOOSAC TUNNEL. Bottom of the central shaft
+showing elevator car and rock skip; pumps at far right. In the center,
+the top bench is being drilled by a single column-mounted Burleigh
+drill. MHT model--1/2" scale. (Smithsonian photo 49260-N.)]
+
+The practice of this method below ground, of course, produced a
+fearfully vitiated atmosphere. It is difficult to imagine whether the
+smoke, the steam, or the toxic fumes from the roasting ore was
+the more distressing to the miners. Even when performed by labor
+considered more or less expendable, the method could be employed only
+where there was ventilation of some sort: natural chimneys and
+convection currents were the chief sources of air circulation. Despite
+the drawbacks of the fire system, its simplicity and efficacy weighed
+so heavily in its favor that its history of use is unbroken almost to
+the present day. Fire setting was of greatest importance during the
+years of intensive mining in Europe before the advent of explosive
+blasting, but its use in many remote areas hardly slackened until the
+early 20th century because of its low cost when compared to powder.
+For this same reason, it did have limited application in actual tunnel
+work until about 1900.
+
+Direct handwork with pick, chisel and hammer, and fire setting were
+the principal means of rock removal for centuries. Although various
+wedging systems were also in favor in some situations, their
+importance was so slight that they were not shown in the model.
+
+
+HOOSAC TUNNEL
+
+It was possible in the model series, without neglecting any major
+advancement in the art of rock tunneling, to complete the sequence of
+development with only a single additional model. Many of the greatest
+works of civil engineering have been those concerned directly with
+transport, and hence are the product of the present era, beginning in
+the early 19th century. The development of the ancient arts of route
+location, bridge construction, and tunnel driving received a powerful
+stimulation after 1800 under the impetus of the modern canal, highway,
+and, especially, the railroad.
+
+The Hoosac Tunnel, driven through Hoosac Mountain in the very
+northwest corner of Massachusetts between 1851 and 1875, was the first
+major tunneling work in the United States. Its importance is due not
+so much to this as to its being literally the fountainhead of modern
+rock-tunneling technology. The remarkable thing is that the work was
+begun using methods of driving almost unchanged during centuries
+previous, and was completed twenty years later by techniques which
+were, for the day, almost totally mechanized. The basic pattern of
+operation set at Hoosac, using pneumatic rock drills and efficient
+explosives, remains practically unchanged today.
+
+The general history of the Hoosac project is so thoroughly recorded
+that the briefest outline of its political aspects will suffice here.
+Hoosac Mountain was the chief obstacle in the path of a railroad
+projected between Greenfield, Massachusetts, and Troy, New York.
+The line was launched by a group of Boston merchants to provide a
+direct route to the rapidly developing West, in competition with the
+coastal routes via New York. The only route economically reasonable
+included a tunnel of nearly five miles through the mountain--a
+length absolutely without precedent, and an immense undertaking in
+view of the relatively primitive rock-working methods then available.
+
+[Illustration: Figure 5.--BURLEIGH ROCK DRILL, improved model of about
+1870, mounted on frame for surface work. (Catalog and price list: The
+Burleigh Rock Drill Company, 1876.)]
+
+The bore's great length and the desire for rapid exploitation inspired
+innovation from the outset of the work. The earliest attempts at
+mechanization, although ineffectual and without influence on tunnel
+engineering until many years later, are of interest. These took the
+form of several experimental machines of the "full area" type,
+intended to excavate the entire face of the work in a single operation
+by cutting one or more concentric grooves in the rock. The rock
+remaining between the grooves was to be blasted out. The first such
+machine tested succeeded in boring a 24-foot diameter opening for 10
+feet before its total failure. Several later machines proved of equal
+merit.[2] It was the Baltimore and Ohio's eminent chief engineer,
+Benjamin H. Latrobe, who in his _Report on the Hoosac Tunnel_
+(Baltimore, Oct. 1, 1862, p. 125) stated that such apparatus contained
+in its own structure the elements of failure, "... as they require
+the machines to do too much and the powder too little of the work,
+thus contradicting the fundamental principles upon which all
+labor-saving machinery is framed ... I could only look upon it as a
+misapplication of mechanical genius."
+
+[Illustration: Figure 6.--HOOSAC TUNNEL. Flash-powder photograph of
+Burleigh drills at the working face. (_Photo courtesy of State
+Library, Commonwealth of Massachusetts._)]
+
+Latrobe stated the basic philosophy of rock-tunnel work. No mechanical
+agent has ever been able to improve upon the efficiency of explosives
+for the shattering of rock. For this reason, the logical application
+of machinery to tunneling was not in replacing or altering the
+fundamental process itself, but in enabling it to be conducted with
+greater speed by mechanically drilling the blasting holes to receive
+the explosive.
+
+Actual work on the Hoosac Tunnel began at both ends of the tunnel in
+about 1854, but without much useful effect until 1858 when a contract
+was let to the renowned civil engineer and railroad builder, Herman
+Haupt of Philadelphia. Haupt immediately resumed investigations of
+improved tunneling methods, both full-area machines and mechanical
+rock drills. At this time mechanical rock-drill technology was in a
+state beyond, but not far beyond, initial experimentation. There
+existed one workable American machine, the Fowle drill, invented in
+1851. It was steam-driven, and had been used in quarry work, although
+apparently not to any commercial extent. However, it was far too
+large and cumbersome to find any possible application in tunneling.
+Nevertheless, it contained in its operating principle, the seed of a
+practical rock drill in that the drill rod was attached directly to
+and reciprocated by a double-acting steam piston. A point of great
+importance was the independence of its operation on gravity,
+permitting drilling in any direction.
+
+While experimenting, Haupt drove the work onward by the classical
+methods, shown in the left-hand section of the model (fig. 2). At
+the far right an advance heading or adit is being formed by pick and
+hammer work; this is then deepened into a top heading with enough
+height to permit hammer drilling, actually the basic tunneling
+operation. A team is shown "double jacking," i.e., using two-handed
+hammers, the steel held by a third man. This was the most efficient of
+the several hand-drilling methods. The top-heading plan was followed
+so that the bulk of the rock could be removed in the form of a bottom
+bench, and the majority of drilling would be downward, obviously the
+most effective direction. Blasting was with black powder and its
+commercial variants. Some liberty was taken in depicting these steps
+so that both operations might be shown within the scope of the model:
+in practice the heading was kept between 400 and 600 feet in advance
+of the bench so that heading blasts would not interfere with the bench
+work. The bench carriage simply facilitated handling of the blasted
+rock. It was rolled back during blasts.
+
+[Illustration: Figure 7.--HOOSAC TUNNEL. GROUP OF MINERS descending
+the west shaft with a Burleigh drill. (_Photo courtesy of State
+Library, Commonwealth of Massachusetts._)]
+
+The experiments conducted by Haupt with machine drills produced no
+immediate useful results. A drill designed by Haupt and his associate,
+Stuart Gwynn, in 1858 bored hard granite at the rate of 5/8 inch per
+minute, but was not substantial enough to bear up in service. Haupt
+left the work in 1861, victim of intense political pressures and
+totally unjust accusations of corruption and mismanagement. The
+work was suspended until taken over by a state commission in 1862.
+Despite frightful ineptitude and very real corruption, this period
+was exceedingly important in the long history both of Hoosac Tunnel
+and of rock tunneling in general.
+
+The merely routine criticism of the project had by this time become
+violent due to the inordinate length of time already elapsed and the
+immense cost, compared to the small portion of work completed. This
+served to generate in the commission a strong sense of urgency to
+hurry the project along. Charles S. Storrow, a competent engineer, was
+sent to Europe to report on the progress of tunneling there, and in
+particular on mechanization at the Mont Cenis Tunnel then under
+construction between France and Italy. Germain Sommeiller, its chief
+engineer, had, after experimentation similar to Haupt's, invented a
+reasonably efficient drilling machine which had gone into service at
+Mont Cenis in March 1861. It was a distinct improvement over hand
+drilling, almost doubling the drilling rate, but was complex and
+highly unreliable. Two hundred drills were required to keep 16 drills
+at work. But the vital point in this was the fact that Sommeiller
+drove his drills not with steam, but air, compressed at the tunnel
+portals and piped to the work face. It was this single factor, one of
+application rather than invention, that made the mechanical drill
+feasible for tunneling.
+
+All previous effort in the field of machine drilling, on both sides
+of the Atlantic, had been directed toward steam as the motive power.
+In deep tunnels, with ventilation already an inherent problem, the
+exhaust of a steam drill into the atmosphere was inadmissible.
+Further, steam could not be piped over great distances due to serious
+losses of energy from radiation of heat, and condensation. Steam
+generation within the tunnel itself was obviously out of the question.
+It was the combination of a practical drill, and the parallel
+invention by Sommeiller of a practical air compressor that resulted in
+the first workable application of machine rock drilling to tunneling.
+
+[Illustration: Figures 8 & 9.--HOOSAC TUNNEL. CONTEMPORARY
+ENGRAVINGS. As such large general areas could not be sufficiently
+illuminated for photography, the Museum model was based primarily on
+artists' versions of the work. (_Science Record_, 1872; _Leslie's
+Weekly_, 1873.)]
+
+The Sommeiller drills greatly impressed Storrow, and his report of
+November 1862 strongly favored their adoption at Hoosac. It is curious
+however, that not a single one was brought to the U.S., even on trial.
+Storrow does speak of Sommeiller's intent to keep the details of the
+machine to himself until it had been further improved, with a view to
+its eventual exploitation. The fact is, that although workable, the
+Sommeiller drill proved to be a dead end in rock-drill development
+because of its many basic deficiencies. It did exert the indirect
+influence of inspiration which, coupled with a pressing need for
+haste, led to renewed trials of drilling machinery at Hoosac. Thomas
+Doane, chief engineer under the state commission, carried this program
+forth with intensity, seeking and encouraging inventors, and himself
+working on the problem. The pattern of the Sommeiller drill was
+generally followed; that is, the drill was designed as a separate,
+relatively light mechanical element, adapted for transportation by
+several miners, and attachable to a movable frame or carriage during
+operation. Air was of course the presumed power. To be effective, it
+was necessary that a drill automatically feed the drill rod as the
+hole deepened, and also rotate the rod automatically to maintain a
+round, smooth hole. Extreme durability was essential, and usually
+proved the source of a machine's failure. The combination of these
+characteristics into a machine capable of driving the drill rod into
+the rock with great force, perhaps five times per second, was a
+severe test of ingenuity and materials. Doane in 1864 had three
+different experimental drills in hand, as well as various steam
+and water-powered compressors.
+
+Success finally came in 1865 with the invention of a drill by Charles
+Burleigh, a mechanical engineer at the well-known Putnam Machine Works
+of Fitchburg, Massachusetts. The drills were first applied in the east
+heading in June of 1866. Although working well, their initial success
+was limited by lack of reliability and a resulting high expense for
+repairs. They were described as having "several weakest points." In
+November, these drills were replaced by an improved Burleigh drill
+which was used with total success to the end of the work. The era of
+modern rock tunneling was thus launched by Sommeiller's insight in
+initially applying pneumatic power to a machine drill, by Doane's
+persistence in searching for a thoroughly practical drill, and by
+Burleigh's mechanical talent in producing one. The desperate need to
+complete the Hoosac Tunnel may reasonably be considered the greatest
+single spur to the development of a successful drill.
+
+The significance of this invention was far reaching. Burleigh's was
+the first practical mechanical rock drill in America and, in view of
+its dependability, efficiency, and simplicity when compared to the
+Sommeiller drill, perhaps in the world. The Burleigh drill achieved
+success almost immediately. It was placed in production by Putnam for
+the Burleigh Rock Drill Company before completion of Hoosac in 1876,
+and its use spread throughout the western mining regions and other
+tunnel works. For a major invention, its adoption was, in relative
+terms, instantaneous. It was the prototype of all succeeding
+piston-type drills, which came to be known generically as "burleighs,"
+regardless of manufacture. Walter Shanley, the Canadian contractor who
+ultimately completed the Hoosac, reported in 1870, after the drills
+had been in service for a sufficient time that the techniques for
+their most efficient use were fully understood and effectively
+applied, that the Burleigh drills saved about half the drilling costs
+over hand drilling. The per-inch cost of machine drilling averaged 5.5
+cents, all inclusive, vs. 11.2 cents for handwork. The more important
+point, that of speed, is shown by the reports of average monthly
+progress of the tunnel itself, before and after use of the air drills.
+
+  _Year_     _Average monthly
+              progress in feet_
+
+  1865              55
+  1866              48
+  1867              99
+  1868              --
+  1869             138
+  1870             126
+  1871             145
+  1872             124
+
+[Illustration: Figure 10.--TRINITROGLYCERINE BLAST at Hoosac Tunnel.
+(_Leslie's Weekly_, 1873.)]
+
+The right portion of the model (fig. 3) represents the workings during
+the final period. The bottom heading system was generally used after
+the Burleigh drills had been introduced. Four to six drills were
+mounted on a carriage designed by Doane. These drove the holes for
+the first blast in the center of the heading in about six hours. The
+full width of the heading, the 24-foot width of the tunnel, was then
+drilled and blasted out in two more stages. As in the early section,
+the benches to the rear were later removed to the full-tunnel height
+of about 20 feet. This operation is shown by a single drill (fig. 4)
+mounted on a screw column. Three 8-hour shifts carried the work
+forward: drilling occupied half the time and half was spent in running
+the carriage back, blasting, and mucking (clearing the broken rock).
+
+[Illustration: Figure 11.--HOOSAC TUNNEL survey crew at engineering
+office. The highest accuracy of the aboveground and underground survey
+work was required to insure proper vertical and horizontal alignment
+and meeting of the several separately driven sections. (_Photo
+courtesy of State Library, Commonwealth of Massachusetts._)]
+
+The tunnel's 1028-foot central shaft, completed under the Shanley
+contract in 1870 to provide two additional work faces as well as a
+ventilation shaft is shown at the far right side of this half of the
+model. Completed so near the end of the project, only 15 percent of
+the tunnel was driven from the shaft.
+
+The enormous increase in rate of progress was not due entirely to
+machine drilling. From the outset of his jurisdiction, Doane undertook
+experiments with explosives as well as drills, seeking an agent more
+effective than black powder. In this case, the need for speed was not
+the sole stimulus. As the east and west headings advanced further and
+further from the portals, the problem of ventilation grew more acute,
+and it became increasingly difficult to exhaust the toxic fumes
+produced by the black powder blasts.
+
+In 1866, Doane imported from Europe a sample of trinitroglycerine,
+the liquid explosive newly introduced by Nobel, known in Europe as
+"glonoïn oil" and in the United States as "nitroglycerine." It already
+had acquired a fearsome reputation from its tendency to decompose with
+heat and age and to explode with or without the slightest provocation.
+Nevertheless, its tremendous power and characteristic of almost
+complete smokelessness led Doane to employ the chemist George W.
+Mowbray, who had blasted for Drake in the Pennsylvania oil fields, to
+develop techniques for the bulk manufacture of the new agent and for
+its safe employment in the tunnel.
+
+Mowbray established a works on the mountain and shortly developed
+a completely new blasting practice based on the explosive. Its
+stability was greatly increased by maintaining absolute purity in the
+manufacturing process. Freezing the liquid to reduce its sensitivity
+during transport to the headings, and extreme caution in its handling
+further reduced the hazard of its use. At the heading, the liquid was
+poured into cylindrical cartridges for placement in the holes. As with
+the Burleigh drill, the general adoption of nitroglycerine was
+immediate once its qualities had been demonstrated. The effect on the
+work was notable. Its explosive characteristics permitted fewer blast
+holes over a given frontal area of working face, and at the same time
+it was capable of effectively blowing from a deeper drill hole, 42
+inches against 30 inches for black powder, so that under ideal
+conditions 40 percent more tunnel length was advanced per cycle of
+operations. A new fuse and a system of electric ignition were
+developed which permitted simultaneous detonation and resulted
+in a degree of effectiveness impossible with the powder train and
+cord fusing used with the black powder. Over a million pounds of
+nitroglycerine were produced by Mowbray between 1866 and completion
+of the tunnel.
+
+[Illustration: Figure 12.--WORKS AT THE CENTRAL SHAFT, HOOSAC TUNNEL,
+for hoisting, pumping and air compressing machinery, and general
+repair, 1871. (_Photo courtesy of State Library, Commonwealth of
+Massachusetts._)]
+
+[Illustration: Figure 13.--HOOSAC TUNNEL. AIR-COMPRESSOR BUILDING on
+Hoosac River near North Adams. The compressors were driven partially
+by waterpower, derived from the river. (_Photo courtesy of State
+Library, Commonwealth of Massachusetts._)]
+
+[Illustration: Figure 14.--WEST PORTAL OF HOOSAC TUNNEL before
+completion, 1868, showing six rings of lining brick. (_Photo
+courtesy of State Library, Commonwealth of Massachusetts._)]
+
+When the Shanleys took the work over in 1868, following political
+difficulties attending operation by the State, the period of
+experimentation was over. The tunnel was being advanced by totally
+modern methods, and to the present day the overall concepts have
+remained fundamentally unaltered: the Burleigh piston drill has been
+replaced by the lighter hammer drill; the Doane drill carriage by the
+more flexible "jumbo"; nitroglycerine by its more stable descendant
+dynamite and its alternatives; and static-electric blasting machines
+by more dependable magnetoelectric. But these are all in the nature of
+improvements, not innovations.
+
+Unlike the preceding model, there was good documentation for this one.
+Also, the Hoosac was apparently the first American tunnel to be well
+recorded photographically. Early flashlight views exist of the drills
+working at the heading (fig. 6) as well as of the portals, the winding
+and pumping works at the central shaft, and much of the machinery and
+associated aspects of the project. These and copies of drawings of
+much of Doane's experimental apparatus, a rare technological record,
+are preserved at the Massachusetts State Library.
+
+
+
+
+Soft-Ground Tunneling
+
+
+So great is the difference between hard-rock and soft-ground tunneling
+that they constitute two almost separate branches of the field. In
+penetrating ground lacking the firmness or cohesion to support itself
+above an opening, the miner's chief concern is not that of removing
+the material, but of preventing its collapse into his excavation. The
+primitive methods depending upon brute strength and direct application
+of fire and human force were suitable for assault on rock, but lacked
+the artifice needed for delving into less stable material. Roman
+engineers were accomplished in spanning subterranean ways with masonry
+arches, but apparently most of their work was done by cut-and-cover
+methods rather than by actual mining.
+
+Not until the Middle Ages did the skill of effectively working
+openings in soft ground develop, and not until the Renaissance
+was this development so consistently successful that it could
+be considered a science.
+
+
+RENAISSANCE MINING
+
+From the earliest periods of rock working, the quest for minerals
+and metals was the primary force that drove men underground. It was
+the technology of mining, the product of slow evolution over the
+centuries, that became the technology of the early tunnel, with no
+significant modification except in size of workings.
+
+Every aspect of 16th century mining is definitively detailed in
+Georgius Agricola's remarkable _De re Metallica_, first published in
+Basel in 1556. During its time of active influence, which extended
+for two centuries, it served as the authoritative work on the subject.
+It remains today an unparalleled early record of an entire branch
+of technology. The superb woodcuts of mine workings and tools in
+themselves constitute a precise description of the techniques of the
+period, and provided an ideal source of information upon which to base
+the first model in the soft-ground series.
+
+[Illustration: Figure 15.--CENTERING FOR PLACEMENT OF FINISHED
+STONEWORK at west portal, 1874. At top-right are the sheds where
+the lining brick was produced. (_Photo courtesy of State Library,
+Commonwealth of Massachusetts._)]
+
+The model, representing a typical European mine, demonstrates the
+early use of timber frames or "sets" to support the soft material of
+the walls and roof. In areas of only moderate instability, the sets
+alone were sufficient to counteract the earth pressure, and were
+spaced according to the degree of support required. In more extreme
+conditions, a solid lagging of small poles or boards was set outside
+the frames, as shown in the model, to provide absolute support of the
+ground. Details of the framing, the windlass, and all tools and
+appliances were supplied by Agricola, with no need for interpretation
+or interpolation.
+
+The basic framing pattern of sill, side posts and cap piece, all
+morticed together, with lagging used where needed, was translated
+unaltered into tunneling practice, particularly in small exploratory
+drifts. It remained in this application until well into the 20th
+century.
+
+The pressure exerted upon tunnels of large area was countered during
+construction by timbering systems of greater elaboration, evolved
+from the basic one. By the time that tunnels of section large
+enough to accommodate canals and railways were being undertaken as
+matter-of-course civil engineering works, a series of nationally
+distinguishable systems had emerged, each possessing characteristic
+points of favor and fault. As might be suspected, the English system
+of tunnel timbering, for instance, was rarely applied on the
+Continent, nor were the German, Austrian or Belgian systems normally
+seen in Great Britain. All were used at one time or another in this
+country, until the American system was introduced in about 1855.
+While the timbering commonly remained in place in mines, it would be
+followed up by permanent masonry arching and lining in tunnel work.
+
+Overhead in the museum Hall of Civil Engineering are frames
+representing the English, Austrian and American systems. Nearby, a
+series of small relief models (fig. 19) is used to show the sequence
+of enlargement in a soft-ground railroad tunnel of about 1855, using
+the Austrian system. Temporary timber support of tunnels fell from use
+gradually after the advent of shield tunneling in conjunction with
+cast-iron lining. This formed a perfect support immediately behind the
+shield, as well as the permanent lining of the tunnel.
+
+
+BRUNEL'S THAMES TUNNEL
+
+The interior surfaces of tunnels through ground merely unstable are
+amenable to support by various systems of timbering and arching. This
+becomes less true as the fluidity of the ground increases. The soft
+material which normally comprises the beds of rivers can approach an
+almost liquid condition resulting in a hydraulic head from the
+overbearing water sufficient to prevent the driving of even the most
+carefully worked drift, supported by simple timbering. The basic
+defect of the timbering systems used in mining and tunneling was
+that there was inevitably a certain amount of the face or ceiling
+unsupported just previous to setting a frame, or placing over it the
+necessary section of lagging. In mine work, runny soil could, and did,
+break through such gaps, filling the working. For this reason, there
+were no serious attempts made before 1825 to drive subaqueous tunnels.
+
+In that year, work was started on a tunnel under the Thames between
+the Rotherhithe and Wapping sections of London, under guidance of the
+already famous engineer Marc Isambard Brunel (1769-1849), father of
+I. K. Brunel. The undertaking is of great interest in that Brunel
+employed an entirely novel apparatus of his own invention to provide
+continuous and reliable support of the soft water-bearing clay which
+formed the riverbed. By means of this "shield," Brunel was able to
+drive the world's first subaqueous tunnel.[3]
+
+[Illustration: Figure 16.--WEST PORTAL UPON COMPLETION, 1876.
+(_Photo courtesy of New-York Historical Society._)]
+
+The shield was of cast-iron, rectangular in elevation, and was
+propelled forward by jackscrews. Shelves at top, bottom, and sides
+supported the tunnel roof, floor, and walls until the permanent brick
+lining was placed. The working face, the critical area, was supported
+by a large number of small "breasting boards," held against the ground
+by small individual screws bearing against the shield framework. The
+shield itself was formed of 12 separate frames, each of which could be
+advanced independently of the others. The height was 22 feet 3 inches:
+the width 37 feet 6 inches.
+
+The progress was piecemeal. In operation the miners would remove one
+breasting board at a time, excavate in front of it, and then replace
+it in the advanced position--about 6 inches forward. This was repeated
+with the next board above or below, and the sequence continued until
+the ground for the entire height of one of the 12 sections had been
+removed. The board screws for that section were shifted to bear on the
+adjacent frames, relieving the frame of longitudinal pressure. It
+could then be screwed forward by the amount of advance, the screws
+bearing to the rear on the completed masonry. Thus, step by step the
+tunnel progressed slowly, the greatest weekly advance being 14 feet.
+
+In the left-hand portion of the model is the shaft sunk to begin
+operations; here also is shown the bucket hoist for removing the
+spoil. The V-type steam engine powering the hoist was designed by
+Brunel. At the right of the main model is an enlarged detail of the
+shield, actually an improved version built in 1835.
+
+The work continued despite setbacks of every sort. The financial ones
+need no recounting here. Technically, although the shield principle
+proved workable, the support afforded was not infallible. Four or five
+times the river broke through the thin cover of silt and flooded
+the workings, despite the utmost caution in excavating. When this
+occurred, masses of clay, sandbags, and mats were dumped over the
+opening in the riverbed to seal it, and the tunnel pumped out. I. K.
+Brunel acted as superintendent and nearly lost his life on a number of
+occasions. After several suspensions of work resulting from withdrawal
+or exhaustion of support, one lasting seven years, the work was
+completed in 1843.
+
+Despite the fact that Brunel had, for the first time, demonstrated a
+practical method for tunneling in firm and water-bearing ground, the
+enormous cost of the work and the almost overwhelming problems
+encountered had a discouraging effect rather than otherwise. Not for
+another quarter of a century was a similar project undertaken.
+
+The Thames Tunnel was used for foot and light highway traffic until
+about 1870 when it was incorporated into the London Underground
+railway system, which it continues to serve today. The roofed-over
+top sections of the two shafts may still be seen from the river.
+
+A number of contemporary popular accounts of the tunnel exist, but one
+of the most thorough and interesting expositions on a single tunnel
+work of any period is Henry Law's _A Memoir of the Thames Tunnel_,
+published in 1845-1846 by John Weale. Law, an eminent civil engineer,
+covers the work in incredible detail from its inception until the
+major suspension in late 1828 when slightly more than half completed.
+The most valuable aspect of his record is a series of plates of
+engineering drawings of the shield and its components, which, so
+far as is known, exist nowhere else. These formed the basis of the
+enlarged section of the shield, shown to the right of the model of the
+tunnel itself. A vertical section through the shield is reproduced
+here from Law for comparison with the model (figs. 21 and 23).
+
+[Illustration: Figure 17.--SOFT-GROUND TUNNELING. The support of
+walls and roof of mine shaft by simple timbering; 16th century.
+MHT model--3/4" scale. (Smithsonian photo 49260-J.)]
+
+[Illustration: Figure 18.--SOFT-GROUND TUNNELING. The model of a 16th
+century mine in the Museum of History and Technology was constructed
+from illustrations in such works as G. E. von Löhneyss' _Bericht vom
+Bergwerck_, 1690, as well as the better known ones from _De re
+Metallica_.]
+
+[Illustration: Figure 19.--THE SUCCESSIVE STAGES in the enlargement
+of a mid-19th century railroad tunnel, using the Austrian system of
+timbering. MHT model.]
+
+[Illustration: Figure 20.--M. I. BRUNEL'S THAMES TUNNEL, 1825-1843,
+the first driven beneath a body of water. MHT model--1/4" scale.
+(Smithsonian photo 49260-F.)]
+
+
+THE TOWER SUBWAY
+
+Various inventors attempted to improve upon the Brunel shield, aware
+of the fundamental soundness of the shield principle. Almost all
+bypassed the rectangular sectional construction used in the Thames
+Tunnel, and took as a starting point a sectional shield of circular
+cross section, advanced by Brunel in his original patent of 1818.
+James Henry Greathead (1844-1896), rightfully called the father of
+modern subaqueous tunneling, surmised in later years that Brunel had
+chosen a rectangular configuration for actual use, as one better
+adapted to the sectional type of shield. The English civil engineer,
+Peter W. Barlow, in 1864 and 1868 patented a circular shield, of one
+piece, which was the basis of one used by him in constructing a small
+subway of 1350 feet beneath the Thames in 1869, the first work to
+follow the lead of Brunel. Greathead, acting as Barlow's contractor,
+was the designer of the shield actually used in the work, but it was
+obviously inspired by Barlow's patents.
+
+The reduction of the multiplicity of parts in the Brunel shield to
+a single rigid unit was of immense advantage and an advance perhaps
+equal to the shield concept of tunneling itself. The Barlow-Greathead
+shield was like the cap of a telescope with a sharpened circular ring
+on the front to assist in penetrating the ground. The diaphragm
+functioned, as did Brunel's breasting boards, to resist the
+longitudinal earth pressure of the face, and the cylindrical portion
+behind the diaphragm bore the radial pressure of roof and walls. Here
+also for the first time, a permanent lining formed of cast-iron
+segments was used, a second major advancement in soft-ground tunneling
+practice. Not only could the segments be placed and bolted together
+far more rapidly than masonry lining could be laid up, but unlike the
+green masonry, they could immediately bear the full force of the
+shield-propelling screws.
+
+Barlow, capitalizing on Brunel's error in burrowing so close to the
+riverbed, maintained an average cover of 30 feet over the tunnel,
+driving through a solid stratum of firm London clay which was
+virtually impervious to water. As the result of this, combined with
+the advantages of the solid shield and the rapidly placed iron lining,
+the work moved forward at a pace and with a facility in startling
+contrast to that of the Thames Tunnel, although in fairness it must be
+recalled that the face area was far less.
+
+The clay was found sufficiently sound that it could be readily
+excavated without the support of the diaphragm, and normally three
+miners worked in front of the shield, digging out the clay and passing
+it back through a doorway in the plate. This could be closed in case
+of a sudden settlement or break in. Following excavation, the shield
+was advanced 18 inches into the excavated area by means of 6 screws,
+and a ring of lining segments 18 inches in length bolted to the
+previous ring under cover of the overlapping rear skirt of the shield.
+The small annular space left between the outside of the lining and the
+clay by the thickness and clearance of the skirt--about an inch--was
+filled with thin cement grout. The tunnel was advanced 18 inches
+during each 8-hour shift. The work continued around the clock, and the
+900-foot river section was completed in only 14 weeks.[4] The entire
+work was completed almost without incident in just under a year, a
+remarkable performance for the world's second subaqueous tunnel.
+
+[Illustration: Figure 21.--ENLARGED DETAIL of Brunel's tunneling
+shield, vertical section. The first two and part of the third of the
+twelve frames are shown. To the left is the tunnel's completed brick
+lining and to the right, the individual breasting boards and screws
+for supporting the face. The propelling screws are seen at top and
+bottom, bearing against the lining. Three miners worked in each frame,
+one above the other. MHT model--3/4" scale. (Smithsonian photo
+49260-G.)]
+
+[Illustration: Figure 22.--BROADSIDE PUBLISHED AFTER COMMENCEMENT
+OF WORK on the Thames Tunnel, 1827. (MHT collections.)
+
+  OPEN TO THE PUBLIC EVERY DAY (_Sundays excepted_) _from Seven in the
+  Morning, until Eight in the Evening_,
+
+  THE THAMES TUNNEL.
+
+  Fig. 1 shows a transverse section of the Thames, and beneath it a
+  longitudinal section of the Tunnel, as it will be when completed;
+  with the ascents in the inclinations in which they will be finished.
+
+  Fig. 2 shows the two arched entrances of the Tunnel from the shaft.
+
+  Fig. 3 is a representation of the iron shield, and shows a workman
+  in each of the compartments.
+
+  The Entrance to the Tunnel is near to Rotherhithe Church, and nearly
+  opposite to the London-Docks. The nearest landing place from the river
+  is Church Stairs. The Greenwich and Deptford coaches which go the
+  lower road, start hourly from Charing-cross, and Gracechurch-street,
+  and pass close by the works at Rotherhithe.
+
+  Books relative to the Tunnel may be had at the works.
+
+  The Public may view the Tunnel every day (Sundays excepted) from
+  Seven in the morning until Eight in the Evening, upon payment of
+  One Shilling each Person.
+
+  The extreme northern end of the Tunnel is for the present secured
+  by a strong wall; but visitors will find a dry, warm, and gravelled
+  promenade, as far as to almost the centre of the river, and
+  brilliantly lighted with oil gas.
+
+  The entrance is from Rotherhithe Street, and by a safe, commodious,
+  and easy stair case.
+
+    H. Teape & Son, Printers, Tower-hill, London.]
+
+[Illustration: Figure 23.--VERTICAL SECTION THROUGH BRUNEL'S SHIELD.
+The long lever, x, supported the wood centering for turning the
+masonry arches of the lining. (LAW, _A Memoir of the Thames
+Tunnel._)]
+
+[Illustration: Figure 24.--THAMES TUNNEL. SECTION THROUGH riverbed and
+tunnel following one of the break-throughs of the river. Inspection of
+the damage with a diving bell. (BEAMISH, _A Memoir of the Life of Sir
+Marc Isambard Brunel_.)]
+
+The Tower Subway at first operated with cylindrical cars that nearly
+filled the 7-foot bore; the cars were drawn by cables powered by small
+steam engines in the shafts. This mode of power had previously been
+used in passenger service only on the Greenwich Street elevated
+railway in New York. Later the cars were abandoned as unprofitable and
+the tunnel turned into a footway (fig. 32). This small tunnel, the
+successful driving due entirely to Greathead's skill, was the
+forerunner of the modern subaqueous tunnel. In it, two of the three
+elements essential to such work thereafter were first applied: the
+one-piece movable shield of circular section, and the segmental
+cast-iron lining.
+
+The documentation of this work is far thinner than for the Thames
+Tunnel. The most accurate source of technical information is a brief
+historical account in Copperthwaite's classic _Tunnel Shields and the
+Use of Compressed Air in Subaqueous Works_, published in 1906.
+Copperthwaite, a successful tunnel engineer, laments the fact that he
+was able to turn up no drawing or original data on this first shield
+of Greathead's, but he presents a sketch of it prepared in the
+Greathead office in 1895, which is presumably a fair representation
+(fig. 33). The Tower Subway model was built on the basis of this and
+several woodcuts of the working area that appeared contemporaneously
+in the illustrated press. In this and the adjacent model of Beach's
+Broadway Subway, the tunnel axis has been placed on an angle to the
+viewer, projecting the bore into the case so that the complete circle
+of the working face is included for a more suggestive effect. This was
+possible because of the short length of the work included.
+
+Henry S. Drinker, also a tunnel engineer and author of the most
+comprehensive work on tunneling ever published, treats rock tunneling
+in exhaustive detail up to 1878. His notice of what he terms
+"submarine tunneling" is extremely brief. He does, however, draw a
+most interesting comparison between the first Thames Tunnel, built by
+Brunel, and the second, built by Greathead 26 years later:
+
+    FIRST THAMES TUNNEL             SECOND THAMES TUNNEL
+                                     (TOWER SUBWAY)
+
+  Brickwork lining, 38 feet       Cast-iron lining of 8 feet
+  wide by 22-1/2 feet high.       outside diameter.
+
+  120-ton cast-iron shield,       2-1/2-ton, wrought-iron shield,
+  accommodating 36 miners.        accommodating at most 3 men.
+
+  Workings filled by irruption   "Water encountered at almost
+  of river five times.            any time could have been
+                                  gathered in a stable pail."
+
+  Eighteen years elapsed between  Work completed in about
+  start and finish of work.       eleven months.
+
+  Cost: $3,000,000.               Cost: $100,000.
+
+[Illustration: Figure 25.--TRANSVERSE SECTION THROUGH SHIELD, after
+inundation. Such disasters, as well as the inconsistency of the
+riverbed's composition, seriously disturbed the alignment of the
+shield's individual sections. (LAW, _A Memoir of the Thames Tunnel_.)]
+
+[Illustration: Figure 26.--LONGITUDINAL SECTION THROUGH THAMES TUNNEL
+after sandbagging to close a break in the riverbed. The tunnel is
+filled with silt and water. (LAW, _A Memoir of the Thames Tunnel_.)]
+
+[Illustration: Figure 27.--INTERIOR OF THE THAMES TUNNEL shortly after
+completion in 1843. (_Photo courtesy of New York Public Library
+Picture Collection._)]
+
+[Illustration: Figure 28.--THAMES TUNNEL in use by London Underground
+railway. (_Illustrated London News_, 1869?)]
+
+[Illustration: Figure 29.--PLACING A segment of cast-iron lining in
+Greathead's Tower Subway, 1869. To the rear is the shield's diaphragm
+or bulkhead. MHT model--1-1/2" scale. (Smithsonian photo 49260-B.)]
+
+
+BEACH'S BROADWAY SUBWAY
+
+Almost simultaneously with the construction of the Tower Subway,
+the first American shield tunnel was driven by Alfred Ely Beach
+(1826-1896). Beach, as editor of the _Scientific American_ and
+inventor of, among other things, a successful typewriter as early as
+1856, was well known and respected in technical circles. He was not
+a civil engineer, but had become concerned with New York's pressing
+traffic problem (even then) and as a solution, developed plans for a
+rapid-transit subway to extend the length of Broadway. He invented a
+shield as an adjunct to this system, solely to permit driving of the
+tunnel without disturbing the overlying streets.
+
+An active patent attorney as well, Beach must certainly have known of
+and studied the existing patents for tunneling shields, which were,
+without exception, British. In certain aspects his shield resembled
+the one patented by Barlow in 1864, but never built. However, work on
+the Beach tunnel started in 1869, so close in time to that on the
+Tower Subway, that it is unlikely that there was any influence from
+that source. Beach had himself patented a shield, in June 1869, a
+two-piece, sectional design that bore no resemblance to the one used.
+His subway plan had been first introduced at the 1867 fair of the
+American Institute in the form of a short plywood tube through which
+a small, close-fitting car was blown by a fan. The car carried 12
+passengers. Sensing opposition to the subway scheme from Tammany, in
+1868 Beach obtained a charter to place a small tube beneath Broadway
+for transporting mail and small packages pneumatically, a plan he
+advocated independently of the passenger subway.
+
+[Illustration: Figure 30.--CONTEMPORARY ILLUSTRATIONS of Tower
+Subway works used as basis of the model in the Museum of History
+and Technology. (_Illustrated London News_, 1869.)
+
+  ADVANCING THE SHIELD. FITTING THE CASTINGS.]
+
+[Illustration: Figure 31.--EXCAVATION IN FRONT OF SHIELD, Tower
+Subway. This was possible because of the stiffness of the clay
+encountered. MHT model--front of model shown in fig. 29.
+(Smithsonian photo 49260-A.)]
+
+Under this thin pretense of legal authorization, the sub-rosa
+excavation began from the basement of a clothing store on Warren
+Street near Broadway. The 8-foot-diameter tunnel ran eastward a short
+distance, made a 90-degree turn, and thence southward under Broadway
+to stop a block away under the south side of Murray Street. The total
+distance was about 312 feet. Work was carried on at night in total
+secrecy, the actual tunneling taking 58 nights. At the Warren Street
+terminal, a waiting room was excavated and a large Roots blower
+installed for propulsion of the single passenger car. The plan was
+similar to that used with the model in 1867: the cylindrical car
+fitted the circular tunnel with only slight circumferential clearance.
+The blower created a plenum within the waiting room and tunnel area
+behind the car of about 0.25 pounds per square inch, resulting in a
+thrust on the car of almost a ton, not accounting for blowby. The car
+was thus blown along its course, and was returned by reversing the
+blower's suction and discharge ducts to produce an equivalent vacuum
+within the tunnel.
+
+[Illustration: Figure 32.--INTERIOR OF COMPLETED TOWER SUBWAY.
+(THORNBURY, _Old and New London, 1887, vol. 1, p. 126_.)]
+
+The system opened in February of 1870 and remained in operation for
+about a year. Beach was ultimately subdued by the hostile influences
+of Boss Tweed, and the project was completely abandoned. Within a very
+few more years the first commercially operated elevated line was
+built, but the subway did not achieve legitimate status in New York
+until the opening of the Interborough line in 1904. Ironically, its
+route traversed Broadway for almost the length of the island.
+
+[Illustration: Figure 33.--VERTICAL SECTION through the Greathead
+shield used at the Tower Subway, 1869. The first one-piece shield of
+circular section. (COPPERTHWAITE, _Tunnel Shields and the Use of
+Compressed Air in Subaqueous Works_.)]
+
+The Beach shield operated with perfect success in this brief trial,
+although the loose sandy soil encountered was admittedly not a severe
+test of its qualities. No diaphragm was used; instead a series of 8
+horizontal shelves with sharpened leading edges extended across the
+front opening of the shield. The outstanding feature of the machine
+was the substitution for the propelling screws used by Brunel and
+Greathead of 18 hydraulic rams, set around its circumference. These
+were fed by a single hand-operated pump, seen in the center of figure
+34. By this means the course of the shield's forward movement could be
+controlled with a convenience and precision not attainable with
+screws. Vertical and horizontal deflection was achieved by throttling
+the supply of water to certain of the rams, which could be
+individually controlled, causing greater pressure on one portion of
+the shield than another. This system has not changed in the ensuing
+time, except, of course, in the substitution of mechanically produced
+hydraulic pressure for hand.
+
+[Illustration: Figure 34.--BEACH'S Broadway Subway. Advancing the
+shield by hydraulic rams, 1869. MHT model--1-1/2" scale. (Smithsonian
+photo 49260-E.)]
+
+[Illustration: Figure 35.--VERTICAL SECTION through the Beach shield
+used on the Broadway Subway, showing the horizontal shelves (C), iron
+cutting ring (B), hydraulic rams (D), hydraulic pump (F), and rear
+protective skirt (H). (_Scientific American_, March 5, 1870.)]
+
+Unlike the driving of the Tower Subway, no excavation was done in
+front of the shield. Rather, the shield was forced by the rams into
+the soil for the length of their stroke, the material which entered
+being supported by the shelves. This was removed from the shelves and
+hauled off. The ram plungers then were withdrawn and a 16-inch length
+of the permanent lining built up within the shelter of the shield's
+tail ring. Against this, the rams bore for the next advance. Masonry
+lining was used in the straight section; cast-iron in the curved. The
+juncture is shown in the model.
+
+[Illustration: Figure 36.--INTERIOR of Beach Subway showing iron
+lining on curved section and the pneumatically powered passenger car.
+View from waiting room. (_Scientific American_, March 5, 1870.)]
+
+Enlarged versions of the Beach shield were used in a few tunnels in
+the Midwest in the early 1870's, but from then until 1886 the shield
+method, for no clear reason, again entered a period of disuse finding
+no application on either side of the Atlantic despite its virtually
+unqualified proof at the hands of Greathead and Beach. Little precise
+information remains on this work. The Beach system of pneumatic
+transit is described fully in a well-illustrated booklet published
+by him in January 1868, in which the American Institute model is
+shown, and many projected systems of pneumatic propulsion as well
+as of subterranean and subaqueous tunneling described. Beach again
+(presumably) is author of the sole contemporary account of the
+Broadway Subway, which appeared in _Scientific American_ following its
+opening early in 1870. Included are good views of the tunnel and car,
+of the shield in operation, and, most important, a vertical sectional
+view through the shield (fig. 35).
+
+It is interesting to note that optical surveys for maintenance of the
+course apparently were not used. The article illustrated and described
+the driving each night of a jointed iron rod up through the tunnel
+roof to the street, twenty or so feet above, for "testing the
+position."
+
+
+THE FIRST HUDSON RIVER TUNNEL
+
+Despite the ultimate success of Brunel's Thames Tunnel in 1843, the
+shield in that case afforded only moderately reliable protection
+because of the fluidity of the soil driven through, and its tendency
+to enter the works through the smallest opening in the shield's
+defense. An English doctor who had made physiological studies of the
+effects on workmen of the high air pressure within diving bells is
+said to have recommended to Brunel in 1828 that he introduce an
+atmosphere of compressed air into the tunnel to exclude the water
+and support the work face.
+
+This plan was first formally described by Sir Thomas Cochrane
+(1775-1860) in a British patent of 1830. Conscious of Brunel's
+problems, he proposed a system of shaft sinking, mining, and tunneling
+in water-bearing materials by filling the excavated area with air
+sufficiently above atmospheric pressure to prevent the water from
+entering and to support the earth. In this, and his description of air
+locks for passage of men and materials between the atmosphere and the
+pressurized area, Cochrane fully outlined the essential features of
+pneumatic excavation as developed since.
+
+[Illustration: Figure 37.--THE GIANT ROOTS LOBE-TYPE BLOWER used for
+propelling the car.]
+
+In 1839, a French engineer first used the system in sinking a mine
+shaft through a watery stratum. From then on, the sinking of shafts,
+and somewhat later the construction of bridge pier foundations, by the
+pneumatic method became almost commonplace engineering practice in
+Europe and America. Not until 1879 however, was the system tried in
+tunneling work, and then, as with the shield ten years earlier, almost
+simultaneously here and abroad. The first application was in a small
+river tunnel in Antwerp, only 5 feet in height. This project was
+successfully completed relying on compressed air alone to support the
+earth, no shield being used. The importance of the work cannot be
+considered great due to its lack of scope.
+
+[Illustration: Figure 38.--TESTING ALIGNMENT of the Broadway Subway at
+night by driving a jointed rod up to street level. (_Scientific
+American_, March 5, 1870.)]
+
+In 1871 Dewitt C. Haskin (1822-1900), a west coast mine and railroad
+builder, became interested in the pneumatic caissons then being used
+to found the river piers of Eads' Mississippi River bridge at St.
+Louis. In apparent total ignorance of the Cochrane patent, he evolved
+a similar system for tunneling water-bearing media, and in 1873
+proposed construction of a tunnel through the silt beneath the Hudson
+to provide rail connection between New Jersey and New York City.
+
+[Illustration: Figure 39.--HASKIN'S pneumatically driven tunnel
+under the Hudson River, 1880. In the engine room at top left was the
+machinery for hoisting, generating electricity for lighting, and air
+compressing. The air lock is seen in the wall of the brick shaft.
+MHT model--0.3" scale. (Smithsonian photo 49260.)]
+
+[Illustration: Figure 40.--ARTIST'S CONCEPTION OF MINERS escaping
+into the air lock during the blowout in Haskin's tunnel.]
+
+It would be difficult to imagine a site more in need of such
+communication. All lines from the south terminated along the west
+shore of the river and the immense traffic--cars, freight and
+passengers--was carried across to Manhattan Island by ferry and barge
+with staggering inconvenience and at enormous cost. A bridge would
+have been, and still is, almost out of the question due not only to
+the width of the crossing, but to the flatness of both banks. To
+provide sufficient navigational clearance (without a drawspan),
+impracticably long approaches would have been necessary to obtain
+a permissibly gentle grade.
+
+Haskin formed a tunneling company and began work with the sinking of
+a shaft in Hoboken on the New Jersey side. In a month it was halted
+because of an injunction by, curiously, the D L & W Railroad, who
+feared for their vast investment in terminal and marine facilities.
+Not until November of 1879 was the injunction lifted and work again
+commenced. The shaft was completed and an air lock located in one wall
+from which the tunnel proper was to be carried forward. It was
+Haskin's plan to use no shield, relying solely on the pressure of
+compressed air to maintain the work faces and prevent the entry of
+water. The air was admitted in late December, and the first
+large-scale pneumatic tunneling operation launched. A single 26-foot,
+double-track bore was at first undertaken, but a work face of such
+diameter proved unmanageable and two oval tubes 18 feet high by 16
+feet wide were substituted, each to carry a single track. Work went
+forward with reasonable facility, considering the lack of precedent.
+A temporary entrance was formed of sheet-iron rings from the air lock
+down to the tunnel grade, at which point the permanent work of the
+north tube was started. Immediately behind the excavation at the face,
+a lining of thin wrought-iron plates was built up, to provide form for
+the 2-foot, permanent brick lining that followed. The three stages are
+shown in the model in about their proper relationship of progress. The
+work is shown passing beneath an old timber-crib bulkhead, used for
+stabilizing the shoreline.
+
+The silt of the riverbed was about the consistency of putty and under
+good conditions formed a secure barrier between the excavation and the
+river above. It was easily excavated, and for removal was mixed with
+water and blown out through a pipe into the shaft by the higher
+pressure in the tunnel. About half was left in the bore for removal
+later. The basic scheme was workable, but in operation an extreme
+precision was required in regulating the air pressure in the work
+area.[5] It was soon found that there existed an 11-psi difference
+between the pressure of water on the top and the bottom of the working
+face, due to the 22-foot height of the unlined opening. Thus, it was
+impossible to maintain perfect pneumatic balance of the external
+pressure over the entire face. It was necessary to strike an average
+with the result that some water entered at the bottom of the face
+where the water pressure was greatest, and some air leaked out at the
+top where the water pressure was below the air pressure. Constant
+attention was essential: several men did nothing but watch the
+behavior of the leaks and adjusted the pressure as the ground density
+changed with advance. Air was supplied by several steam-driven
+compressors at the surface.
+
+The air lock permitted passage back and forth of men and supplies
+between the atmosphere and the work area, without disturbing the
+pressure differential. This principle is demonstrated by an animated
+model set into the main model, to the left of the shaft (fig. 39). The
+variation of pressure within the lock chamber to match the atmosphere
+or the pressurized area, depending on the direction of passage, is
+clearly shown by simplified valves and gauges, and by the use of light
+in varying color density. In the Haskin tunnel, 5 to 10 minutes were
+taken to pass the miners through the lock so as to avoid too abrupt a
+physiological change.
+
+Despite caution, a blowout occurred in July 1880 due to air leakage
+not at the face, but around the temporary entrance. One door of the
+air lock jammed and twenty men drowned, resulting in an inquiry which
+brought forth much of the distrust with which Haskin was regarded by
+the engineering profession. His ability and qualifications were
+subjected to the bitterest attack in and by the technical press. There
+is some indication that, although the project began with a staff of
+competent engineers, they were alienated by Haskin in the course of
+work and at least one withdrew. Haskin's remarks in his own defense
+indicate that some of the denunciation was undoubtedly justified. And
+yet, despite this reaction, the fundamental merit of the pneumatic
+tunneling method had been demonstrated by Haskin and was immediately
+recognized and freely acknowledged. It was apparent at the same time,
+however, that air by itself did not provide a sufficiently reliable
+support for large-area tunnel works in unstable ground, and this
+remains the only major subaqueous tunnel work driven with air alone.
+
+[Illustration: Figure 41.--LOCATION OF HUDSON RIVER TUNNEL. (_Leslie's
+Weekly_, 1879.)]
+
+After the accident, work continued under Haskin until 1882 when funds
+ran out. About 1600 feet of the north tube and 600 feet of the south
+tube had been completed. Greathead resumed operations with a shield
+for a British company in 1889, but exhaustion of funds again caused
+stoppage in 1891. The tunnel was finally completed in 1904, and is
+now in use as part of the Hudson and Manhattan rapid-transit system,
+never providing the sought-after rail link. A splendid document of the
+Haskin portion of the work is S. D. V. Burr's _Tunneling Under the
+Hudson River_ published in 1885. It is based entirely upon firsthand
+material and contains drawings of most of the work, including the
+auxiliary apparatus. It is interesting to note that electric
+illumination (arc, not incandescent, lights) and telephones were used,
+unquestionably the first employment of either in tunnel work.
+
+[Illustration: Figure 42.--ST. CLAIR TUNNEL. View of front of shield
+showing method of excavation in firm strata. Incandescent electric
+illumination was used. 1889-90. MHT model--1" scale. (Smithsonian
+photo 49260-D.)]
+
+
+THE ST. CLAIR TUNNEL
+
+The final model of the soft-ground series reflects, as did the Hoosac
+Tunnel model for hard-rock tunneling, final emergence into the modern
+period. Although the St. Clair Tunnel was completed over 70 years ago,
+it typifies in its method of construction, the basic procedures of
+subaqueous work in the present day. The Thames Tunnel of Brunel, and
+Haskin's efforts beneath the Hudson, had clearly shown that by
+themselves, both the shield and pneumatic systems of driving through
+fluid ground were defective in practice for tunnels of large area.
+Note that the earliest successful works by each method had been of
+very small area, so that the influence of adverse conditions was
+greatly diminished.
+
+The first man to perceive and seize upon the benefits to be gained by
+combining the two systems was, most fittingly, Greathead. Although he
+had projected the technique earlier, in driving the underground City
+and South London Railway in 1886, he brought together for the first
+time the three fundamental elements essential for the practical
+tunneling of soft, water-bearing ground: compressed-air support of the
+work during construction, the movable shield, and cast-iron, permanent
+lining. The marriage was a happy one indeed; the limitations of each
+system were almost perfectly overcome by the qualities of the others.
+
+The conditions prevailing in 1882 at the Sarnia, Ontario, terminal of
+the Grand Trunk Railway, both operational and physical, were almost
+precisely the same as those which inspired the undertaking of the
+Hudson River Tunnel. The heavy traffic at this vital U.S.--Canada rail
+interchange was ferried inconveniently across the wide St. Clair
+River, and the bank and river conditions precluded construction of a
+bridge. A tunnel was projected by the railway in that year, the time
+when Haskin's tribulations were at their height. Perhaps because of
+this lack of precedent for a work of such size, nothing was done
+immediately. In 1884 the railway organized a tunnel company; in 1886
+test borings were made in the riverbed and small exploratory drifts
+were started across from both banks by normal methods of mine
+timbering. The natural gas, quicksand, and water encountered soon
+stopped the work.
+
+[Illustration: Figure 43.--REAR VIEW OF ST. CLAIR SHIELD showing the
+erector arm placing a cast-iron lining segment. The three motions of
+the arm--axial, radial, and rotational, were manually powered.
+(Smithsonian photo 49260-C.)]
+
+It was at this time that the railway's president visited Greathead's
+City and South London workings. The obvious answer to the St. Clair
+problem lay in the successful conduct of this subway. Joseph Hobson,
+chief engineer of the Grand Trunk and of the tunnel project, in
+designing a shield, is said to have searched for drawings of the
+shields used in the Broadway and Tower Subways of 1868-9, but unable
+to locate any, he relied to a limited extent on the small drawings of
+those in Drinker's volume. There is no explanation as to why he did
+not have drawings of the City and South London shield at that moment
+in use, unless one considers the rather unlikely possibility that
+Greathead maintained its design in secrecy.
+
+[Illustration: Figure 44.--OPENING OF THE ST. CLAIR TUNNEL, 1891.
+(_Photo courtesy of Detroit Library, Burton Historical Collection._)]
+
+The Hobson shield followed Greathead's as closely as any other, in
+having a diaphragm with closable doors, but a modification of Beach's
+sharpened horizontal shelves was also used. However, these functioned
+more as working platforms than supports for the earth. The machine was
+21-1/2 feet in diameter, an unprecedented size and almost twice that
+of Greathead's current one. It was driven by 24 hydraulic rams.
+Throughout the entire preliminary consideration of the project there
+was a marked sense of caution that amounted to what seems an almost
+total lack of confidence in success. Commencement of the work from
+vertical shafts was planned so that if the tunnel itself failed, no
+expenditure would have been made for approach work. In April 1888,
+the shafts were started near both riverbanks, but before reaching
+proper depth the almost fluid clay and silt flowed up faster than it
+could be excavated and this plan was abandoned. After this second
+inauspicious start, long open approach cuts were made and the work
+finally began. The portals were established in the cuts, several
+thousand feet back from each bank and there the tunneling itself
+began. The portions under the shore were driven without air. When the
+banks were reached, brick bulkheads containing air locks were built
+across the opening and the section beneath the river, about 3,710 feet
+long, driven under air pressure of 10 to 28 pounds above atmosphere.
+For most of the way, the clay was firm and there was little air
+leakage. It was found that horses could not survive in the compressed
+air, and so mules were used under the river.
+
+In the firm clay, excavation was carried on several feet in front of
+the shield, as shown in the model (fig. 42). About twelve miners
+worked at the face. However, in certain strata the clay encountered
+was so fluid that the shield could be simply driven forward by the
+rams, causing the muck to flow in at the door openings without
+excavation. After each advance, the rams were retracted and a ring of
+iron lining segments built up, as in the Tower Subway. Here, for the
+first time, an "erector arm" was used for placing the segments, which
+weighed about half a ton. In all respects, the work advanced with
+wonderful facility and lack of operational difficulty. Considering
+the large area, no subaqueous tunnel had ever been driven with such
+speed. The average monthly progress for the American and Canadian
+headings totaled 455 feet, and at top efficiency 10 rings or a length
+of 15.3 feet could be set in a 24-hour day in each heading. The 6,000
+feet of tunnel was driven in just a year; the two shields met
+vis-a-vis in August of 1890.
+
+The transition was complete. The work had been closely followed by the
+technical journals and the reports of its successful accomplishment
+thus were brought to the attention of the entire civil engineering
+profession. As the first major subaqueous tunnel completed in America
+and the first in the world of a size able to accommodate full-scale
+rail traffic, the St. Clair Tunnel served to dispel the doubts
+surrounding such work, and established the pattern for a mode of
+tunneling which has since changed only in matters of detail.
+
+Of the eight models, only this one was built under the positive
+guidance of original documents. In the possession of the Canadian
+National Railways are drawings not only of all elements of the shield
+and lining, but of much of the auxiliary apparatus used in
+construction. Such materials rarely survive, and do so in this case
+only because of the foresight of the railway which, to avoid paying a
+high profit margin to a private contractor as compensation for the
+risk and uncertainty involved, carried the contract itself and,
+therefore, preserved all original drawing records.
+
+While the engineering of tunnels has been comprehensively treated in
+this paper from the historical standpoint, it is well to still reflect
+that the advances made in tunneling have not perceptibly removed the
+elements of uncertainty but have only provided more positive and
+effective means of countering their forces. Still to be faced are the
+surprises of hidden streams, geologic faults, shifts of strata,
+unstable materials, and areas of extreme pressure and temperature.
+
+
+
+
+BIBLIOGRAPHY
+
+
+  AGRICOLA, GEORGIUS. _De re Metallica._ [English transl. H. C. and L.
+     H. Hoover (_The Mining Magazine_, London, 1912).] Basel: Froben,
+     1556.
+
+  BEACH, ALFRED ELY. _The pneumatic dispatch._ New York: The American
+     News Company, 1868.
+
+  BEAMISH, RICHARD. _A memoir of the life of Sir Marc Isambard
+     Brunel._ London: Longmans, Green, Longmans and Roberts, 1862.
+
+  BURR, S. D. V. _Tunneling under the Hudson River._ New York: John
+     Wiley and Sons, 1885.
+
+  COPPERTHWAITE, WILLIAM CHARLES. _Tunnel shields and the use of
+     compressed air in subaqueous works._ New York: D. Van Nostrand
+     Company, 1906.
+
+  DRINKER, HENRY STURGESS. _Tunneling, explosive compounds and rock
+     drills._ New York: John Wiley and Sons, 1878.
+
+  LATROBE, BENJAMIN H. Report on the Hoosac Tunnel (Baltimore, October
+     1, 1862). Pp. 125-139, app. 2, in _Report of the commissioners upon
+     the Troy and Greenfield Railroad and Hoosac Tunnel_. Boston, 1863.
+
+  LAW, HENRY. A memoir of the Thames Tunnel. _Weale's Quarterly Papers
+     on Engineering_ (London, 1845-46), vol. 3, pp. 1-25 and vol. 5,
+     pp. 1-86.
+
+  The pneumatic tunnel under Broadway, N.Y. _Scientific American_
+     (March 5, 1870), pp. 154-156.
+
+  _Report of the commissioners upon the Troy and Greenfield Railroad
+     and Hoosac Tunnel to his excellency the governor and the honorable
+     the executive council of the state of Massachusetts, February 28,
+     1863._ Boston, 1863.
+
+  STORROW, CHARLES S. Report on European tunnels (Boston, November 28,
+     1862). Pp. 5-122, app. 1, in _Report of the commissioners upon the
+     Troy and Greenfield Railroad and Hoosac Tunnel...._ Boston, 1863.
+
+  The St. Clair Tunnel. _Engineering News_ (in series running October
+     4 to December 27, 1890).
+
+
+
+
+FOOTNOTES
+
+  [1] There are two important secondary techniques for opening
+      subterranean and subaqueous ways, neither a method truly of
+      tunneling. One of these, of ancient origin, used mainly in the
+      construction of shallow subways and utility ways, is the "cut and
+      cover" system, whereby an open trench is excavated and then roofed
+      over. The result is, in effect, a tunnel. The concept of the other
+      method was propounded in the early 19th century but only used
+      practically in recent years. This is the "trench" method, a sort
+      of subaqueous equivalent of cut and cover. A trench is dredged in
+      the bed of a body of water, into which prefabricated sections of
+      large diameter tube are lowered, in a continuous line. The joints
+      are then sealed by divers, the trench is backfilled over the tube,
+      the ends are brought up to dryland portals, the water is pumped
+      out, and a subterranean passage results. The Chesapeake Bay Bridge
+      Tunnel (1960-1964) is a recent major work of this character.
+
+  [2] In 1952 a successful machine was developed on this plan, with
+      hardened rollers on a revolving cutting head for disintegrating
+      the rock. The idea is basically sound, possessing advantages in
+      certain situations over conventional drilling and blasting
+      systems.
+
+  [3] In 1807 the noted Cornish engineer Trevithick commenced a small
+      timbered drift beneath the Thames, 5 feet by 3 feet, as an
+      exploratory passage for a larger vehicular tunnel. Due to the
+      small frontal area, he was able to successfully probe about 1000
+      feet, but the river then broke in and halted the work. Mine
+      tunnels had also reached beneath the Irish Sea and various rivers
+      in the coal regions of Newcastle, but these were so far below the
+      surface as to be in perfectly solid ground and can hardly be
+      considered subaqueous workings.
+
+  [4] Unlike the Brunel tunnel, this was driven from both ends
+      simultaneously, the total overall progress thus being 3 feet per
+      shift rather than 18 inches. A top speed of 9 feet per day could
+      be advanced by each shield under ideal conditions.
+
+  [5] Ideally, the pressure of air within the work area of a
+      pneumatically driven tunnel should just balance the hydrostatic
+      head of the water without, which is a function of its total height
+      above the opening. If the air pressure is not high enough, water
+      will, of course, enter, and if very low, there is danger of
+      complete collapse of the unsupported ground areas. If too high,
+      the air pressure will overcome that due to the water and the air
+      will force its way out through the ground, through increasingly
+      larger openings, until it all rushes out suddenly in a "blowout."
+      The pressurized atmosphere gone, the water then is able to pour
+      in through the same opening, flooding the workings.
+
+
+
+
+INDEX
+
+
+  Agricola, Georgius, 215, 216
+
+
+  Barlow, Peter W., 221, 227
+  Beach, Alfred Ely, 224, 227-229, 231, 237
+  Brunel, Marc Isambard (the elder), 204, 205, 217, 218, 221,
+    224, 229, 231, 236
+  Burleigh, Charles, 212, 213
+  Burleigh Rock Drill Company, 212
+  Burr, S. D. V., 236
+
+  Cochrane, Sir Thomas, 231, 232
+  Copperthwaite, William Charles, 224
+
+  Doane, Thomas, 210, 212, 213, 215
+  Drinker, Henry S., 224, 237
+
+  Greathead, James Henry, 204, 218, 221, 224, 229, 231, 235-237
+  Gwynn, Stuart, 210
+
+  Haskin, DeWitt C., 204, 232, 234-236
+  Haupt, Herman, 204, 209, 210
+  Hobson, Joseph, 237
+
+  Latrobe, Benjamin H., 208, 209
+  Law, Henry, 218
+
+  Mowbray, George W., 213, 215
+
+  Nobel, Alfred B., 213
+
+  Putnam Machine Works, 212
+
+  Shanley, Walter, 212
+  Shanley Bros., 215
+  Sommeiller, Germain, 210
+  Storrow, Charles S., 210
+
+  Tweed, William Marcy (Boss), 229
+
+  Weale, John, 218
+
+
+       *       *       *       *       *
+
+
+Transcriber's Notes
+
+All obvious typographical errors corrected. Formatting inconsistancies
+and spelling were standardized. Paragraphs split by illustrations were
+rejoined. The text in the reproduced handbill for the Thames Tunnel
+was transcribed with a slight modification to the figure description
+portion. The Index was extracted from the full publication Index.
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Tunnel Engineering. A Museum Treatment, by 
+Robert M. Vogel
+
+*** END OF THIS PROJECT GUTENBERG EBOOK TUNNEL ENGINEERING ***
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+<pre>
+
+The Project Gutenberg EBook of Tunnel Engineering. A Museum Treatment, by 
+Robert M. Vogel
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Tunnel Engineering. A Museum Treatment
+
+Author: Robert M. Vogel
+
+Release Date: May 24, 2012 [EBook #39785]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TUNNEL ENGINEERING ***
+
+
+
+
+Produced by Chris Curnow, Tom Cosmas, Joseph Cooper and
+the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+</pre>
+
+
+<div class="fig_center" style="width: 242px; margin-bottom:2em;">
+<a name="cover" id="cover"></a>
+<img src="images/cover.jpg" width="312" height="435" alt="" title="" />
+</div>
+
+<div class="trans_notes">
+<p>This is Paper 41 from the <i>Smithsonian Institution United States
+National Museum Bulletin 240</i>, comprising Papers 34-44, which will
+also be available as a complete e-book.</p>
+
+<p>The front material, introduction and relevant index entries from
+the <i>Bulletin</i> are included in each single-paper e-book.</p>
+
+<p>For most images, clicking on them will open a larger version.</p>
+</div>
+
+<p class="caption1nc tdr">SMITHSONIAN INSTITUTION<br />
+UNITED STATES NATIONAL MUSEUM<br />
+BULLETIN 240</p>
+
+<div class="fig_right">
+  <img src="images/i_002b.png" width="172" height="237" alt="Smithsonian Press Logo" title="" />
+</div>
+
+<p class="tdr pmb4" style="clear: both;">SMITHSONIAN PRESS</p>
+
+<div class="center" style="font-size:150%;">
+MUSEUM OF HISTORY AND TECHNOLOGY<br />
+
+<div class="tdl p0 pmt4 pmb4" style="padding-left:2em; line-height:150%; margin-left:20%">
+<span class="smcap pmb2" style="font-size:150%;">Contributions<br />
+From the<br />
+Museum<br />
+of History and<br />
+Technology</span><br />
+<br />
+<i>Papers 34-44<br />
+On Science and Technology</i>
+</div>
+
+<span class="p0 pmt2">SMITHSONIAN INSTITUTION · WASHINGTON, D.C. 1966</span>
+</div>
+
+<hr class="chap" />
+
+<p class="center larger"><i>Publications of the United States National Museum</i></p>
+
+<p>The scholarly and scientific publications of the United States National Museum
+include two series, <i>Proceedings of the United States National Museum</i> and <i>United States
+National Museum Bulletin</i>.</p>
+
+<p>In these series, the Museum publishes original articles and monographs dealing
+with the collections and work of its constituent museums&mdash;The Museum of Natural
+History and the Museum of History and Technology&mdash;setting forth newly acquired
+facts in the fields of anthropology, biology, history, geology, and technology. Copies
+of each publication are distributed to libraries, to cultural and scientific organizations,
+and to specialists and others interested in the different subjects.</p>
+
+<p>The <i>Proceedings</i>, begun in 1878, are intended for the publication, in separate
+form, of shorter papers from the Museum of Natural History. These are gathered
+in volumes, octavo in size, with the publication date of each paper recorded in the
+table of contents of the volume.</p>
+
+<p>In the <i>Bulletin</i> series, the first of which was issued in 1875, appear longer, separate
+publications consisting of monographs (occasionally in several parts) and volumes
+in which are collected works on related subjects. <i>Bulletins</i> are either octavo or
+quarto in size, depending on the needs of the presentation. Since 1902 papers relating
+to the botanical collections of the Museum of Natural History have been
+published in the <i>Bulletin</i> series under the heading <i>Contributions from the United States
+National Herbarium</i>, and since 1959, in <i>Bulletins</i> titled &ldquo;Contributions from the Museum
+of History and Technology,&rdquo; have been gathered shorter papers relating to the collections
+and research of that Museum.</p>
+
+<p>The present collection of Contributions, Papers 34-44, comprises Bulletin 240.
+Each of these papers has been previously published in separate form. The year of
+publication is shown on the last page of each paper.</p>
+
+<p class="tdr"><span class="smcap">Frank A. Taylor</span><br />
+<em>Director, United States National Museum</em></p>
+
+<hr class="full" />
+
+<p><span class="pagenum"><a name="Page_201" id="Page_201">[201]</a></span><br />
+<a name="Paper_41" id="Paper_41"></a></p>
+
+<p class="tdr" style="font-size:1.5em; line-height:150%;">
+<span class="smcap">Contributions from <br />
+The Museum of History and Technology.</span><br />
+<span class="smcap">Paper</span> 41<br />
+<br />
+<br />
+<span class="smcap">Tunnel Engineering&mdash;A Museum Treatment</span></p>
+
+<p class="tdr" style="font-size:1.5em;;"><i>Robert M. Vogel</i></p>
+
+<table class="pmt4 pmb4" style="width:100%;" summary="ToC">
+<tr>
+  <td class="tdr" style="padding-bottom:2em;"><a href="#Introduction">INTRODUCTION</a></td>
+  <td class="tdr vtop">203</td>
+</tr>
+<tr>
+  <td class="tdr" style="padding-bottom:2em;"><a href="#Rock_Tunneling">ROCK TUNNELING</a></td>
+  <td class="tdr vtop" style="width:3em">206</td>
+</tr>
+<tr>
+  <td class="tdr" style="padding-bottom:2em;"><a href="#Soft-Ground_Tunneling">SOFT-GROUND TUNNELING</a></td>
+  <td class="tdr vtop">215</td>
+</tr>
+<tr>
+  <td class="tdr" style="padding-bottom:2em;"><a href="#BIBLIOGRAPHY">BIBLIOGRAPHY</a></td>
+  <td class="tdr vtop">239</td>
+</tr>
+<tr>
+  <td class="tdr" style="padding-bottom:2em;"><a href="#FOOTNOTES">FOOTNOTES</a></td>
+  <td></td>
+</tr>
+<tr>
+  <td class="tdr" style="padding-bottom:2em;"><a href="#INDEX">INDEX</a></td>
+  <td></td>
+</tr>
+</table>
+
+<p><span class="pagenum"><a name="Page_202" id="Page_202">[202]</a></span></p>
+<div class="fig_center" style="width: 537px;">
+<a name="Fig_41_1" id="Fig_41_1"></a>
+<a href="images/fig_41_1_lrg.png"><img src="images/fig_41_1.png" width="537" height="610" alt="" /></a>
+<p class="fig_caption pmb4">Figure 1.&mdash;<span class="smcap">Mining by early European civilizations</span>,
+using fire setting and hand chiseling to break out ore and rock.
+MHT model&mdash;&frac34;" scale. (Smithsonian photo 49260-H.)</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_203" id="Page_203">[203]</a></span></p>
+<p class="caption2nc tdr pmb2"><i>Robert M. Vogel</i></p>
+
+<p class="caption1"><a name="TUNNEL_ENGINEERING_A" id="TUNNEL_ENGINEERING_A"></a>
+TUNNEL ENGINEERING&mdash;A MUSEUM TREATMENT</p>
+
+<blockquote><p><i>During the years from 1830 to 1900, extensive developments took
+place in the field of tunneling, which today is an important,
+firmly established branch of civil engineering. This paper offers
+a picture of its growth from the historical standpoint, based on
+a series of models constructed for the Hall of Civil Engineering
+in the new Museum of History and Technology. The eight
+models described highlight the fundamental advances which have
+occurred between primitive man&rsquo;s first systematic use of fire for
+excavating rock in mining, and the use in combination of compressed
+air, an iron lining, and a movable shield in a subaqueous
+tunnel at the end of the 19th century.</i></p>
+
+<p><span class="smcap">The Author</span>: <i>Robert M. Vogel is curator of heavy machinery
+and civil engineering, in the Smithsonian Institution&rsquo;s Museum
+of History and Technology.</i></p></blockquote>
+
+<p class="caption2"><a name="Introduction"
+id="Introduction"></a>Introduction</p>
+
+<div class="dropcap">W</div>
+<p class="p0">ith few exceptions, civil engineering is a field
+in which the ultimate goal is the assemblage of
+materials into a useful structural form according to a
+scientifically derived plan which is based on various
+natural and man-imposed conditions. This is true
+whether the result be, for example, a dam, a building,
+a bridge, or even the fixed plant of a railroad. However,
+one principal branch of the field is based upon
+an entirely different concept. In the engineering of
+tunnels the utility of the &ldquo;structure&rdquo; is derived not
+from the bringing together of elements but from the
+separation of one portion of naturally existing material
+from another to permit passage through a former
+barrier.</p>
+
+<p>In tunneling hard, firm rock, this is practically
+the entire compass of the work: breaking away the
+rock from the mother mass, and, coincidently, removing
+it from the workings. The opposite extreme
+in conditions is met in the soft-ground tunnel, driven
+through material incapable of supporting itself above
+<span class="pagenum"><a name="Page_204" id="Page_204">[204]</a></span>
+the tunnel opening. Here, the excavation of the
+tunneled substance is of relatively small concern,
+eclipsed by the problem of preventing the surrounding
+material from collapsing into the bore.</p>
+
+<div class="fig_center" style="width: 547px;">
+<a name="Fig_41_2" id="Fig_41_2"></a>
+<a href="images/fig_41_2_lrg.png">
+<img src="images/fig_41_2.png" width="547" height="155" alt="" /></a>
+<p class="fig_caption">Figure 2.&mdash;<span class="smcap">Hoosac Tunnel. Method of working early sections</span>
+of the project; blast holes drilled by hand jacking. MHT
+model&mdash;&frac12;" scale. (Smithsonian photo 49260-L.)</p>
+</div>
+
+<p>In one other principal respect does tunnel engineering
+differ widely from its collateral branches of
+civil engineering. Few other physical undertakings
+are approached with anything like the uncertainty
+attending a tunnel work. This is even more true in
+mountain tunnels, for which test borings frequently
+cannot be made to determine the nature of the
+material and the geologic conditions which will be
+encountered.</p>
+
+<p>The course of tunnel work is not subject to an overall
+preliminary survey; the engineer is faced with not
+only the inability to anticipate general contingencies
+common to all engineering work, but with the peculiar
+and often overwhelming unpredictability of the
+very basis of his work.</p>
+
+<p>Subaqueous and soft-ground work on the other
+hand, while still subject to many indeterminates, is
+now far more predictable than during its early history,
+simply because the nature of the adverse condition
+prevailing eventually was understood to be quite
+predictable. The steady pressures of earth and water
+to refill the excavated area are today overcome with
+relative ease and consistency by the tunneler.</p>
+
+<p>In tunneling as in no other branch of civil engineering
+did empiricism so long resist the advance of scientific
+theory; in no other did the &ldquo;practical engineer&rdquo;
+remain to such an extent the key figure in establishing
+the success or failure of a project. The Hoosac Tunnel,
+after 25 years of legislative, financial, and technical
+difficulties, in 1875 was finally driven to successful
+completion only by the efforts of a group who, while
+in the majority were trained civil engineers, were to
+an even greater extent men of vast practical ability,
+more at home in field than office.</p>
+
+<p>DeWitt C. Haskin (see <a href="#Haskin">p. 234</a>), during the inquest
+that followed the death of a number of men in a
+blowout of his pneumatically driven Hudson River
+Tunnel in 1880, stated in his own defense: &ldquo;I am not
+a scientific engineer, but a practical one ... I know
+nothing of mathematics; in my experience I have
+grasped such matters as a whole; I believe that the
+study of mathematics in that kind of work [tunneling]
+has a tendency to dwarf the mind rather than enlighten
+it....&rdquo; An extreme attitude perhaps, and
+one which by no means adds to Haskin&rsquo;s stature, but
+a not unusual one in tunnel work at the time. It
+would not of course be fair to imply that such men
+as Herman Haupt, Brunel the elder, and Greathead
+were not accomplished theoretical engineers. But it
+was their innate ability to evaluate and control the
+overlying physical conditions of the site and work that
+made possible their significant contributions to the
+development of tunnel engineering.</p>
+
+<p>Tunneling remained largely independent of the
+realm of mathematical analysis long after the time
+when all but the most insignificant engineering works
+were designed by that means. Thus, as structural
+engineering has advanced as the result of a flow of
+new theoretical concepts, new, improved, and strengthened
+materials, and new methods of fastening, the
+progress of tunnel engineering has been due more to
+the continual refinement of constructional techniques.</p>
+
+<p class="caption3">A NEW HALL OF CIVIL ENGINEERING</p>
+
+<p>In the Museum of History and Technology has
+recently been established a Hall of Civil Engineering
+<span class="pagenum"><a name="Page_205" id="Page_205">[205]</a></span>
+in which the engineering of tunnels is comprehensively
+treated from the historical standpoint&mdash;something
+not previously done in an American museum.
+The guiding precept of the exhibit has not been to
+outline exhaustively the entire history of tunneling,
+but rather to show the fundamental advances which
+have occurred between primitive man&rsquo;s first systematic
+use of fire for excavating rock in mining, and the use
+in combination of compressed air, iron lining, and a
+movable shield in a subaqueous tunnel at the end of
+the 19th century. This termination date was selected
+because it was during the period from about 1830 to
+1900 that the most concentrated development took
+place, and during which tunneling became a firmly
+established and important branch of civil engineering
+and indeed, of modern civilization. The techniques
+of present-day tunneling are so fully related in current
+writing that it was deemed far more useful to
+devote the exhibit entirely to a segment of the field&rsquo;s
+history which is less commonly treated.</p>
+
+<div class="fig_center" style="width: 534px;">
+<a name="Fig_41_3" id="Fig_41_3"></a>
+<a href="images/fig_41_3_lrg.png">
+<img src="images/fig_41_3.png" width="534" height="139" alt="" /></a>
+<p class="fig_caption">Figure 3.&mdash;<span class="smcap">Hoosac Tunnel. Working of later stages</span>
+with Burleigh pneumatic drills mounted on carriages. The bottom heading
+is being drilled in preparation for blasting out with nitroglycerine.
+MHT model&mdash;&frac12;" scale. (Smithsonian photo 49260-M.)</p>
+</div>
+
+<p>The major advances, which have already been
+spoken of as being ones of technique rather than
+theory, devolve quite naturally into two basic classifications:
+the one of supporting a mass of loose,
+unstable, pressure-exerting material&mdash;soft-ground tunneling;
+and the diametrically opposite problem of
+separating rock from the basic mass when it is so
+firm and solid that it can support its own overbearing
+weight as an opening is forced through it&mdash;rock, or
+hard-ground tunneling.</p>
+
+<p>To exhibit the sequence in a thorough manner,
+inviting and capable of easy and correct interpretation
+by the nonprofessional viewer, models offered
+the only logical means of presentation. Six tunnels
+were selected, all driven in the 19th century. Each
+represents either a fundamental, new concept of
+tunneling technique, or an important, early application
+of one. Models of these works form the basis of
+the exhibit. No effort was made to restrict the work
+to projects on American soil. This would, in fact, have
+been quite impossible if an accurate picture of tunnel
+technology was to be drawn; for as in virtually all
+other areas of technology, the overall development in
+this field has been international. The art of mining
+was first developed highly in the Middle Ages in the
+Germanic states; the tunnel shield was invented by
+a Frenchman residing in England, and the use of
+compressed air to exclude the water from subaqueous
+tunnels was first introduced on a major work by an
+American. In addition, the two main subdivisions,
+rock and soft-ground tunneling, are each introduced
+by a model not of an actual working, but of one
+typifying early classical methods which were in use
+for centuries until the comparatively recent development
+of more efficient systems of earth support and
+rock breaking. Particular attention is given to accuracy
+of detail throughout the series of eight models;
+original sources of descriptive and graphic information
+were used in their construction wherever possible. In
+all cases except the introductory model in the rock-tunneling
+series, representing copper mining by early
+civilizations, these sources were contemporary
+accounts.</p>
+
+<p>The plan to use a uniform scale of reduction
+throughout, in order to facilitate the viewers&rsquo; interpretation,
+unfortunately proved impractical, due to
+the great difference in the amount of area to be
+encompassed in different models, and the necessity
+that the cases holding them be of uniform height. The
+related models of the Broadway and Tower Subways
+represent short sections of tunnels only 8 feet or so in
+diameter enabling a relatively large scale, 1&frac12; inches
+to the foot, to be used. Conversely, in order that the
+model of Brunel&rsquo;s Thames Tunnel be most effective,
+<span class="pagenum"><a name="Page_206" id="Page_206">[206]</a></span>
+it was necessary to include one of the vertical terminal
+shafts used in its construction. These were about 60
+feet in depth, and thus the much smaller scale of
+&frac14; inch to the foot was used. This variation is not
+as confusing as might be thought, for the human
+figures in each model provide an immediate and
+positive sense of proportion and scale.</p>
+
+<p>Careful thought was devoted to the internal lighting
+of the models, as this was one of the critical factors in
+establishing, so far as is possible in a model, an
+atmosphere convincingly representative of work
+conducted solely by artificial light. Remarkable
+realism was achieved by use of plastic rods to conduct
+light to the tiny sources of tunnel illumination, such
+as the candles on the miners&rsquo; hats in the Hoosac
+Tunnel, and the gas lights in the Thames Tunnel.
+No overscaled miniature bulbs, generally applied in
+such cases, were used. At several points where the
+general lighting within the tunnel proper has been
+kept at a low level to simulate the natural atmosphere
+of the work, hidden lamps can be operated by push-button
+in order to bring out detail which otherwise
+would be unseen.</p>
+
+<p>The remainder of the material in the Museum&rsquo;s
+tunneling section further extends the two major aspects
+of tunneling. Space limitations did not permit
+treatment of the many interesting ancillary matters
+vital to tunnel engineering, such as the unique problems
+of subterranean surveying, and the extreme
+accuracy required in the triangulation and subsequent
+guidance of the boring in long mountain tunnels;
+nor the difficult problems of ventilating long workings,
+both during driving and in service; nor the several
+major methods developed through the years for
+driving or constructing tunnels in other than the
+conventional manner.<a name="FNanchor_1_1" id="FNanchor_1_1"></a>
+<a href="#Footnote_1_1" class="fnanchor">[1]</a></p>
+
+
+<p class="caption2"><a name="Rock_Tunneling"
+id="Rock_Tunneling"></a>Rock Tunneling</p>
+
+<p>While the art of tunneling soft ground is of relatively
+recent origin, that of rock tunneling is deeply
+rooted in antiquity. However, the line of its development
+is not absolutely direct, but is more
+logically followed through a closely related branch
+of technology&mdash;mining. The development of mining
+techniques is a practically unbroken one, whereas
+there appears little continuity or relationship between
+the few works undertaken before about the 18th
+century for passage through the earth.</p>
+
+<p>The Egyptians were the first people in recorded
+history to have driven openings, often of considerable
+magnitude, through solid rock. As is true of all
+major works of that nation, the capability of such
+grand proportion was due solely to the inexhaustible
+supply of human power and the casual evaluation of
+life. The tombs and temples won from the rock
+masses of the Nile Valley are monuments of perseverance
+rather than technical skill. Neither the
+Egyptians nor any other peoples before the Middle
+Ages have left any consistent evidence that they were
+able to pierce ground that would not support itself
+above the opening as would firm rock. In Egypt
+were established the methods of rock breaking that
+were to remain classical until the first use of gun-powder
+blasting in the 17th century which formed the
+basis of the ensuing technology of mining.</p>
+
+<p>Notwithstanding the religious motives which inspired
+the earliest rock excavations, more constant and
+universal throughout history has been the incentive
+to obtain the useful and decorative minerals hidden
+beneath the earth&rsquo;s surface. It was the miner who
+developed the methods introduced by the early civilizations
+to break rock away from the primary mass,
+and who added the refinements of subterranean surveying
+and ventilating, all of which were later to be
+assimilated into the new art of driving tunnels of large
+diameter. The connection is the more evident from
+the fact that tunnelmen are still known as miners.</p>
+
+
+<p class="caption3">COPPER MINING, B.C.</p>
+
+<p>Therefore, the first model of the sequence, reflecting
+elemental rock-breaking techniques, depicts a hard-rock
+copper mine (<a href="#Fig_41_1">fig. 1</a>). Due to the absence of
+specific information about such works during the
+pre-Christian eras, this model is based on no particular
+period or locale, but represents in a general way,
+a mine in the Rio Tinto area of Spain where copper
+<span class="pagenum"><a name="Page_207" id="Page_207">[207]</a></span>
+has been extracted since at least 1000 B.C. Similar
+workings existed in the Tirol as early as about 1600
+B.C. Two means of breaking away the rock are
+shown: to the left is the most primitive of all methods,
+the hammer and chisel, which require no further
+description. At the right side, the two figures are
+shown utilizing the first rock-breaking method in
+which a force beyond that of human muscles was
+employed, the age-old &ldquo;fire-setting&rdquo; method. The
+rock was thoroughly heated by a fierce fire built
+against its face and then suddenly cooled by dashing
+water against it. The thermal shock disintegrated the
+rock or ore into bits easily removable by hand.</p>
+
+<div class="fig_center" style="width: 551px;">
+<a name="Fig_41_4" id="Fig_41_4"></a>
+<a href="images/fig_41_4_lrg.png">
+<img src="images/fig_41_4.png" width="551" height="380" alt="" /></a>
+<p class="fig_caption">Figure 4.&mdash;<span class="smcap">Hoosac Tunnel.</span> Bottom of the
+central shaft showing elevator car and rock
+skip; pumps at far right. In the center, the
+top bench is being drilled by a single column-mounted
+Burleigh drill. MHT model&mdash;&frac12;"
+scale. (Smithsonian photo 49260-N.)</p>
+</div>
+
+<p>The practice of this method below ground, of
+course, produced a fearfully vitiated atmosphere. It
+is difficult to imagine whether the smoke, the steam,
+or the toxic fumes from the roasting ore was the more
+distressing to the miners. Even when performed by
+labor considered more or less expendable, the method
+could be employed only where there was ventilation
+of some sort: natural chimneys and convection currents
+were the chief sources of air circulation. Despite
+the drawbacks of the fire system, its simplicity and
+efficacy weighed so heavily in its favor that its history
+of use is unbroken almost to the present day. Fire
+setting was of greatest importance during the years of
+intensive mining in Europe before the advent of
+explosive blasting, but its use in many remote areas
+hardly slackened until the early 20th century because
+of its low cost when compared to powder. For this
+same reason, it did have limited application in actual
+tunnel work until about 1900.</p>
+
+<p>Direct handwork with pick, chisel and hammer, and
+fire setting were the principal means of rock removal
+for centuries. Although various wedging systems
+were also in favor in some situations, their importance
+was so slight that they were not shown in the
+model.</p>
+
+<p><span class="pagenum"><a name="Page_208" id="Page_208">[208]</a></span></p>
+<p class="caption3">HOOSAC TUNNEL</p>
+
+<p>It was possible in the model series, without neglecting
+any major advancement in the art of rock
+tunneling, to complete the sequence of development
+with only a single additional model. Many of the
+greatest works of civil engineering have been those
+concerned directly with transport, and hence are the
+product of the present era, beginning in the early
+19th century. The development of the ancient arts of
+route location, bridge construction, and tunnel
+driving received a powerful stimulation after 1800
+under the impetus of the modern canal, highway, and,
+especially, the railroad.</p>
+
+<p>The Hoosac Tunnel, driven through Hoosac
+Mountain in the very northwest corner of Massachusetts
+between 1851 and 1875, was the first major
+tunneling work in the United States. Its importance
+is due not so much to this as to its being literally
+the fountainhead of modern rock-tunneling technology.
+The remarkable thing is that the work was
+begun using methods of driving almost unchanged
+during centuries previous, and was completed twenty
+years later by techniques which were, for the day,
+almost totally mechanized. The basic pattern of
+operation set at Hoosac, using pneumatic rock drills
+and efficient explosives, remains practically unchanged
+today.</p>
+
+<p>The general history of the Hoosac project is so
+thoroughly recorded that the briefest outline of its
+political aspects will suffice here. Hoosac Mountain
+was the chief obstacle in the path of a railroad projected
+between Greenfield, Massachusetts, and Troy,
+New York. The line was launched by a group of
+Boston merchants to provide a direct route to the
+rapidly developing West, in competition with the
+coastal routes via New York. The only route
+economically reasonable included a tunnel of nearly
+five miles through the mountain&mdash;a length absolutely
+without precedent, and an immense undertaking in
+view of the relatively primitive rock-working methods
+then available.</p>
+
+<div class="fig_right" style="width: 262px;">
+<a name="Fig_41_5" id="Fig_41_5"></a>
+<a href="images/fig_41_5_lrg.png">
+<img src="images/fig_41_5.png" width="273" height="439" alt="" title="" /></a>
+<p class="fig_caption">Figure 5.&mdash;<span class="smcap">Burleigh rock drill</span>, improved model
+of about 1870, mounted on frame for surface work. (Catalog and price list:
+The Burleigh Rock Drill Company, 1876.)</p>
+</div>
+
+<p>The bore&rsquo;s great length and the desire for rapid
+exploitation inspired innovation from the outset of
+the work. The earliest attempts at mechanization,
+although ineffectual and without influence on tunnel
+engineering until many years later, are of interest.
+These took the form of several experimental machines
+of the &ldquo;full area&rdquo; type, intended to excavate the entire
+face of the work in a single operation by cutting one
+or more concentric grooves in the rock. The rock
+remaining between the grooves was to be blasted out.
+The first such machine tested succeeded in boring a
+24-foot diameter opening for 10 feet before its total
+failure. Several later machines proved of equal
+merit.<a name="FNanchor_2_2" id="FNanchor_2_2"></a>
+<a href="#Footnote_2_2" class="fnanchor">[2]</a>
+It was the Baltimore and Ohio&rsquo;s eminent
+chief engineer, Benjamin H. Latrobe, who in his
+<i>Report on the Hoosac Tunnel</i> (Baltimore, Oct. 1, 1862,
+p. 125) stated that such apparatus contained in its
+own structure the elements of failure, &ldquo; ... as they
+require the machines to do too much and the powder
+too little of the work, thus contradicting the fundamental
+principles upon which all labor-saving
+machinery is framed ... I could only look upon it
+as a misapplication of mechanical genius.&rdquo;</p>
+
+<p><span class="pagenum"><a name="Page_209" id="Page_209">[209]</a></span></p>
+<div class="fig_left" style="width: 450px;">
+<a name="Fig_41_6" id="Fig_41_6"></a>
+<img src="images/fig_41_6.png" width="450" height="406" alt="" />
+<p class="fig_caption">Figure 6.&mdash;<span class="smcap">Hoosac Tunnel.</span> Flash-powder
+photograph of Burleigh drills
+at the working face. (<i>Photo courtesy of
+State Library, Commonwealth of Massachusetts.</i>)</p>
+</div>
+
+<p>Latrobe stated the basic philosophy of rock-tunnel
+work. No mechanical agent has ever been able to
+improve upon the efficiency of explosives for the shattering
+of rock. For this reason, the logical application
+of machinery to tunneling was not in replacing or
+altering the fundamental process itself, but in enabling
+it to be conducted with greater speed by mechanically
+drilling the blasting holes to receive the explosive.</p>
+
+<p>Actual work on the Hoosac Tunnel began at both
+ends of the tunnel in about 1854, but without much
+useful effect until 1858 when a contract was let to
+the renowned civil engineer and railroad builder,
+Herman Haupt of Philadelphia. Haupt immediately
+resumed investigations of improved tunneling methods,
+both full-area machines and mechanical rock
+drills. At this time mechanical rock-drill technology
+was in a state beyond, but not far beyond, initial
+experimentation. There existed one workable American
+machine, the Fowle drill, invented in 1851. It
+was steam-driven, and had been used in quarry work,
+although apparently not to any commercial extent.
+However, it was far too large and cumbersome to
+find any possible application in tunneling. Nevertheless,
+it contained in its operating principle, the
+seed of a practical rock drill in that the drill rod was
+attached directly to and reciprocated by a double-acting
+steam piston. A point of great importance
+was the independence of its operation on gravity,
+permitting drilling in any direction.</p>
+
+<p>While experimenting, Haupt drove the work onward
+by the classical methods, shown in the left-hand
+section of the model (<a href="#Fig_41_2">fig. 2</a>). At the far right an
+advance heading or adit is being formed by pick and
+hammer work; this is then deepened into a top
+heading with enough height to permit hammer
+drilling, actually the basic tunneling operation. A
+team is shown &ldquo;double jacking,&rdquo; i.e., using two-handed
+hammers, the steel held by a third man.
+This was the most efficient of the several hand-drilling
+methods. The top-heading plan was followed so
+that the bulk of the rock could be removed in the
+form of a bottom bench, and the majority of drilling
+would be downward, obviously the most effective
+direction. Blasting was with black powder and its
+commercial variants. Some liberty was taken in depicting
+these steps so that both operations might be
+shown within the scope of the model: in practice
+<span class="pagenum"><a name="Page_210" id="Page_210">[210]</a></span>
+the heading was kept between 400 and 600 feet in
+advance of the bench so that heading blasts would
+not interfere with the bench work. The bench
+carriage simply facilitated handling of the blasted
+rock. It was rolled back during blasts.</p>
+
+<div class="fig_left" style="width: 337px;">
+<a name="Fig_41_7" id="Fig_41_7"></a>
+<img src="images/fig_41_7.png" width="337" height="424" alt="" />
+<p class="fig_caption">Figure 7.&mdash;<span class="smcap">Hoosac Tunnel. Group of miners</span>
+descending the west shaft with a Burleigh drill.
+(<i>Photo courtesy of State Library, Commonwealth of
+Massachusetts.</i>)</p>
+</div>
+
+<p>The experiments conducted by Haupt with machine
+drills produced no immediate useful results. A drill
+designed by Haupt and his associate, Stuart Gwynn,
+in 1858 bored hard granite at the rate of <sup>5</sup>/<sub>8</sub> inch per
+minute, but was not substantial enough to bear up
+in service. Haupt left the work in 1861, victim of
+intense political pressures and totally unjust accusations
+of corruption and mismanagement. The work
+was suspended until taken over by a state commission
+in 1862. Despite frightful ineptitude and very real
+corruption, this period was exceedingly important
+in the long history both of Hoosac Tunnel and of rock
+tunneling in general.</p>
+
+<p>The merely routine criticism of the project had by
+this time become violent due to the inordinate length
+of time already elapsed and the immense cost, compared
+to the small portion of work completed. This
+served to generate in the commission a strong sense
+of urgency to hurry the project along. Charles S.
+Storrow, a competent engineer, was sent to Europe
+to report on the progress of tunneling there, and in
+particular on mechanization at the Mont Cenis Tunnel
+then under construction between France and Italy.
+Germain Sommeiller, its chief engineer, had, after
+experimentation similar to Haupt&rsquo;s, invented a
+reasonably efficient drilling machine which had gone
+into service at Mont Cenis in March 1861. It was
+a distinct improvement over hand drilling, almost
+doubling the drilling rate, but was complex and highly
+unreliable. Two hundred drills were required to keep
+16 drills at work. But the vital point in this was the
+fact that Sommeiller drove his drills not with steam,
+but air, compressed at the tunnel portals and piped
+to the work face. It was this single factor, one of
+application rather than invention, that made the
+mechanical drill feasible for tunneling.</p>
+
+<p>All previous effort in the field of machine drilling, on
+both sides of the Atlantic, had been directed toward
+steam as the motive power. In deep tunnels, with
+ventilation already an inherent problem, the exhaust
+of a steam drill into the atmosphere was inadmissible.
+Further, steam could not be piped over great distances
+due to serious losses of energy from radiation of heat,
+and condensation. Steam generation within the
+tunnel itself was obviously out of the question. It
+was the combination of a practical drill, and the
+parallel invention by Sommeiller of a practical air
+compressor that resulted in the first workable application
+of machine rock drilling to tunneling.</p>
+
+<p><span class="pagenum"><a name="Page_211" id="Page_211">[211]</a></span></p>
+<div class="fig_center" style="width: 480px;">
+<a name="Fig_41_8" id="Fig_41_8"></a>
+<a name="Fig_41_9" id="Fig_41_9"></a>
+<img src="images/fig_41_8.png" width="478" height="299" alt="" />
+<img src="images/fig_41_9.png" width="480" height="315" alt="" />
+<p class="fig_caption">Figures 8 &amp; 9.&mdash;<span class="smcap">Hoosac Tunnel. Contemporary engravings.</span> As such large general areas could not be sufficiently
+illuminated for photography, the Museum model was based primarily on
+artists&#39; versions of the work. (<i>Science Record</i>, 1872;
+<i>Leslie&#39;s Weekly</i>, 1873.)</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_212" id="Page_212">[212]</a></span></p>
+<p>The Sommeiller drills greatly impressed Storrow,
+and his report of November 1862 strongly favored
+their adoption at Hoosac. It is curious however, that
+not a single one was brought to the U.S., even on
+trial. Storrow does speak of Sommeiller&rsquo;s intent to
+keep the details of the machine to himself until it had
+been further improved, with a view to its eventual
+exploitation. The fact is, that although workable, the
+Sommeiller drill proved to be a dead end in rock-drill
+development because of its many basic deficiencies.
+It did exert the indirect influence of inspiration which,
+coupled with a pressing need for haste, led to renewed
+trials of drilling machinery at Hoosac. Thomas
+Doane, chief engineer under the state commission,
+carried this program forth with intensity, seeking and
+encouraging inventors, and himself working on the
+problem. The pattern of the Sommeiller drill was
+generally followed; that is, the drill was designed as
+a separate, relatively light mechanical element,
+adapted for transportation by several miners, and
+attachable to a movable frame or carriage during
+operation. Air was of course the presumed power.
+To be effective, it was necessary that a drill automatically
+feed the drill rod as the hole deepened, and
+also rotate the rod automatically to maintain a round,
+smooth hole. Extreme durability was essential, and
+usually proved the source of a machine&rsquo;s failure.
+The combination of these characteristics into a
+machine capable of driving the drill rod into the rock
+with great force, perhaps five times per second, was a
+severe test of ingenuity and materials. Doane in 1864
+had three different experimental drills in hand, as
+well as various steam and water-powered compressors.</p>
+
+<p>Success finally came in 1865 with the invention of
+a drill by Charles Burleigh, a mechanical engineer
+at the well-known Putnam Machine Works of Fitchburg,
+Massachusetts. The drills were first applied
+in the east heading in June of 1866. Although
+working well, their initial success was limited by
+lack of reliability and a resulting high expense for
+repairs. They were described as having &ldquo;several
+weakest points.&rdquo; In November, these drills were
+replaced by an improved Burleigh drill which was
+used with total success to the end of the work. The
+era of modern rock tunneling was thus launched by
+Sommeiller&rsquo;s insight in initially applying pneumatic
+power to a machine drill, by Doane&rsquo;s persistence in
+searching for a thoroughly practical drill, and by
+Burleigh&rsquo;s mechanical talent in producing one. The
+desperate need to complete the Hoosac Tunnel may
+reasonably be considered the greatest single spur to
+the development of a successful drill.</p>
+
+<p>The significance of this invention was far reaching.
+Burleigh&rsquo;s was the first practical mechanical rock
+drill in America and, in view of its dependability,
+efficiency, and simplicity when compared to the
+Sommeiller drill, perhaps in the world. The Burleigh
+drill achieved success almost immediately. It was
+placed in production by Putnam for the Burleigh
+Rock Drill Company before completion of Hoosac in
+1876, and its use spread throughout the western
+mining regions and other tunnel works. For a major
+invention, its adoption was, in relative terms, instantaneous.
+It was the prototype of all succeeding
+piston-type drills, which came to be known generically
+as &ldquo;burleighs,&rdquo; regardless of manufacture. Walter
+Shanley, the Canadian contractor who ultimately
+completed the Hoosac, reported in 1870, after the
+drills had been in service for a sufficient time that
+the techniques for their most efficient use were fully
+understood and effectively applied, that the Burleigh
+drills saved about half the drilling costs over hand
+drilling. The per-inch cost of machine drilling
+averaged 5.5 cents, all inclusive, vs. 11.2 cents for
+handwork. The more important point, that of
+speed, is shown by the reports of average monthly
+progress of the tunnel itself, before and after use of
+the air drills.</p>
+
+<table style="width:15em;" summary="Tunnel data">
+<tr>
+  <td><i>Year</i></td>
+  <td class="center"><i>Average monthly<br />progress in feet</i></td>
+</tr>
+<tr>
+  <td>1865</td>
+  <td class="tdr" style="padding-right:5em">55</td>
+</tr>
+<tr>
+  <td>1866</td>
+  <td class="tdr" style="padding-right:5em">48</td>
+</tr>
+<tr>
+  <td>1867</td>
+  <td class="tdr" style="padding-right:5em">99</td>
+</tr>
+<tr>
+  <td>1868</td>
+  <td class="tdr" style="padding-right:5em">&mdash;
+</td>
+</tr>
+<tr>
+  <td>1869</td>
+  <td class="tdr" style="padding-right:5em">138</td>
+</tr>
+<tr>
+  <td>1870</td>
+  <td class="tdr" style="padding-right:5em">126</td>
+</tr>
+<tr>
+  <td>1871</td>
+  <td class="tdr" style="padding-right:5em">145</td>
+</tr>
+<tr>
+  <td>1872</td>
+  <td class="tdr" style="padding-right:5em">124</td>
+</tr>
+</table>
+
+<div class="fig_right" style="width: 305px;">
+<a name="Fig_41_10" id="Fig_41_10"></a>
+<a href="images/fig_41_10_lrg.png"><img src="images/fig_41_10.png" width="305" height="345" alt="" /></a>
+<p class="fig_caption">Figure 10.&mdash;<span class="smcap">Trinitroglycerine blast</span> at Hoosac
+Tunnel. (<i>Leslie&#39;s Weekly</i>, 1873.)</p>
+</div>
+
+<p>The right portion of the model (<a href="#Fig_41_3">fig. 3</a>) represents
+the workings during the final period. The bottom
+heading system was generally used after the Burleigh
+drills had been introduced. Four to six drills were
+<span class="pagenum"><a name="Page_213" id="Page_213">[213]</a></span>
+mounted on a carriage designed by Doane. These
+drove the holes for the first blast in the center of the
+heading in about six hours. The full width of the
+heading, the 24-foot width of the tunnel, was then
+drilled and blasted out in two more stages. As in
+the early section, the benches to the rear were later
+removed to the full-tunnel height of about 20 feet.
+This operation is shown by a single drill (<a href="#Fig_41_4">fig. 4</a>)
+mounted on a screw column. Three 8-hour shifts carried
+the work forward: drilling occupied half the
+time and half was spent in running the carriage back,
+blasting, and mucking (clearing the broken rock).</p>
+
+<p>The tunnel&rsquo;s 1028-foot central shaft, completed
+under the Shanley contract in 1870 to provide two
+additional work faces as well as a ventilation shaft is
+shown at the far right side of this half of the model.
+Completed so near the end of the project, only 15
+percent of the tunnel was driven from the shaft.</p>
+
+<p>The enormous increase in rate of progress was not
+due entirely to machine drilling. From the outset
+of his jurisdiction, Doane undertook experiments with
+explosives as well as drills, seeking an agent more
+effective than black powder. In this case, the need
+for speed was not the sole stimulus. As the east and
+west headings advanced further and further from the
+portals, the problem of ventilation grew more acute,
+and it became increasingly difficult to exhaust the
+toxic fumes produced by the black powder blasts.</p>
+
+<p>In 1866, Doane imported from Europe a sample
+of trinitroglycerine, the liquid explosive newly introduced
+by Nobel, known in Europe as &ldquo;glonoïn oil&rdquo;
+and in the United States as &ldquo;nitroglycerine.&rdquo; It
+already had acquired a fearsome reputation from its
+tendency to decompose with heat and age and to
+explode with or without the slightest provocation.
+Nevertheless, its tremendous power and characteristic
+of almost complete smokelessness led Doane to
+employ the chemist George W. Mowbray, who had
+blasted for Drake in the Pennsylvania oil fields, to
+develop techniques for the bulk manufacture of the
+new agent and for its safe employment in the tunnel.</p>
+
+<div class="fig_center" style="width: 407px;">
+<a name="Fig_41_11" id="Fig_41_11"></a>
+<img src="images/fig_41_11.png" width="407" height="295" alt="" />
+<p class="fig_caption">Figure 11.&mdash;<span class="smcap">Hoosac Tunnel</span> survey
+crew at engineering office. The highest
+accuracy of the aboveground and
+underground survey work was required
+to insure proper vertical and horizontal
+alignment and meeting of the several
+separately driven sections. (<i>Photo courtesy
+of State Library, Commonwealth of
+Massachusetts.</i>)</p>
+</div>
+
+<p>Mowbray established a works on the mountain and
+shortly developed a completely new blasting practice
+based on the explosive. Its stability was greatly increased
+by maintaining absolute purity in the manufacturing
+process. Freezing the liquid to reduce its
+sensitivity during transport to the headings, and
+extreme caution in its handling further reduced the
+hazard of its use. At the heading, the liquid was
+poured into cylindrical cartridges for placement in
+the holes. As with the Burleigh drill, the general
+adoption of nitroglycerine was immediate once its
+qualities had been demonstrated. The effect on the
+work was notable. Its explosive characteristics permitted
+fewer blast holes over a given frontal area of
+working face, and at the same time it was capable
+of effectively blowing from a deeper drill hole, 42
+inches against 30 inches for black powder, so that
+under ideal conditions 40 percent more tunnel length
+was advanced per cycle of operations. A new fuse
+and a system of electric ignition were developed which
+permitted simultaneous detonation and resulted in
+a degree of effectiveness impossible with the powder
+train and cord fusing used with the black powder.
+Over a million pounds of nitroglycerine were produced
+by Mowbray between 1866 and completion of
+the tunnel.</p>
+
+<p><span class="pagenum"><a name="Page_214" id="Page_214">[214]</a></span></p>
+<div class="fig_center" style="width: 656px;">
+<a name="Fig_41_12" id="Fig_41_12"></a>
+<img src="images/fig_41_12.png" width="656" height="286" alt="" title="" />
+<p class="fig_caption">Figure 12.&mdash;<span class="smcap">Works at the central shaft, Hoosac Tunnel</span>,
+for hoisting, pumping and air compressing machinery,
+and general repair, 1871. (<i>Photo courtesy of State Library,
+Commonwealth of Massachusetts.</i>)</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_215" id="Page_215">[215]</a></span></p>
+<div class="fig_center" style="width: 659px;">
+<a name="Fig_41_13" id="Fig_41_13"></a>
+<img src="images/fig_41_13.png" width="659" height="418" alt="" />
+<p class="fig_caption">Figure 13.&mdash;<span class="smcap">Hoosac Tunnel. Air-compressor building</span>
+on Hoosac River near North Adams. The compressors were driven partially by
+waterpower, derived from the river. (<i>Photo courtesy of State Library,
+Commonwealth of Massachusetts.</i>)</p>
+</div>
+
+<div class="fig_left" style="width: 313px;">
+<a name="Fig_41_14" id="Fig_41_14"></a>
+<img src="images/fig_41_14.png" width="313" height="351" alt="" title="" />
+<p class="fig_caption">Figure 14.&mdash;<span class="smcap">West portal of Hoosac Tunnel</span> before
+completion, 1868, showing six rings of lining brick.
+(<i>Photo courtesy of State Library, Commonwealth of Massachusetts.</i>)</p>
+</div>
+
+<p>When the Shanleys took the work over in 1868,
+following political difficulties attending operation by
+the State, the period of experimentation was over.
+The tunnel was being advanced by totally modern
+methods, and to the present day the overall concepts
+have remained fundamentally unaltered: the Burleigh
+piston drill has been replaced by the lighter
+hammer drill; the Doane drill carriage by the more
+flexible &ldquo;jumbo&rdquo;;nitroglycerine by its more stable
+descendant dynamite and its alternatives; and static-electric
+blasting machines by more dependable
+magnetoelectric. But these are all in the nature of
+improvements, not innovations.</p>
+
+<p>Unlike the preceding model, there was good documentation
+for this one. Also, the Hoosac was apparently
+the first American tunnel to be well recorded
+photographically. Early flashlight views exist of the
+drills working at the heading (<a href="#Fig_41_6">fig. 6</a>) as well as of the
+portals, the winding and pumping works at the central
+shaft, and much of the machinery and associated
+aspects of the project. These and copies of drawings
+of much of Doane&rsquo;s experimental apparatus, a rare
+technological record, are preserved at the Massachusetts
+State Library.</p>
+
+
+<p class="caption2"><a name="Soft-Ground_Tunneling" id="Soft-Ground_Tunneling"></a>
+Soft-Ground Tunneling</p>
+
+<p>So great is the difference between hard-rock and
+soft-ground tunneling that they constitute two almost
+separate branches of the field. In penetrating ground
+lacking the firmness or cohesion to support itself above
+an opening, the miner&rsquo;s chief concern is not that of
+removing the material, but of preventing its collapse
+into his excavation. The primitive methods depending
+upon brute strength and direct application of fire
+and human force were suitable for assault on rock,
+but lacked the artifice needed for delving into less
+stable material. Roman engineers were accomplished
+in spanning subterranean ways with masonry arches,
+but apparently most of their work was done by cut-and-cover
+methods rather than by actual mining.</p>
+
+<p>Not until the Middle Ages did the skill of effectively
+working openings in soft ground develop, and not
+until the Renaissance was this development so consistently
+successful that it could be considered a
+science.</p>
+
+<p class="caption3">RENAISSANCE MINING</p>
+
+<p>From the earliest periods of rock working, the quest
+for minerals and metals was the primary force that
+drove men underground. It was the technology of
+mining, the product of slow evolution over the
+centuries, that became the technology of the early
+tunnel, with no significant modification except in
+size of workings.</p>
+
+<p>Every aspect of 16th century mining is definitively
+detailed in Georgius Agricola&rsquo;s remarkable <i>De re
+Metallica</i>, first published in Basel in 1556. During
+its time of active influence, which extended for two
+centuries, it served as the authoritative work on the
+subject. It remains today an unparalleled early record
+of an entire branch of technology. The superb
+woodcuts of mine workings and tools in themselves
+constitute a precise description of the techniques of
+the period, and provided an ideal source of information
+upon which to base the first model in the soft-ground
+series.</p>
+
+<p><span class="pagenum"><a name="Page_216" id="Page_216">[216]</a></span></p>
+<div class="fig_center" style="width: 663px;">
+<a name="Fig_41_15" id="Fig_41_15"></a>
+<a href="images/fig_41_15_lrg.png">
+<img src="images/fig_41_15.png" width="663" height="478" alt="" /></a>
+<p class="fig_caption">Figure 15.&mdash;<span class="smcap">Centering for placement of finished stonework</span>
+at west portal, 1874. At top-right are the sheds where
+the lining brick was produced. (<i>Photo courtesy of State Library,
+Commonwealth of Massachusetts.</i>)</p>
+</div>
+
+<p>The model, representing a typical European mine,
+demonstrates the early use of timber frames or
+&ldquo;sets&rdquo; to support the soft material of the walls and roof.
+In areas of only moderate instability, the sets alone
+were sufficient to counteract the earth pressure, and
+were spaced according to the degree of support
+required. In more extreme conditions, a solid lagging
+of small poles or boards was set outside the
+frames, as shown in the model, to provide absolute
+support of the ground. Details of the framing, the
+windlass, and all tools and appliances were supplied
+by Agricola, with no need for interpretation or
+interpolation.</p>
+
+<p>The basic framing pattern of sill, side posts and cap
+piece, all morticed together, with lagging used where
+needed, was translated unaltered into tunneling practice,
+particularly in small exploratory drifts. It
+remained in this application until well into the 20th
+century.</p>
+
+<p>The pressure exerted upon tunnels of large area
+was countered during construction by timbering
+systems of greater elaboration, evolved from the basic
+one. By the time that tunnels of section large
+enough to accommodate canals and railways were
+being undertaken as matter-of-course civil engineering
+works, a series of nationally distinguishable
+systems had emerged, each possessing characteristic
+points of favor and fault. As might be suspected, the
+English system of tunnel timbering, for instance, was
+rarely applied on the Continent, nor were the German,
+Austrian or Belgian systems normally seen in Great
+Britain. All were used at one time or another in
+this country, until the American system was introduced
+in about 1855. While the timbering commonly
+remained in place in mines, it would be followed up
+by permanent masonry arching and lining in tunnel
+work.</p>
+
+<p>Overhead in the museum Hall of Civil Engineering
+<span class="pagenum"><a name="Page_217" id="Page_217">[217]</a></span>
+are frames representing the English, Austrian and
+American systems. Nearby, a series of small relief
+models (<a href="#Fig_41_19">fig. 19</a>) is used to show the sequence of enlargement
+in a soft-ground railroad tunnel of about
+1855, using the Austrian system. Temporary timber
+support of tunnels fell from use gradually after the
+advent of shield tunneling in conjunction with cast-iron
+lining. This formed a perfect support immediately
+behind the shield, as well as the permanent
+lining of the tunnel.</p>
+
+<div class="fig_right" style="width: 294px;">
+<a name="Fig_41_16" id="Fig_41_16"></a>
+<a href="images/fig_41_16_co.png">
+<img src="images/fig_41_16.png" width="294" height="401" alt="" title="" /></a>
+<p class="fig_caption">Figure 16.&mdash;<span class="smcap">West portal upon completion</span>, 1876.
+(<i>Photo courtesy of New-York Historical Society.</i>)
+ Click on image for a color version of poster.</p>
+</div>
+
+<p class="caption3">BRUNEL&rsquo;S THAMES TUNNEL</p>
+
+<p>The interior surfaces of tunnels through ground
+merely unstable are amenable to support by various
+systems of timbering and arching. This becomes less
+true as the fluidity of the ground increases. The
+soft material which normally comprises the beds of
+rivers can approach an almost liquid condition
+resulting in a hydraulic head from the overbearing
+water sufficient to prevent the driving of even the
+most carefully worked drift, supported by simple
+timbering. The basic defect of the timbering systems
+used in mining and tunneling was that there was
+inevitably a certain amount of the face or ceiling
+unsupported just previous to setting a frame, or
+placing over it the necessary section of lagging. In
+mine work, runny soil could, and did, break through
+such gaps, filling the working. For this reason,
+there were no serious attempts made before 1825 to
+drive subaqueous tunnels.</p>
+
+<p>In that year, work was started on a tunnel under the
+Thames between the Rotherhithe and Wapping
+sections of London, under guidance of the already
+famous engineer Marc Isambard Brunel (1769-1849),
+father of I. K. Brunel. The undertaking is of great
+interest in that Brunel employed an entirely novel
+apparatus of his own invention to provide continuous
+and reliable support of the soft water-bearing clay
+which formed the riverbed. By means of this
+&ldquo;shield,&rdquo; Brunel was able to drive the world&rsquo;s first
+subaqueous tunnel.<a name="FNanchor_3_3" id="FNanchor_3_3"></a>
+<a href="#Footnote_3_3" class="fnanchor">[3]</a></p>
+
+<p>The shield was of cast-iron, rectangular in elevation,
+and was propelled forward by jackscrews. Shelves at
+top, bottom, and sides supported the tunnel roof,
+floor, and walls until the permanent brick lining was
+placed. The working face, the critical area, was
+supported by a large number of small &ldquo;breasting
+boards,&rdquo; held against the ground by small individual
+screws bearing against the shield framework. The
+shield itself was formed of 12 separate frames, each
+of which could be advanced independently of the
+others. The height was 22 feet 3 inches: the width
+37 feet 6 inches.</p>
+
+<p>The progress was piecemeal. In operation the
+miners would remove one breasting board at a time,
+excavate in front of it, and then replace it in the
+advanced position&mdash;about 6 inches forward. This
+was repeated with the next board above or below, and
+the sequence continued until the ground for the
+entire height of one of the 12 sections had been
+removed. The board screws for that section were
+shifted to bear on the adjacent frames, relieving the
+<span class="pagenum"><a name="Page_218" id="Page_218">[218]</a></span>
+frame of longitudinal pressure. It could then be
+screwed forward by the amount of advance, the
+screws bearing to the rear on the completed masonry.
+Thus, step by step the tunnel progressed slowly, the
+greatest weekly advance being 14 feet.</p>
+
+<div class="fig_right" style="width: 242px;">
+<a name="Fig_41_17" id="Fig_41_17"></a>
+<a href="images/fig_41_17_lrg.png">
+<img src="images/fig_41_17.png" width="242" height="389" alt="" title="" /></a>
+<p class="fig_caption">Figure 17.&mdash;<span class="smcap">Soft-ground tunneling.</span> The support
+of walls and roof of mine shaft by simple
+timbering; 16th century. MHT model&mdash;&frac34;" scale.
+(Smithsonian photo 49260-J.)</p>
+</div>
+
+<p>In the left-hand portion of the model is the shaft
+sunk to begin operations; here also is shown the bucket
+hoist for removing the spoil. The V-type steam engine
+powering the hoist was designed by Brunel. At the
+right of the main model is an enlarged detail of the
+shield, actually an improved version built in 1835.</p>
+
+<p>The work continued despite setbacks of every sort.
+The financial ones need no recounting here. Technically,
+although the shield principle proved workable,
+the support afforded was not infallible. Four or five
+times the river broke through the thin cover of silt
+and flooded the workings, despite the utmost caution
+in excavating. When this occurred, masses of clay,
+sandbags, and mats were dumped over the opening
+in the riverbed to seal it, and the tunnel pumped out.
+I. K. Brunel acted as superintendent and nearly lost
+his life on a number of occasions. After several suspensions
+of work resulting from withdrawal or
+exhaustion of support, one lasting seven years, the
+work was completed in 1843.</p>
+
+<p>Despite the fact that Brunel had, for the first time,
+demonstrated a practical method for tunneling in firm
+and water-bearing ground, the enormous cost of the
+work and the almost overwhelming problems encountered
+had a discouraging effect rather than otherwise.
+Not for another quarter of a century was a similar
+project undertaken.</p>
+
+<p>The Thames Tunnel was used for foot and light
+highway traffic until about 1870 when it was incorporated
+into the London Underground railway
+system, which it continues to serve today. The
+roofed-over top sections of the two shafts may still be
+seen from the river.</p>
+
+<p>A number of contemporary popular accounts of the
+tunnel exist, but one of the most thorough and
+interesting expositions on a single tunnel work of any
+period is Henry Law&rsquo;s <i>A Memoir of the Thames Tunnel</i>,
+published in 1845-1846 by John Weale. Law, an
+eminent civil engineer, covers the work in incredible
+detail from its inception until the major suspension
+in late 1828 when slightly more than half completed.
+The most valuable aspect of his record is a series of
+plates of engineering drawings of the shield and its
+components, which, so far as is known, exist nowhere
+else. These formed the basis of the enlarged section
+of the shield, shown to the right of the model of the
+tunnel itself. A vertical section through the shield
+is reproduced here from Law for comparison with
+the model (figs. 21 and 23).</p>
+
+<p><span class="pagenum"><a name="Page_219" id="Page_219">[219]</a></span></p>
+<div class="fig_center" style="width: 599px;">
+<a name="Fig_41_18" id="Fig_41_18"></a>
+<a href="images/fig_41_18_lrg.png">
+<img src="images/fig_41_18.png" width="599" height="654" alt="" /></a>
+<p class="fig_caption">Figure 18.&mdash;<span class="smcap">Soft-ground tunneling.</span> The model of
+a 16th century mine in the Museum of History and Technology was
+constructed from illustrations in such works as G. E. von Löhneyss&#39;
+<i>Bericht vom Bergwerck</i>, 1690, as well as the better known ones from
+<i>De re Metallica</i>.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_220" id="Page_220">[220]</a></span></p>
+<div class="fig_center" style="width: 646px;">
+<a name="Fig_41_19" id="Fig_41_19"></a>
+<a href="images/fig_41_19_lrg.png">
+<img src="images/fig_41_19.png" width="646" height="429" alt="" /></a>
+<p class="center fig_caption">Figure 19.&mdash;<span class="smcap">The successive stages</span> in the
+enlargement of a mid-19th century railroad tunnel, using the Austrian
+system of timbering.<br />MHT model.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_221" id="Page_221">[221]</a></span></p>
+<div class="fig_center" style="width: 598px;">
+<a name="Fig_41_20" id="Fig_41_20"></a>
+<a href="images/fig_41_20_lrg.png">
+<img src="images/fig_41_20.png" width="598" height="321" alt="" /></a>
+<p class="center fig_caption">Figure 20.&mdash;<span class="smcap">M. I. Brunel&#39;s Thames Tunnel</span>,
+1825-1843, the first driven beneath a body of water.<br />MHT model&mdash;&frac14;"
+scale. (Smithsonian photo 49260-F.)</p>
+</div>
+
+<p class="caption3">THE TOWER SUBWAY</p>
+
+<div class="fig_right" style="width: 277px;">
+<a name="Fig_41_21" id="Fig_41_21"></a>
+<a href="images/fig_41_21_lrg.png">
+<img src="images/fig_41_21.png" width="277" height="424" alt="" title="" /></a>
+<p class="fig_caption">Figure 21.&mdash;<span class="smcap">Enlarged detail</span> of Brunel&#39;s
+tunneling shield, vertical section. The
+first two and part of the third of the
+twelve frames are shown. To the left is
+the tunnel&#39;s completed brick lining and to
+the right, the individual breasting boards
+and screws for supporting the face. The
+propelling screws are seen at top and
+bottom, bearing against the lining.
+Three miners worked in each frame, one
+above the other. MHT model&mdash;&frac34;" scale.
+(Smithsonian photo 49260-G.)</p>
+</div>
+
+<p>Various inventors attempted to improve upon the
+Brunel shield, aware of the fundamental soundness
+of the shield principle. Almost all bypassed the
+rectangular sectional construction used in the Thames
+Tunnel, and took as a starting point a sectional
+shield of circular cross section, advanced by Brunel
+in his original patent of 1818. James Henry Greathead
+(1844-1896), rightfully called the father of modern
+subaqueous tunneling, surmised in later years that
+Brunel had chosen a rectangular configuration for
+actual use, as one better adapted to the sectional
+type of shield. The English civil engineer, Peter W.
+Barlow, in 1864 and 1868 patented a circular shield,
+of one piece, which was the basis of one used by him in
+constructing a small subway of 1350 feet beneath the
+Thames in 1869, the first work to follow the lead of
+Brunel. Greathead, acting as Barlow&rsquo;s contractor,
+was the designer of the shield actually used in the
+work, but it was obviously inspired by Barlow&rsquo;s
+patents.</p>
+
+<p>The reduction of the multiplicity of parts in the
+Brunel shield to a single rigid unit was of immense
+advantage and an advance perhaps equal to the
+shield concept of tunneling itself. The Barlow-Greathead
+shield was like the cap of a telescope with
+a sharpened circular ring on the front to assist in
+penetrating the ground. The diaphragm functioned,
+as did Brunel&rsquo;s breasting boards, to resist the longitudinal
+earth pressure of the face, and the cylindrical
+portion behind the diaphragm bore the radial pressure
+of roof and walls. Here also for the first time, a
+permanent lining formed of cast-iron segments was
+used, a second major advancement in soft-ground
+tunneling practice. Not only could the segments be
+placed and bolted together far more rapidly than
+masonry lining could be laid up, but unlike the
+green masonry, they could immediately bear the full
+force of the shield-propelling screws.</p>
+
+<p>Barlow, capitalizing on Brunel&rsquo;s error in burrowing
+so close to the riverbed, maintained an average cover
+of 30 feet over the tunnel, driving through a solid
+stratum of firm London clay which was virtually
+impervious to water. As the result of this, combined
+with the advantages of the solid shield and the
+rapidly placed iron lining, the work moved forward
+at a pace and with a facility in startling contrast to
+that of the Thames Tunnel, although in fairness it
+must be recalled that the face area was far less.</p>
+
+<p>The clay was found sufficiently sound that it could
+be readily excavated without the support of the diaphragm,
+and normally three miners worked in front
+of the shield, digging out the clay and passing it
+back through a doorway in the plate. This could be
+closed in case of a sudden settlement or break in.
+Following excavation, the shield was advanced 18
+inches into the excavated area by means of 6 screws,
+and a ring of lining segments 18 inches in length
+bolted to the previous ring under cover of the overlapping
+rear skirt of the shield. The small annular
+space left between the outside of the lining and the
+clay by the thickness and clearance of the skirt&mdash;about
+an inch&mdash;was filled with thin cement grout. The
+tunnel was advanced 18 inches during each 8-hour
+shift. The work continued around the clock, and the
+900-foot river section was completed in only 14 weeks.
+<a name="FNanchor_4_4" id="FNanchor_4_4"></a>
+<a href="#Footnote_4_4" class="fnanchor">[4]</a>
+The entire work was completed almost without
+incident in just under a year, a remarkable performance
+for the world&rsquo;s second subaqueous tunnel.</p>
+
+<p><span class="pagenum" style="clear:both;">
+<a name="Page_222" id="Page_222">[222]</a></span></p>
+<div class="fig_center" style="width: 594px; clear:both;">
+<a name="Fig_41_22" id="Fig_41_22"></a>
+<a href="images/fig_41_22_lrg.png">
+<img src="images/fig_41_22.png" width="594" height="648" alt="" /></a>
+<p class="fig_caption center">Figure 22.&mdash;<span class="smcap">Broadside published after commencement of work</span>
+on the Thames Tunnel, 1827.<br />(MHT collections.)
+<a href="#Transcription">Transcription</a> of the text is
+presented in the Transcriber&#39;s Notes below.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_223" id="Page_223">[223]</a></span></p>
+<div class="fig_center" style="width: 477px;">
+<a name="Fig_41_23" id="Fig_41_23"></a>
+<a href="images/fig_41_23_lrg.png">
+<img src="images/fig_41_23.png" width="477" height="625" alt="" /></a>
+<p class="fig_caption">Figure 23.&mdash;<span class="smcap">Vertical section through Brunel&#39;s shield.</span> The long lever, x, supported the wood centering for turning
+the masonry arches of the lining. (<span class="smcap">Law</span>,
+<i>A Memoir of the Thames Tunnel.</i>)</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_224" id="Page_224">[224]</a></span></p>
+<div class="fig_left" style="width: 283px;">
+<a name="Fig_41_24" id="Fig_41_24"></a>
+<a href="images/fig_41_24_lrg.png">
+<img src="images/fig_41_24.png" width="283" height="460" alt="" /></a>
+<p class="fig_caption">Figure 24.&mdash;<span class="smcap">Thames Tunnel. Section through</span>
+riverbed and tunnel following one of the break-throughs
+of the river. Inspection of the damage
+with a diving bell. (<span class="smcap">Beamish</span>, <i>A Memoir of the
+Life of Sir Marc Isambard Brunel</i>.)</p>
+</div>
+
+<p>The Tower Subway at first operated with cylindrical
+cars that nearly filled the 7-foot bore; the cars
+were drawn by cables powered by small steam
+engines in the shafts. This mode of power had previously
+been used in passenger service only on the
+Greenwich Street elevated railway in New York.
+Later the cars were abandoned as unprofitable and
+the tunnel turned into a footway (<a href="#Fig_41_32">fig. 32</a>). This small
+tunnel, the successful driving due entirely to Greathead&rsquo;s
+skill, was the forerunner of the modern subaqueous
+tunnel. In it, two of the three elements
+essential to such work thereafter were first applied:
+the one-piece movable shield of circular section, and
+the segmental cast-iron lining.</p>
+
+<p>The documentation of this work is far thinner than
+for the Thames Tunnel. The most accurate source of
+technical information is a brief historical account in
+Copperthwaite&rsquo;s classic <i>Tunnel Shields and the Use of
+Compressed Air in Subaqueous Works</i>, published in 1906.
+Copperthwaite, a successful tunnel engineer, laments
+the fact that he was able to turn up no drawing or
+original data on this first shield of Greathead&rsquo;s, but
+he presents a sketch of it prepared in the Greathead
+office in 1895, which is presumably a fair representation
+(<a href="#Fig_41_33">fig. 33</a>). The Tower Subway model was built
+on the basis of this and several woodcuts of the working
+area that appeared contemporaneously in the
+illustrated press. In this and the adjacent model of
+Beach&rsquo;s Broadway Subway, the tunnel axis has been
+placed on an angle to the viewer, projecting the bore
+into the case so that the complete circle of the working
+face is included for a more suggestive effect. This
+was possible because of the short length of the work
+included.</p>
+
+<p>Henry S. Drinker, also a tunnel engineer and author
+of the most comprehensive work on tunneling ever
+published, treats rock tunneling in exhaustive detail
+up to 1878. His notice of what he terms &ldquo;submarine
+tunneling&rdquo; is extremely brief. He does, however,
+draw a most interesting comparison between the first
+Thames Tunnel, built by Brunel, and the second,
+built by Greathead 26 years later:</p>
+
+<div class="tdtb">
+<table summary="Thames Tunnel Costs">
+<tr>
+  <td class="vtop center" style="width:45%">FIRST THAMES TUNNEL</td>
+  <td class="center" style="width:45%">SECOND THAMES TUNNEL<br />(TOWER SUBWAY)</td>
+</tr>
+<tr>
+  <td>Brickwork lining, 38 feet wide by 22&frac12; feet high.</td>
+  <td class="vtop">Cast-iron lining of 8 feet outside diameter.</td>
+</tr>
+<tr>
+  <td class="vtop">120-ton cast-iron shield, accommodating 36 miners.</td>
+  <td>2&frac12;-ton, wrought-iron shield, accommodating at most 3 men.</td>
+</tr>
+<tr>
+  <td class="vtop">Workings filled by irruption of river five times.</td>
+  <td>&ldquo;Water encountered at almost any time could have
+    been gathered in a stable pail.&rdquo;</td>
+</tr>
+<tr>
+  <td>Eighteen years elapsed between start and finish of work.</td>
+  <td class="vtop">Work completed in about eleven months.</td>
+</tr>
+<tr>
+  <td>Cost: $3,000,000.</td>
+  <td>Cost: $100,000.</td>
+</tr>
+</table>
+</div>
+
+<p><span class="pagenum"><a name="Page_225" id="Page_225">[225]</a></span></p>
+<div class="fig_center" style="width: 597px;">
+<a name="Fig_41_25" id="Fig_41_25"></a>
+<a href="images/fig_41_25_lrg.png">
+<img src="images/fig_41_25.png" width="597" height="613" alt="" /></a>
+<p class="fig_caption">Figure 25.&mdash;<span class="smcap">Transverse section through shield</span>,
+after inundation. Such disasters, as well as the inconsistency of the
+riverbed&#39;s composition, seriously disturbed the alignment of the
+shield&#39;s individual sections. (<span class="smcap">Law</span>, <i>A Memoir of the Thames
+Tunnel</i>.)</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_226" id="Page_226">[226]</a></span></p>
+<div class="fig_center" style="width: 644px;">
+<a name="Fig_41_26" id="Fig_41_26"></a>
+<a href="images/fig_41_26_lrg.png">
+<img src="images/fig_41_26.png" width="644" height="332" alt="" /></a>
+<p class="fig_caption">Figure 26.&mdash;<span class="smcap">Longitudinal section through Thames Tunnel</span>
+after sandbagging to close a break in the riverbed. The tunnel
+is filled with silt and water. (<span class="smcap">Law</span>, <i>A Memoir of the Thames
+Tunnel</i>.)</p>
+</div>
+
+<div class="fig_center" style="width: 639px;">
+<a name="Fig_41_27" id="Fig_41_27"></a>
+<a href="images/fig_41_27_lrg.png">
+<img src="images/fig_41_27.png" width="639" height="443" alt="" title="" /></a>
+<p class="fig_caption">Figure 27.&mdash;<span class="smcap">Interior of the Thames Tunnel</span> shortly
+after completion in 1843. (<i>Photo courtesy of New York Public Library
+Picture Collection.</i>)</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_227" id="Page_227">[227]</a></span></p>
+<div class="fig_center" style="width: 644px;">
+<a name="Fig_41_28" id="Fig_41_28"></a>
+<a href="images/fig_41_28_lrg.png">
+<img src="images/fig_41_28.png" width="644" height="372" alt="" /></a>
+<p class="fig_caption">Figure 28.&mdash;<span class="smcap">Thames Tunnel</span> in use by London
+Underground railway. (<i>Illustrated London News</i>, 1869?)</p>
+</div>
+
+<div class="fig_left" style="width: 341px;">
+<a name="Fig_41_29" id="Fig_41_29"></a>
+<a href="images/fig_41_29_lrg.png">
+<img src="images/fig_41_29.png" width="341" height="273" alt="" title="" /></a>
+<p class="fig_caption">Figure 29.&mdash;<span class="smcap">Placing a</span> segment of cast-iron lining
+in Greathead&#39;s Tower Subway, 1869. To the rear is the shield&#39;s diaphragm
+or bulkhead. MHT model&mdash;1&frac12;" scale. (Smithsonian photo 49260-B.)</p>
+</div>
+
+<p class="caption3">BEACH&rsquo;S BROADWAY SUBWAY</p>
+
+<p>Almost simultaneously with the construction of the
+Tower Subway, the first American shield tunnel was
+driven by Alfred Ely Beach (1826-1896). Beach, as
+editor of the <i>Scientific American</i> and inventor of, among
+other things, a successful typewriter as early as 1856,
+was well known and respected in technical circles.
+He was not a civil engineer, but had become concerned
+with New York&rsquo;s pressing traffic problem
+(even then) and as a solution, developed plans for a
+rapid-transit subway to extend the length of Broadway.
+He invented a shield as an adjunct to this system,
+solely to permit driving of the tunnel without disturbing
+the overlying streets.</p>
+
+<p>An active patent attorney as well, Beach must
+certainly have known of and studied the existing
+patents for tunneling shields, which were, without
+exception, British. In certain aspects his shield
+resembled the one patented by Barlow in 1864, but
+never built. However, work on the Beach tunnel
+started in 1869, so close in time to that on the Tower
+<span class="pagenum"><a name="Page_228" id="Page_228">[228]</a></span>
+Subway, that it is unlikely that there was any influence
+from that source. Beach had himself patented
+a shield, in June 1869, a two-piece, sectional design
+that bore no resemblance to the one used. His
+subway plan had been first introduced at the 1867
+fair of the American Institute in the form of a short
+plywood tube through which a small, close-fitting
+car was blown by a fan. The car carried 12 passengers.
+Sensing opposition to the subway scheme
+from Tammany, in 1868 Beach obtained a charter
+to place a small tube beneath Broadway for transporting
+mail and small packages pneumatically, a
+plan he advocated independently of the passenger
+subway.</p>
+<div style="clear: both;"></div>
+
+<div class="center">
+<table summary="Tower Subway Illustrations">
+<tr>
+  <td colspan="2"><a name="Fig_41_30" id="Fig_41_30"></a>
+    <a href="images/fig_41_30_lrg.png">
+    <img src="images/fig_41_30.png" width="627" height="336" alt="" /></a>
+  </td>
+</tr>
+<tr>
+  <td class="fig_caption">ADVANCING THE SHIELD.</td>
+  <td class="fig_caption">FITTING THE CASTINGS.</td>
+</tr>
+<tr>
+  <td class="fig_caption" colspan="2">Figure 30.&mdash;    <span class="smcap">Contemporary illustrations</span> of Tower Subway works used
+    as basis of the model in the Museum of History and Technology.
+    (<i>Illustrated London News</i>, 1869.)
+  </td>
+</tr>
+</table>
+</div>
+
+<div class="fig_left" style="width: 311px;">
+<a name="Fig_41_31" id="Fig_41_31"></a>
+<img src="images/fig_41_31.png" width="311" height="443" alt="" title="" />
+<p class="fig_caption">Figure 31.&mdash;<span class="smcap">Excavation in front of shield</span>,
+Tower Subway. This was possible because of the
+stiffness of the clay encountered. MHT model&mdash;front
+of model shown in <a href="#Fig_41_29">fig. 29</a>. (Smithsonian
+photo 49260-A.)</p>
+</div>
+
+<p>Under this thin pretense of legal authorization, the
+sub-rosa excavation began from the basement of a
+clothing store on Warren Street near Broadway. The
+8-foot-diameter tunnel ran eastward a short distance,
+made a 90-degree turn, and thence southward under
+<span class="pagenum"><a name="Page_229" id="Page_229">[229]</a></span>
+Broadway to stop a block away under the south side
+of Murray Street. The total distance was about 312
+feet. Work was carried on at night in total secrecy,
+the actual tunneling taking 58 nights. At the Warren
+Street terminal, a waiting room was excavated and a
+large Roots blower installed for propulsion of the single
+passenger car. The plan was similar to that used
+with the model in 1867: the cylindrical car fitted the
+circular tunnel with only slight circumferential clearance.
+The blower created a plenum within the waiting
+room and tunnel area behind the car of about
+0.25 pounds per square inch, resulting in a thrust on
+the car of almost a ton, not accounting for blowby.
+The car was thus blown along its course, and was
+returned by reversing the blower&rsquo;s suction and discharge
+ducts to produce an equivalent vacuum within
+the tunnel.</p>
+
+<div class="fig_right" style="width: 292px;">
+<a name="Fig_41_33" id="Fig_41_33"></a>
+<img src="images/fig_41_33.png" width="292" height="449" alt="" title="" />
+<p class="fig_caption">Figure 33.&mdash;<span class="smcap">Vertical section</span> through the Greathead
+shield used at the Tower Subway, 1869. The
+first one-piece shield of circular section.
+(<span class="smcap">Copperthwaite</span>,
+<i>Tunnel Shields and the Use of Compressed
+Air in Subaqueous Works</i>.)</p>
+</div>
+
+
+<p>The system opened in February of 1870 and remained
+in operation for about a year. Beach was
+ultimately subdued by the hostile influences of Boss
+Tweed, and the project was completely abandoned.
+Within a very few more years the first commercially
+operated elevated line was built, but the subway did
+not achieve legitimate status in New York until the
+opening of the Interborough line in 1904. Ironically,
+its route traversed Broadway for almost the length of
+the island.</p>
+
+
+<div class="fig_left" style="width: 353px;">
+<a name="Fig_41_32" id="Fig_41_32"></a>
+<a href="images/fig_41_32_lrg.png">
+<img src="images/fig_41_32.png" width="353" height="351" alt="" title="" /></a>
+<p class="fig_caption">Figure 32.&mdash;<span class="smcap">Interior of completed Tower Subway.</span>
+(<span class="smcap">Thornbury</span>, <i>Old and New London, 1887,
+vol. 1, p. 126</i>.)</p>
+</div>
+
+
+<p>The Beach shield operated with perfect success in
+this brief trial, although the loose sandy soil encountered
+was admittedly not a severe test of its qualities.
+No diaphragm was used; instead a series of 8 horizontal
+shelves with sharpened leading edges extended
+across the front opening of the shield. The outstanding
+feature of the machine was the substitution
+for the propelling screws used by Brunel and
+Greathead of 18 hydraulic rams, set around its
+circumference. These were fed by a single hand-operated
+pump, seen in the center of figure 34. By
+this means the course of the shield&rsquo;s forward movement
+could be controlled with a convenience and
+<span class="pagenum"><a name="Page_230" id="Page_230">[230]</a></span>
+precision not attainable with screws. Vertical and
+horizontal deflection was achieved by throttling the
+supply of water to certain of the rams, which could
+be individually controlled, causing greater pressure
+on one portion of the shield than another. This
+system has not changed in the ensuing time, except,
+of course, in the substitution of mechanically produced
+hydraulic pressure for hand.</p>
+
+<div class="fig_right" style="width: 400px;">
+<a name="Fig_41_34" id="Fig_41_34"></a>
+<a href="images/fig_41_34_lrg.png">
+<img src="images/fig_41_34.png" width="400" height="358" alt="" title="" /></a>
+<p class="fig_caption">Figure 34.&mdash;<span class="smcap">Beach&#39;s</span> Broadway Subway.
+Advancing the shield by
+hydraulic rams, 1869. MHT
+model&mdash;1&frac12;" scale. (Smithsonian
+photo 49260-E.)</p>
+</div>
+
+<p>Unlike the driving of the Tower Subway, no excavation
+was done in front of the shield. Rather,
+the shield was forced by the rams into the soil for
+the length of their stroke, the material which entered
+being supported by the shelves. This was removed
+from the shelves and hauled off. The ram plungers
+then were withdrawn and a 16-inch length of the
+permanent lining built up within the shelter of the
+shield&rsquo;s tail ring. Against this, the rams bore for the
+next advance. Masonry lining was used in the
+straight section; cast-iron in the curved. The juncture
+is shown in the model.</p>
+
+<p><span class="pagenum"><a name="Page_231" id="Page_231">[231]</a></span></p>
+<div class="fig_right" style="width: 354px;">
+<a name="Fig_41_36" id="Fig_41_36"></a>
+<a href="images/fig_41_36_lrg.png">
+<img src="images/fig_41_36.png" width="354" height="366" alt="" title="" /></a>
+<p class="fig_caption">Figure 36.&mdash;<span class="smcap">Interior</span> of Beach Subway showing iron
+lining on curved section and the pneumatically powered passenger car.
+View from waiting room. (<i>Scientific American</i>, March 5, 1870.)</p>
+</div>
+
+
+<div class="fig_left" style="width: 298px;">
+<a name="Fig_41_35" id="Fig_41_35"></a>
+<a href="images/fig_41_35_lrg.png">
+<img src="images/fig_41_35.png" width="298" height="419" alt="" title="" /></a>
+<p class="fig_caption">Figure 35.&mdash;<span class="smcap">Vertical section</span> through the Beach
+shield used on the Broadway Subway, showing the
+horizontal shelves (C), iron cutting ring (B),
+hydraulic rams (D), hydraulic pump (F), and rear
+protective skirt (H). (<i>Scientific American</i>, March 5,
+1870.)</p>
+</div>
+
+
+<p>Enlarged versions of the Beach shield were used in a
+few tunnels in the Midwest in the early 1870&rsquo;s, but
+from then until 1886 the shield method, for no clear
+reason, again entered a period of disuse finding no
+application on either side of the Atlantic despite its
+virtually unqualified proof at the hands of Greathead
+and Beach. Little precise information remains on
+this work. The Beach system of pneumatic transit is
+described fully in a well-illustrated booklet published
+by him in January 1868, in which the American Institute
+model is shown, and many projected systems
+of pneumatic propulsion as well as of subterranean
+and subaqueous tunneling described. Beach again
+(presumably) is author of the sole contemporary account
+of the Broadway Subway, which appeared in
+<i>Scientific American</i> following its opening early in 1870.
+Included are good views of the tunnel and car, of the
+shield in operation, and, most important, a vertical
+sectional view through the shield (<a href="#Fig_41_35">fig. 35</a>).</p>
+
+<p>It is interesting to note that optical surveys for
+maintenance of the course apparently were not used.
+The article illustrated and described the driving each
+night of a jointed iron rod up through the tunnel
+roof to the street, twenty or so feet above, for &ldquo;testing
+the position.&rdquo;</p>
+
+
+<p class="caption3" style="clear:both;">THE FIRST HUDSON RIVER TUNNEL</p>
+
+<p>Despite the ultimate success of Brunel&rsquo;s Thames
+Tunnel in 1843, the shield in that case afforded only
+moderately reliable protection because of the fluidity
+of the soil driven through, and its tendency to enter
+the works through the smallest opening in the shield&rsquo;s
+defense. An English doctor who had made physiological
+studies of the effects on workmen of the high
+air pressure within diving bells is said to have recommended
+to Brunel in 1828 that he introduce an
+atmosphere of compressed air into the tunnel to exclude
+the water and support the work face.</p>
+
+<p>This plan was first formally described by Sir Thomas
+Cochrane (1775-1860) in a British patent of 1830.
+Conscious of Brunel&rsquo;s problems, he proposed a system
+of shaft sinking, mining, and tunneling in water-bearing
+materials by filling the excavated area with
+<span class="pagenum"><a name="Page_232" id="Page_232">[232]</a></span>
+air sufficiently above atmospheric pressure to prevent
+the water from entering and to support the earth. In
+this, and his description of air locks for passage of men
+and materials between the atmosphere and the pressurized
+area, Cochrane fully outlined the essential
+features of pneumatic excavation as developed since.</p>
+
+<table summary="tunnel details">
+<tr>
+  <td style="width:267px; padding:15px;"><a name="Fig_41_37" id="Fig_41_37"></a>
+    <a href="images/fig_41_37_lrg.png">
+    <img src="images/fig_41_37.png" width="267" height="377" alt="" title="" /></a>
+  </td>
+  <td style="width:250px; padding:15px;"><a name="Fig_41_38" id="Fig_41_38"></a>
+    <a href="images/fig_41_38_lrg.png">
+    <img src="images/fig_41_38.png" width="250" height="448" alt="" title="" /></a>
+  </td>
+</tr>
+<tr>
+  <td class="vtop" style="padding:15px;"><p class="fig_caption">Figure 37.&mdash;    <span class="smcap">The giant Roots lobe-type blower</span>
+    used for propelling the car.</p>
+  </td>
+  <td style="padding:15px;"><p class="fig_caption">Figure 38.&mdash;    <span class="smcap">Testing alignment</span> of the Broadway
+    Subway at night by driving a jointed rod up to
+    street level. (<i>Scientific American</i>, March 5, 1870.)</p>
+  </td>
+</tr>
+</table>
+
+<p style="clear:both;">In 1839, a French engineer first used the system in
+sinking a mine shaft through a watery stratum. From
+then on, the sinking of shafts, and somewhat later the
+construction of bridge pier foundations, by the pneumatic
+method became almost commonplace engineering
+practice in Europe and America. Not until 1879
+however, was the system tried in tunneling work, and
+then, as with the shield ten years earlier, almost
+simultaneously here and abroad. The first application
+was in a small river tunnel in Antwerp, only
+5 feet in height. This project was successfully completed
+relying on compressed air alone to support the
+earth, no shield being used. The importance of the
+work cannot be considered great due to its lack of
+scope.</p>
+
+<p>In 1871 Dewitt C. Haskin (1822-1900), a west
+coast mine and railroad builder, became interested
+in the pneumatic caissons then being used to found
+the river piers of Eads&rsquo; Mississippi River bridge at
+St. Louis. In apparent total ignorance of the Cochrane
+patent, he evolved a similar system for tunneling
+water-bearing media, and in 1873 proposed construction
+of a tunnel through the silt beneath the Hudson
+to provide rail connection between New Jersey and
+New York City.</p>
+
+<p><span class="pagenum"><a name="Page_233" id="Page_233">[233]</a></span></p>
+<div class="fig_center" style="width: 631px;">
+<a name="Fig_41_39" id="Fig_41_39"></a>
+<a href="images/fig_41_39_lrg.png">
+<img src="images/fig_41_39.png" width="631" height="291" alt="" /></a>
+<p class="fig_caption">Figure 39.&mdash;<span class="smcap">Haskin&#39;s</span> pneumatically driven tunnel
+under the Hudson River, 1880. In the engine room at top left was the
+machinery for hoisting, generating electricity for lighting, and air
+compressing. The air lock is seen in the wall of the brick shaft.
+MHT model&mdash;0.3" scale. (Smithsonian photo 49260.)</p>
+</div>
+
+<div class="fig_center" style="width: 626px;">
+<a name="Fig_41_40" id="Fig_41_40"></a>
+<img src="images/fig_41_40.png" width="626" height="470" alt="" />
+<p class="fig_caption">Figure 40.&mdash;<span class="smcap">Artist&#39;s conception of miners</span> escaping
+into the air lock during the blowout in Haskin&#39;s tunnel.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_234" id="Page_234">[234]</a></span></p>
+<p>It would be difficult to imagine a site more in need
+of such communication. All lines from the south
+terminated along the west shore of the river and the
+immense traffic&mdash;cars, freight and passengers&mdash;was
+carried across to Manhattan Island by ferry and
+barge with staggering inconvenience and at enormous
+cost. A bridge would have been, and still is, almost
+out of the question due not only to the width of the
+crossing, but to the flatness of both banks. To provide
+sufficient navigational clearance (without a drawspan),
+impracticably long approaches would have
+been necessary to obtain a permissibly gentle grade.</p>
+
+<p><a name="Haskin" id="Haskin"></a>
+Haskin formed a tunneling company and began
+work with the sinking of a shaft in Hoboken on the
+New Jersey side. In a month it was halted because
+of an injunction by, curiously, the D L &amp; W Railroad,
+who feared for their vast investment in terminal and
+marine facilities. Not until November of 1879 was
+the injunction lifted and work again commenced.
+The shaft was completed and an air lock located in
+one wall from which the tunnel proper was to be
+carried forward. It was Haskin&rsquo;s plan to use no
+shield, relying solely on the pressure of compressed
+air to maintain the work faces and prevent the entry
+of water. The air was admitted in late December,
+and the first large-scale pneumatic tunneling operation
+launched. A single 26-foot, double-track bore
+was at first undertaken, but a work face of such
+diameter proved unmanageable and two oval tubes
+18 feet high by 16 feet wide were substituted, each
+to carry a single track. Work went forward with
+reasonable facility, considering the lack of precedent.
+A temporary entrance was formed of sheet-iron rings
+from the air lock down to the tunnel grade, at which
+point the permanent work of the north tube was
+started. Immediately behind the excavation at the
+face, a lining of thin wrought-iron plates was built up,
+to provide form for the 2-foot, permanent brick lining
+that followed. The three stages are shown in the
+model in about their proper relationship of progress.
+The work is shown passing beneath an old timber-crib
+bulkhead, used for stabilizing the shoreline.</p>
+
+<p>The silt of the riverbed was about the consistency of
+putty and under good conditions formed a secure
+barrier between the excavation and the river above.
+It was easily excavated, and for removal was mixed
+with water and blown out through a pipe into the
+shaft by the higher pressure in the tunnel. About
+half was left in the bore for removal later. The basic
+scheme was workable, but in operation an extreme
+precision was required in regulating the air pressure
+in the work area.<a name="FNanchor_5_5" id="FNanchor_5_5"></a>
+<a href="#Footnote_5_5" class="fnanchor">[5]</a> It was soon found that there
+existed an 11-psi difference between the pressure of
+water on the top and the bottom of the working face,
+due to the 22-foot height of the unlined opening.
+Thus, it was impossible to maintain perfect pneumatic
+balance of the external pressure over the entire face.
+It was necessary to strike an average with the result
+that some water entered at the bottom of the face
+where the water pressure was greatest, and some air
+leaked out at the top where the water pressure was
+below the air pressure. Constant attention was essential:
+several men did nothing but watch the behavior
+of the leaks and adjusted the pressure as the
+ground density changed with advance. Air was supplied
+by several steam-driven compressors at the
+surface.</p>
+
+<p>The air lock permitted passage back and forth of men
+and supplies between the atmosphere and the work
+area, without disturbing the pressure differential.
+This principle is demonstrated by an animated model
+set into the main model, to the left of the shaft
+(<a href="#Fig_41_39">fig. 39</a>). The variation of
+pressure within the lock chamber
+to match the atmosphere or the pressurized area,
+depending on the direction of passage, is clearly
+shown by simplified valves and gauges, and by the
+use of light in varying color density. In the Haskin
+tunnel, 5 to 10 minutes were taken to pass the miners
+through the lock so as to avoid too abrupt a physiological
+change.</p>
+
+<p><span class="pagenum"><a name="Page_235" id="Page_235">[235]</a></span></p>
+<p>Despite caution, a blowout occurred in July 1880
+due to air leakage not at the face, but around the
+temporary entrance. One door of the air lock jammed
+and twenty men drowned, resulting in an inquiry
+which brought forth much of the distrust with which
+Haskin was regarded by the engineering profession.
+His ability and qualifications were subjected to the
+bitterest attack in and by the technical press. There
+is some indication that, although the project began
+with a staff of competent engineers, they were
+alienated by Haskin in the course of work and at least
+one withdrew. Haskin&rsquo;s remarks in his own defense
+indicate that some of the denunciation was undoubtedly
+justified. And yet, despite this reaction, the
+fundamental merit of the pneumatic tunneling method
+had been demonstrated by Haskin and was immediately
+recognized and freely acknowledged. It was
+apparent at the same time, however, that air by itself
+did not provide a sufficiently reliable support for
+large-area tunnel works in unstable ground, and this
+remains the only major subaqueous tunnel work
+driven with air alone.</p>
+
+<div class="fig_center" style="width: 626px;">
+<a name="Fig_41_41" id="Fig_41_41"></a>
+<img src="images/fig_41_41.png" width="626" height="611" alt="" />
+<p class="fig_caption">Figure 41.&mdash;<span class="smcap">Location of Hudson River Tunnel.</span>
+(<i>Leslie&#39;s Weekly</i>, 1879.)</p>
+</div>
+
+<p>After the accident, work continued under Haskin
+until 1882 when funds ran out. About 1600 feet of
+the north tube and 600 feet of the south tube had been
+completed. Greathead resumed operations with a
+shield for a British company in 1889, but exhaustion
+of funds again caused stoppage in 1891. The tunnel
+<span class="pagenum"><a name="Page_236" id="Page_236">[236]</a></span>
+was finally completed in 1904, and is now in use as part
+of the Hudson and Manhattan rapid-transit system,
+never providing the sought-after rail link. A splendid
+document of the Haskin portion of the work is S. D. V.
+Burr&rsquo;s <i>Tunneling Under the Hudson River</i> published in
+1885. It is based entirely upon firsthand material and
+contains drawings of most of the work, including the
+auxiliary apparatus. It is interesting to note that
+electric illumination (arc, not incandescent, lights) and
+telephones were used, unquestionably the first employment
+of either in tunnel work.</p>
+
+<div class="fig_center" style="width: 630px;">
+<a name="Fig_41_42" id="Fig_41_42"></a>
+<a href="images/fig_41_42_lrg.png">
+<img src="images/fig_41_42.png" width="630" height="372" alt="" /></a>
+<p class="fig_caption">Figure 42.&mdash;<span class="smcap">St. Clair Tunnel.</span> View of front of
+shield showing method of excavation in firm strata. Incandescent electric
+illumination was used. 1889-90. MHT model&mdash;1" scale. (Smithsonian photo
+49260-D.)</p>
+</div>
+
+
+<p class="caption3">THE ST. CLAIR TUNNEL</p>
+
+<p>The final model of the soft-ground series reflects, as
+did the Hoosac Tunnel model for hard-rock tunneling,
+final emergence into the modern period. Although
+the St. Clair Tunnel was completed over 70 years ago,
+it typifies in its method of construction, the basic
+procedures of subaqueous work in the present day.
+The Thames Tunnel of Brunel, and Haskin&rsquo;s efforts
+beneath the Hudson, had clearly shown that by
+themselves, both the shield and pneumatic systems of
+driving through fluid ground were defective in practice
+for tunnels of large area. Note that the earliest
+successful works by each method had been of very
+small area, so that the influence of adverse conditions
+was greatly diminished.</p>
+
+<p>The first man to perceive and seize upon the benefits
+to be gained by combining the two systems was,
+most fittingly, Greathead. Although he had projected
+the technique earlier, in driving the underground City
+and South London Railway in 1886, he brought
+together for the first time the three fundamental elements
+essential for the practical tunneling of soft,
+water-bearing ground: compressed-air support of the
+work during construction, the movable shield, and cast-iron,
+permanent lining. The marriage was a happy
+one indeed; the limitations of each system were almost
+perfectly overcome by the qualities of the others.</p>
+
+<p>The conditions prevailing in 1882 at the Sarnia,
+Ontario, terminal of the Grand Trunk Railway, both
+operational and physical, were almost precisely the
+same as those which inspired the undertaking of the
+<span class="pagenum"><a name="Page_237" id="Page_237">[237]</a></span>
+Hudson River Tunnel. The heavy traffic at this
+vital U.S.&mdash;Canada rail interchange was ferried inconveniently
+across the wide St. Clair River, and the
+bank and river conditions precluded construction of a
+bridge. A tunnel was projected by the railway in
+that year, the time when Haskin&rsquo;s tribulations were
+at their height. Perhaps because of this lack of precedent
+for a work of such size, nothing was done
+immediately. In 1884 the railway organized a tunnel
+company; in 1886 test borings were made in the riverbed
+and small exploratory drifts were started across
+from both banks by normal methods of mine timbering.
+The natural gas, quicksand, and water
+encountered soon stopped the work.</p>
+
+<div class="fig_center" style="width: 642px;">
+<a name="Fig_41_43" id="Fig_41_43"></a>
+<a href="images/fig_41_43_lrg.png">
+<img src="images/fig_41_43.png" width="642" height="359" alt="" /></a>
+<p class="fig_caption">Figure 43.&mdash;<span class="smcap">Rear view of St. Clair shield</span> showing
+the erector arm placing a cast-iron lining segment. The three motions of
+the arm&mdash;axial, radial, and rotational, were manually powered.
+(Smithsonian photo 49260-C.)</p>
+</div>
+
+<p>It was at this time that the railway&rsquo;s president
+visited Greathead&rsquo;s City and South London workings.
+The obvious answer to the St. Clair problem lay in
+the successful conduct of this subway. Joseph
+Hobson, chief engineer of the Grand Trunk and of
+the tunnel project, in designing a shield, is said to
+have searched for drawings of the shields used in
+the Broadway and Tower Subways of 1868-9, but
+unable to locate any, he relied to a limited extent
+on the small drawings of those in Drinker&rsquo;s volume.
+There is no explanation as to why he did not have
+drawings of the City and South London shield at that
+moment in use, unless one considers the rather unlikely
+possibility that Greathead maintained its
+design in secrecy.</p>
+
+<div class="fig_center" style="width: 606px;">
+<a name="Fig_41_44" id="Fig_41_44"></a>
+<a href="images/fig_41_44_lrg.png">
+<img src="images/fig_41_44.png" width="606" height="460" alt="" /></a>
+<p class="fig_caption">Figure 44.&mdash;<span class="smcap">Opening of the St. Clair Tunnel, 1891.</span>
+(<i>Photo courtesy of Detroit Library, Burton Historical Collection.</i>)</p>
+</div>
+
+<p>The Hobson shield followed Greathead&rsquo;s as closely
+as any other, in having a diaphragm with closable
+doors, but a modification of Beach&rsquo;s sharpened
+horizontal shelves was also used. However, these
+functioned more as working platforms than supports
+for the earth. The machine was 21&frac12; feet in diameter,
+an unprecedented size and almost twice that of
+Greathead&rsquo;s current one. It was driven by 24
+hydraulic rams. Throughout the entire preliminary
+consideration of the project there was a marked sense
+of caution that amounted to what seems an almost
+total lack of confidence in success. Commencement
+of the work from vertical shafts was planned so that
+if the tunnel itself failed, no expenditure would have
+been made for approach work. In April 1888, the
+<span class="pagenum"><a name="Page_238" id="Page_238">[238]</a></span>
+shafts were started near both riverbanks, but before
+reaching proper depth the almost fluid clay and silt
+flowed up faster than it could be excavated and this
+plan was abandoned. After this second inauspicious
+start, long open approach cuts were made and the
+work finally began. The portals were established in
+the cuts, several thousand feet back from each bank
+and there the tunneling itself began. The portions
+under the shore were driven without air. When the
+banks were reached, brick bulkheads containing air
+locks were built across the opening and the section
+beneath the river, about 3,710 feet long, driven under
+air pressure of 10 to 28 pounds above atmosphere.
+For most of the way, the clay was firm and there was
+little air leakage. It was found that horses could not
+survive in the compressed air, and so mules were
+used under the river.</p>
+
+<p>In the firm clay, excavation was carried on several
+feet in front of the shield, as shown in the model
+(<a href="#Fig_41_42">fig. 42</a>). About twelve
+miners worked at the face. However,
+in certain strata the clay encountered was so
+fluid that the shield could be simply driven forward by
+the rams, causing the muck to flow in at the door
+openings without excavation. After each advance,
+the rams were retracted and a ring of iron lining
+segments built up, as in the Tower Subway. Here,
+for the first time, an &ldquo;erector arm&rdquo; was used for
+placing the segments, which weighed about half a ton.
+In all respects, the work advanced with wonderful
+facility and lack of operational difficulty. Considering
+<span class="pagenum"><a name="Page_239" id="Page_239">[239]</a></span>
+the large area, no subaqueous tunnel had ever
+been driven with such speed. The average monthly
+progress for the American and Canadian headings
+totaled 455 feet, and at top efficiency 10 rings or a
+length of 15.3 feet could be set in a 24-hour day in
+each heading. The 6,000 feet of tunnel was driven in
+just a year; the two shields met vis-a-vis in August of
+1890.</p>
+
+<p>The transition was complete. The work had been
+closely followed by the technical journals and the
+reports of its successful accomplishment thus were
+brought to the attention of the entire civil engineering
+profession. As the first major subaqueous tunnel
+completed in America and the first in the world of
+a size able to accommodate full-scale rail traffic, the
+St. Clair Tunnel served to dispel the doubts surrounding
+such work, and established the pattern for a mode
+of tunneling which has since changed only in matters
+of detail.</p>
+
+<p>Of the eight models, only this one was built under
+the positive guidance of original documents. In the
+possession of the Canadian National Railways are
+drawings not only of all elements of the shield and
+lining, but of much of the auxiliary apparatus used
+in construction. Such materials rarely survive, and
+do so in this case only because of the foresight of the
+railway which, to avoid paying a high profit margin
+to a private contractor as compensation for the risk
+and uncertainty involved, carried the contract itself
+and, therefore, preserved all original drawing records.</p>
+
+<p>While the engineering of tunnels has been comprehensively
+treated in this paper from the historical
+standpoint, it is well to still reflect that the advances
+made in tunneling have not perceptibly removed the
+elements of uncertainty but have only provided more
+positive and effective means of countering their
+forces. Still to be faced are the surprises of hidden
+streams, geologic faults, shifts of strata, unstable materials,
+and areas of extreme pressure and temperature.</p>
+
+
+<p class="caption2"><a name="BIBLIOGRAPHY" id="BIBLIOGRAPHY"></a>BIBLIOGRAPHY</p>
+
+<p class="references"><span class="smcap">Agricola, Georgius.</span> <i>De re Metallica.</i> [English
+transl. H. C. and L. H. Hoover (<i>The Mining Magazine</i>,
+London, 1912).] Basel: Froben, 1556.</p>
+
+<p class="references"><span class="smcap">Beach, Alfred Ely.</span> <i>The pneumatic dispatch.</i> New
+York: The American News Company, 1868.</p>
+
+<p class="references"><span class="smcap">Beamish, Richard.</span> <i>A memoir of the life of Sir Marc
+Isambard Brunel.</i> London: Longmans, Green, Longmans
+and Roberts, 1862.</p>
+
+<p class="references"><span class="smcap">Burr, S. D. V.</span> <i>Tunneling under the Hudson River.</i>
+New York: John Wiley and Sons, 1885.</p>
+
+<p class="references"><span class="smcap">Copperthwaite, William Charles.</span> <i>Tunnel shields
+and the use of compressed air in subaqueous works.</i> New
+York: D. Van Nostrand Company, 1906.</p>
+
+<p class="references"><span class="smcap">Drinker, Henry Sturgess.</span> <i>Tunneling, explosive compounds
+and rock drills.</i> New York: John Wiley and
+Sons, 1878.</p>
+
+<p class="references"><span class="smcap">Latrobe, Benjamin H.</span> Report on the Hoosac
+Tunnel (Baltimore, October 1, 1862). Pp. 125-139,
+app. 2, in <i>Report of the commissioners upon the Troy
+and Greenfield Railroad and Hoosac Tunnel</i>. Boston,
+1863.</p>
+
+<p class="references"><span class="smcap">Law, Henry.</span> A memoir of the Thames Tunnel.
+<i>Weale&rsquo;s Quarterly Papers on Engineering</i> (London,
+1845-46), vol. 3, pp. 1-25 and vol. 5, pp. 1-86.</p>
+
+<p class="references">The pneumatic tunnel under Broadway, N.Y.
+<i>Scientific American</i> (March 5, 1870), pp. 154-156.</p>
+
+<p class="references"><i>Report of the commissioners upon the Troy and Greenfield
+Railroad and Hoosac Tunnel to his excellency the governor
+and the honorable the executive council of the state of
+Massachusetts, February 28, 1863.</i> Boston, 1863.</p>
+
+<p class="references"><span class="smcap">Storrow, Charles S.</span> Report on European tunnels
+(Boston, November 28, 1862). Pp. 5-122, app. 1, in
+<i>Report of the commissioners upon the Troy and Greenfield
+Railroad and Hoosac Tunnel....</i> Boston, 1863.</p>
+
+<p class="references pmb4">The St. Clair Tunnel. <i>Engineering News</i> (in series
+running October 4 to December 27, 1890).</p>
+
+
+<p class="caption2"><a name="FOOTNOTES" id="FOOTNOTES"></a>FOOTNOTES:</p>
+
+<div class="footnote"><p><a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> There are two important secondary techniques for opening
+subterranean and subaqueous ways, neither a method truly of
+tunneling. One of these, of ancient origin, used mainly in the
+construction of shallow subways and utility ways, is the &ldquo;cut
+and cover&rdquo; system, whereby an open trench is excavated and
+then roofed over. The result is, in effect, a tunnel. The concept
+of the other method was propounded in the early 19th
+century but only used practically in recent years. This is the
+&ldquo;trench&rdquo; method, a sort of subaqueous equivalent of cut and
+cover. A trench is dredged in the bed of a body of water,
+into which prefabricated sections of large diameter tube are
+lowered, in a continuous line. The joints are then sealed by
+divers, the trench is backfilled over the tube, the ends are
+brought up to dryland portals, the water is pumped out, and a
+subterranean passage results. The Chesapeake Bay Bridge
+Tunnel (1960-1964) is a recent major work of this character.</p></div>
+
+<div class="footnote"><p><a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> In 1952 a successful machine was developed on this plan,
+with hardened rollers on a revolving cutting head for disintegrating
+the rock. The idea is basically sound, possessing advantages
+in certain situations over conventional drilling and
+blasting systems.</p></div>
+
+<div class="footnote"><p><a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> In 1807 the noted Cornish engineer Trevithick commenced
+a small timbered drift beneath the Thames, 5 feet by 3 feet, as
+an exploratory passage for a larger vehicular tunnel. Due to
+the small frontal area, he was able to successfully probe about
+1000 feet, but the river then broke in and halted the work.
+Mine tunnels had also reached beneath the Irish Sea and various
+rivers in the coal regions of Newcastle, but these were so far
+below the surface as to be in perfectly solid ground and can
+hardly be considered subaqueous workings.</p></div>
+
+<div class="footnote"><p><a name="Footnote_4_4" id="Footnote_4_4"></a><a href="#FNanchor_4_4"><span class="label">[4]</span></a> Unlike the Brunel tunnel, this was driven from both ends
+simultaneously, the total overall progress thus being 3 feet per
+shift rather than 18 inches. A top speed of 9 feet per day could
+be advanced by each shield under ideal conditions.</p></div>
+
+<div class="footnote"><p><a name="Footnote_5_5" id="Footnote_5_5"></a><a href="#FNanchor_5_5"><span class="label">[5]</span></a> Ideally, the pressure of air within the work area of a
+pneumatically driven tunnel should just balance the hydrostatic head of
+the water without, which is a function of its total height above the
+opening. If the air pressure is not high enough, water will, of course,
+enter, and if very low, there is danger of complete collapse of the
+unsupported ground areas. If too high, the air pressure will overcome
+that due to the water and the air will force its way out through the
+ground, through increasingly larger openings, until it all rushes out
+suddenly in a &ldquo;blowout.&rdquo; The pressurized atmosphere gone, the water then
+is able to pour in through the same opening, flooding the workings.</p></div>
+
+
+<p class="caption2"><a name="INDEX" id="Index"></a><i>Index</i></p>
+
+<p style="text-indent:0">
+Agricola, Georgius, <a href="#Page_215">215</a>,<a href="#Page_216">216</a><br />
+<br />
+Barlow, Peter W., <a href="#Page_221">221</a>, <a href="#Page_227">227</a><br />
+Beach, Alfred Ely, <a href="#Page_224">224</a>,
+ <a href="#Page_227">227</a>-<a href="#Page_229">229</a>,
+ <a href="#Page_231">231</a>, <a href="#Page_237">237</a><br />
+Brunel, Marc Isambard (the elder), <a href="#Page_204">204</a>,
+ <a href="#Page_205">205</a>, <a href="#Page_217">217</a>,
+ <a href="#Page_218">218</a>, <a href="#Page_221">221</a>,
+ <a href="#Page_224">224</a>, <a href="#Page_229">229</a>,
+ <a href="#Page_231">231</a>, <a href="#Page_236">236</a><br />
+Burleigh, Charles, <a href="#Page_212">212</a>, <a href="#Page_213">213</a><br />
+Burleigh Rock Drill Company, <a href="#Page_212">212</a><br />
+Burr, S. D. V., <a href="#Page_236">236</a><br />
+<br />
+Cochrane, Sir Thomas, <a href="#Page_231">231</a>, <a href="#Page_232">232</a><br />
+Copperthwaite, William Charles, <a href="#Page_224">224</a><br />
+<br />
+Doane, Thomas, <a href="#Page_210">210</a>, <a href="#Page_212">212</a>,
+ <a href="#Page_213">213</a>, <a href="#Page_215">215</a><br />
+Drinker, Henry S., <a href="#Page_224">224</a>, <a href="#Page_237">237</a><br />
+<br />
+Greathead, James Henry, <a href="#Page_204">204</a>, <a href="#Page_218">218</a>,
+ <a href="#Page_221">221</a>, <a href="#Page_224">224</a>,
+ <a href="#Page_229">229</a>, <a href="#Page_231">231</a>,
+ <a href="#Page_235">235</a>-<a href="#Page_237">237</a><br />
+Gwynn, Stuart, <a href="#Page_210">210</a><br />
+<br />
+Haskin, DeWitt C., <a href="#Page_204">204</a>, <a href="#Page_232">232</a>,
+ <a href="#Page_234">234</a>-<a href="#Page_236">236</a><br />
+Haupt, Herman, <a href="#Page_204">204</a>, <a href="#Page_209">209</a>,
+ <a href="#Page_210">210</a><br />
+Hobson, Joseph, <a href="#Page_237">237</a><br />
+<br />
+Latrobe, Benjamin H., <a href="#Page_208">208</a>, <a href="#Page_209">209</a><br />
+Law, Henry, <a href="#Page_218">218</a><br />
+<br />
+Mowbray, George W., <a href="#Page_213">213</a>, <a href="#Page_215">215</a><br />
+<br />
+Nobel, Alfred B., <a href="#Page_213">213</a><br />
+<br />
+Putnam Machine Works, <a href="#Page_212">212</a><br />
+<br />
+Shanley, Walter, <a href="#Page_212">212</a><br />
+Shanley Bros., <a href="#Page_215">215</a><br />
+Sommeiller, Germain, <a href="#Page_210">210</a><br />
+Storrow, Charles S., <a href="#Page_210">210</a><br />
+<br />
+Tweed, William Marcy (Boss), <a href="#Page_229">229</a><br />
+<br />
+Weale, John, <a href="#Page_218">218</a><br />
+</p>
+
+<div class="trans_notes">
+
+
+<p class="caption2">Transcriber&rsquo;s Notes</p>
+
+<p>All obvious typographical errors corrected. Formatting inconsistancies
+and spelling were standardized. Paragraphs split by illustrations were
+rejoined. The <a href="#INDEX">Index</a> was extracted from the full publication Index.</p>
+
+
+<div style="width:60%; background-color:#f0f080;padding:7px; margin-left:auto; margin-right:auto;">
+<a name="Transcription" id="Transcription"></a>
+<p class="center caption3 pmb2">Transcription of the text in <a href="#Fig_41_22">Figure 22</a>. The text
+was transcribed with a slight modification to the figure description portion.</p>
+
+<p class="center"><b>OPEN TO THE PUBLIC EVERY DAY</b> <i>(Sundays excepted)<br />
+from Seven in the Morning, until Eight in the Evening</i>,</p>
+
+<p class="caption2">THE THAMES TUNNEL.</p>
+
+<p  class="references">Fig. 1 shows a transverse section of the Thames, and beneath it a
+longitudinal section of the Tunnel, as it will be when completed;
+with the ascents in the inclinations in which they will be
+finished.</p>
+
+<p  class="references">Fig. 2 shows the two arched entrances of the Tunnel from the shaft.</p>
+
+<p  class="references">Fig. 3 is a representation of the iron shield, and shows a workman
+in each of the compartments.</p>
+
+<p>The Entrance to the Tunnel is near to Rotherhithe Church, and nearly
+opposite to the London-Docks. The nearest landing place from the river
+is Church Stairs. The Greenwich and Deptford coaches which go the lower
+road, start hourly from Charing-cross, and Gracechurch-street, and pass
+close by the works at Rotherhithe.</p>
+
+<p>Books relative to the Tunnel may be had at the works.</p>
+
+<p>The Public may view the Tunnel every day (Sundays excepted) from Seven
+in the morning until Eight in the Evening, upon payment of One Shilling
+each Person.</p>
+
+<p>The extreme northern end of the Tunnel is for the present secured by a
+strong wall; but visitors will find a dry, warm, and gravelled
+promenade, as far as to almost the centre of the river, and brilliantly
+lighted with oil gas.</p>
+
+<p>The entrance is from Rotherhithe Street, and by a safe, commodious, and
+easy stair case.</p>
+
+<p class="center">H. Teape &amp; Son, Printers, Tower-hill, London.</p>
+</div>
+
+</div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of Tunnel Engineering. A Museum Treatment, by 
+Robert M. Vogel
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@@ -0,0 +1,2139 @@
+The Project Gutenberg EBook of Tunnel Engineering. A Museum Treatment, by
+Robert M. Vogel
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Tunnel Engineering. A Museum Treatment
+
+Author: Robert M. Vogel
+
+Release Date: May 24, 2012 [EBook #39785]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK TUNNEL ENGINEERING ***
+
+
+
+
+Produced by Chris Curnow, Tom Cosmas, Joseph Cooper and
+the Online Distributed Proofreading Team at
+http://www.pgdp.net
+
+
+
+
+
+
+
+Transcriber's note:
+
+This is Paper 41 from the _Smithsonian Institution United States
+National Museum Bulletin 240_, comprising Papers 34-44, which will
+also be available as a complete e-book.
+
+The front material, introduction and relevant index entries from the
+_Bulletin_ are included in each single-paper e-book.
+
+Italic emphasis denoted as _Text_.
+
+Whole numbers and fractions: shown as 1-1/2, 3-1/4, etc.
+
+Please see the end of the book for corrections and changes made.
+
+
+
+
+Smithsonian Institution
+
+United States National Museum
+
+Bulletin 240
+
+
+
+
+[Illustration: Smithsonian Press]
+
+
+
+
+Museum of History and Technology
+
+
+Contributions from the Museum of History and Technology
+
+
+_Papers 34-44_
+
+_On Science and Technology_
+
+
+Smithsonian Institution . Washington, D.C. 1966
+
+
+       *       *       *       *       *
+
+
+_Publications of the United States National Museum_
+
+The scholarly and scientific publications of the United States National
+Museum include two series, _Proceedings of the United States National
+Museum_ and _United States National Museum Bulletin_.
+
+In these series, the Museum publishes original articles and monographs
+dealing with the collections and work of its constituent museums--The
+Museum of Natural History and the Museum of History and
+Technology--setting forth newly acquired facts in the fields of
+anthropology, biology, history, geology, and technology. Copies of
+each publication are distributed to libraries, to cultural and
+scientific organizations, and to specialists and others interested in
+the different subjects.
+
+The _Proceedings_, begun in 1878, are intended for the publication,
+in separate form, of shorter papers from the Museum of Natural History.
+These are gathered in volumes, octavo in size, with the publication
+date of each paper recorded in the table of contents of the volume.
+
+In the _Bulletin_ series, the first of which was issued in 1875, appear
+longer, separate publications consisting of monographs (occasionally in
+several parts) and volumes in which are collected works on related
+subjects. _Bulletins_ are either octavo or quarto in size, depending
+on the needs of the presentation. Since 1902 papers relating to the
+botanical collections of the Museum of Natural History have been
+published in the _Bulletin_ series under the heading _Contributions from
+the United States National Herbarium_, and since 1959, in _Bulletins_
+titled "Contributions from the Museum of History and Technology," have
+been gathered shorter papers relating to the collections and research of
+that Museum.
+
+The present collection of Contributions, Papers 34-44, comprises
+Bulletin 240. Each of these papers has been previously published in
+separate form. The year of publication is shown on the last page of
+each paper.
+
+  FRANK A. TAYLOR
+  _Director, United States National Museum_
+
+
+       *       *       *       *       *
+
+
+
+
+CONTRIBUTIONS FROM
+
+THE MUSEUM OF HISTORY AND TECHNOLOGY.
+
+PAPER 41
+
+
+
+
+TUNNEL ENGINEERING--A MUSEUM TREATMENT
+
+
+_Robert M. Vogel_
+
+
+
+
+                                       INTRODUCTION      203
+
+                                     ROCK TUNNELING      206
+
+                              SOFT-GROUND TUNNELING      215
+
+                                       BIBLIOGRAPHY      239
+
+                                          FOOTNOTES
+
+                                              INDEX
+
+
+
+
+[Illustration: Figure 1.--MINING BY EARLY EUROPEAN CIVILIZATIONS,
+using fire setting and hand chiseling to break out ore and rock.
+MHT model--3/4" scale. (Smithsonian photo 49260-H.)]
+
+
+
+
+_Robert M. Vogel_
+
+TUNNEL ENGINEERING--A MUSEUM TREATMENT
+
+
+     _During the years from 1830 to 1900, extensive developments took
+     place in the field of tunneling, which today is an important,
+     firmly established branch of civil engineering. This paper offers
+     a picture of its growth from the historical standpoint, based on
+     a series of models constructed for the Hall of Civil Engineering
+     in the new Museum of History and Technology. The eight models
+     described highlight the fundamental advances which have occurred
+     between primitive man's first systematic use of fire for excavating
+     rock in mining, and the use in combination of compressed air, an
+     iron lining, and a movable shield in a subaqueous tunnel at the end
+     of the 19th century._
+
+     THE AUTHOR: _Robert M. Vogel is curator of heavy machinery and
+     civil engineering, in the Smithsonian Institution's Museum of
+     History and Technology._
+
+
+
+
+Introduction
+
+
+With few exceptions, civil engineering is a field in which the ultimate
+goal is the assemblage of materials into a useful structural form
+according to a scientifically derived plan which is based on various
+natural and man-imposed conditions. This is true whether the result be,
+for example, a dam, a building, a bridge, or even the fixed plant of a
+railroad. However, one principal branch of the field is based upon an
+entirely different concept. In the engineering of tunnels the utility of
+the "structure" is derived not from the bringing together of elements
+but from the separation of one portion of naturally existing material
+from another to permit passage through a former barrier.
+
+In tunneling hard, firm rock, this is practically the entire compass
+of the work: breaking away the rock from the mother mass, and,
+coincidently, removing it from the workings. The opposite extreme in
+conditions is met in the soft-ground tunnel, driven through material
+incapable of supporting itself above the tunnel opening. Here, the
+excavation of the tunneled substance is of relatively small concern,
+eclipsed by the problem of preventing the surrounding material from
+collapsing into the bore.
+
+[Illustration: Figure 2.--HOOSAC TUNNEL. METHOD OF WORKING EARLY
+SECTIONS of the project; blast holes drilled by hand jacking.
+MHT model--1/2" scale. (Smithsonian photo 49260-L.)]
+
+In one other principal respect does tunnel engineering differ widely
+from its collateral branches of civil engineering. Few other physical
+undertakings are approached with anything like the uncertainty
+attending a tunnel work. This is even more true in mountain tunnels,
+for which test borings frequently cannot be made to determine the
+nature of the material and the geologic conditions which will be
+encountered.
+
+The course of tunnel work is not subject to an overall preliminary
+survey; the engineer is faced with not only the inability to
+anticipate general contingencies common to all engineering work, but
+with the peculiar and often overwhelming unpredictability of the very
+basis of his work.
+
+Subaqueous and soft-ground work on the other hand, while still subject
+to many indeterminates, is now far more predictable than during its
+early history, simply because the nature of the adverse condition
+prevailing eventually was understood to be quite predictable. The
+steady pressures of earth and water to refill the excavated area are
+today overcome with relative ease and consistency by the tunneler.
+
+In tunneling as in no other branch of civil engineering did empiricism
+so long resist the advance of scientific theory; in no other did the
+"practical engineer" remain to such an extent the key figure in
+establishing the success or failure of a project. The Hoosac Tunnel,
+after 25 years of legislative, financial, and technical difficulties,
+in 1875 was finally driven to successful completion only by the
+efforts of a group who, while in the majority were trained civil
+engineers, were to an even greater extent men of vast practical
+ability, more at home in field than office.
+
+DeWitt C. Haskin (see p. 234), during the inquest that followed the
+death of a number of men in a blowout of his pneumatically driven
+Hudson River Tunnel in 1880, stated in his own defense: "I am not a
+scientific engineer, but a practical one ... I know nothing of
+mathematics; in my experience I have grasped such matters as a whole;
+I believe that the study of mathematics in that kind of work
+[tunneling] has a tendency to dwarf the mind rather than enlighten
+it...." An extreme attitude perhaps, and one which by no means adds to
+Haskin's stature, but a not unusual one in tunnel work at the time. It
+would not of course be fair to imply that such men as Herman Haupt,
+Brunel the elder, and Greathead were not accomplished theoretical
+engineers. But it was their innate ability to evaluate and control the
+overlying physical conditions of the site and work that made possible
+their significant contributions to the development of tunnel
+engineering.
+
+Tunneling remained largely independent of the realm of mathematical
+analysis long after the time when all but the most insignificant
+engineering works were designed by that means. Thus, as structural
+engineering has advanced as the result of a flow of new theoretical
+concepts, new, improved, and strengthened materials, and new methods
+of fastening, the progress of tunnel engineering has been due more to
+the continual refinement of constructional techniques.
+
+
+A NEW HALL OF CIVIL ENGINEERING
+
+In the Museum of History and Technology has recently been established
+a Hall of Civil Engineering in which the engineering of tunnels is
+comprehensively treated from the historical standpoint--something not
+previously done in an American museum. The guiding precept of the
+exhibit has not been to outline exhaustively the entire history of
+tunneling, but rather to show the fundamental advances which have
+occurred between primitive man's first systematic use of fire for
+excavating rock in mining, and the use in combination of compressed
+air, iron lining, and a movable shield in a subaqueous tunnel at the
+end of the 19th century. This termination date was selected because it
+was during the period from about 1830 to 1900 that the most
+concentrated development took place, and during which tunneling became
+a firmly established and important branch of civil engineering and
+indeed, of modern civilization. The techniques of present-day
+tunneling are so fully related in current writing that it was deemed
+far more useful to devote the exhibit entirely to a segment of the
+field's history which is less commonly treated.
+
+[Illustration: Figure 3.--HOOSAC TUNNEL. WORKING OF LATER STAGES with
+Burleigh pneumatic drills mounted on carriages. The bottom heading is
+being drilled in preparation for blasting out with nitroglycerine.
+MHT model--1/2" scale. (Smithsonian photo 49260-M.)]
+
+The major advances, which have already been spoken of as being ones of
+technique rather than theory, devolve quite naturally into two basic
+classifications: the one of supporting a mass of loose, unstable,
+pressure-exerting material--soft-ground tunneling; and the
+diametrically opposite problem of separating rock from the basic mass
+when it is so firm and solid that it can support its own overbearing
+weight as an opening is forced through it--rock, or hard-ground
+tunneling.
+
+To exhibit the sequence in a thorough manner, inviting and capable of
+easy and correct interpretation by the nonprofessional viewer, models
+offered the only logical means of presentation. Six tunnels were
+selected, all driven in the 19th century. Each represents either a
+fundamental, new concept of tunneling technique, or an important,
+early application of one. Models of these works form the basis of the
+exhibit. No effort was made to restrict the work to projects on
+American soil. This would, in fact, have been quite impossible if an
+accurate picture of tunnel technology was to be drawn; for as in
+virtually all other areas of technology, the overall development
+in this field has been international. The art of mining was first
+developed highly in the Middle Ages in the Germanic states; the tunnel
+shield was invented by a Frenchman residing in England, and the use of
+compressed air to exclude the water from subaqueous tunnels was first
+introduced on a major work by an American. In addition, the two main
+subdivisions, rock and soft-ground tunneling, are each introduced by
+a model not of an actual working, but of one typifying early classical
+methods which were in use for centuries until the comparatively recent
+development of more efficient systems of earth support and rock
+breaking. Particular attention is given to accuracy of detail
+throughout the series of eight models; original sources of descriptive
+and graphic information were used in their construction wherever
+possible. In all cases except the introductory model in the
+rock-tunneling series, representing copper mining by early
+civilizations, these sources were contemporary accounts.
+
+The plan to use a uniform scale of reduction throughout, in order
+to facilitate the viewers' interpretation, unfortunately proved
+impractical, due to the great difference in the amount of area to be
+encompassed in different models, and the necessity that the cases
+holding them be of uniform height. The related models of the Broadway
+and Tower Subways represent short sections of tunnels only 8 feet or
+so in diameter enabling a relatively large scale, 1-1/2 inches to the
+foot, to be used. Conversely, in order that the model of Brunel's
+Thames Tunnel be most effective, it was necessary to include one of
+the vertical terminal shafts used in its construction. These were
+about 60 feet in depth, and thus the much smaller scale of 1/4 inch
+to the foot was used. This variation is not as confusing as might be
+thought, for the human figures in each model provide an immediate and
+positive sense of proportion and scale.
+
+Careful thought was devoted to the internal lighting of the models, as
+this was one of the critical factors in establishing, so far as is
+possible in a model, an atmosphere convincingly representative of work
+conducted solely by artificial light. Remarkable realism was achieved
+by use of plastic rods to conduct light to the tiny sources of tunnel
+illumination, such as the candles on the miners' hats in the Hoosac
+Tunnel, and the gas lights in the Thames Tunnel. No overscaled
+miniature bulbs, generally applied in such cases, were used. At
+several points where the general lighting within the tunnel proper has
+been kept at a low level to simulate the natural atmosphere of the
+work, hidden lamps can be operated by push-button in order to bring
+out detail which otherwise would be unseen.
+
+The remainder of the material in the Museum's tunneling section
+further extends the two major aspects of tunneling. Space limitations
+did not permit treatment of the many interesting ancillary matters
+vital to tunnel engineering, such as the unique problems of
+subterranean surveying, and the extreme accuracy required in the
+triangulation and subsequent guidance of the boring in long mountain
+tunnels; nor the difficult problems of ventilating long workings, both
+during driving and in service; nor the several major methods developed
+through the years for driving or constructing tunnels in other than
+the conventional manner.[1]
+
+
+
+
+Rock Tunneling
+
+
+While the art of tunneling soft ground is of relatively recent origin,
+that of rock tunneling is deeply rooted in antiquity. However,
+the line of its development is not absolutely direct, but is
+more logically followed through a closely related branch of
+technology--mining. The development of mining techniques is a
+practically unbroken one, whereas there appears little continuity or
+relationship between the few works undertaken before about the 18th
+century for passage through the earth.
+
+The Egyptians were the first people in recorded history to have driven
+openings, often of considerable magnitude, through solid rock. As is
+true of all major works of that nation, the capability of such grand
+proportion was due solely to the inexhaustible supply of human power
+and the casual evaluation of life. The tombs and temples won from the
+rock masses of the Nile Valley are monuments of perseverance rather
+than technical skill. Neither the Egyptians nor any other peoples
+before the Middle Ages have left any consistent evidence that they
+were able to pierce ground that would not support itself above the
+opening as would firm rock. In Egypt were established the methods of
+rock breaking that were to remain classical until the first use of
+gun-powder blasting in the 17th century which formed the basis of the
+ensuing technology of mining.
+
+Notwithstanding the religious motives which inspired the earliest rock
+excavations, more constant and universal throughout history has been
+the incentive to obtain the useful and decorative minerals hidden
+beneath the earth's surface. It was the miner who developed the
+methods introduced by the early civilizations to break rock away from
+the primary mass, and who added the refinements of subterranean
+surveying and ventilating, all of which were later to be assimilated
+into the new art of driving tunnels of large diameter. The connection
+is the more evident from the fact that tunnelmen are still known as
+miners.
+
+
+COPPER MINING, B.C.
+
+Therefore, the first model of the sequence, reflecting elemental
+rock-breaking techniques, depicts a hard-rock copper mine (fig. 1).
+Due to the absence of specific information about such works during the
+pre-Christian eras, this model is based on no particular period or
+locale, but represents in a general way, a mine in the Rio Tinto area
+of Spain where copper has been extracted since at least 1000 B.C.
+Similar workings existed in the Tirol as early as about 1600 B.C. Two
+means of breaking away the rock are shown: to the left is the most
+primitive of all methods, the hammer and chisel, which require no
+further description. At the right side, the two figures are shown
+utilizing the first rock-breaking method in which a force beyond that
+of human muscles was employed, the age-old "fire-setting" method. The
+rock was thoroughly heated by a fierce fire built against its face and
+then suddenly cooled by dashing water against it. The thermal shock
+disintegrated the rock or ore into bits easily removable by hand.
+
+[Illustration: Figure 4.--HOOSAC TUNNEL. Bottom of the central shaft
+showing elevator car and rock skip; pumps at far right. In the center,
+the top bench is being drilled by a single column-mounted Burleigh
+drill. MHT model--1/2" scale. (Smithsonian photo 49260-N.)]
+
+The practice of this method below ground, of course, produced a
+fearfully vitiated atmosphere. It is difficult to imagine whether the
+smoke, the steam, or the toxic fumes from the roasting ore was
+the more distressing to the miners. Even when performed by labor
+considered more or less expendable, the method could be employed only
+where there was ventilation of some sort: natural chimneys and
+convection currents were the chief sources of air circulation. Despite
+the drawbacks of the fire system, its simplicity and efficacy weighed
+so heavily in its favor that its history of use is unbroken almost to
+the present day. Fire setting was of greatest importance during the
+years of intensive mining in Europe before the advent of explosive
+blasting, but its use in many remote areas hardly slackened until the
+early 20th century because of its low cost when compared to powder.
+For this same reason, it did have limited application in actual tunnel
+work until about 1900.
+
+Direct handwork with pick, chisel and hammer, and fire setting were
+the principal means of rock removal for centuries. Although various
+wedging systems were also in favor in some situations, their
+importance was so slight that they were not shown in the model.
+
+
+HOOSAC TUNNEL
+
+It was possible in the model series, without neglecting any major
+advancement in the art of rock tunneling, to complete the sequence of
+development with only a single additional model. Many of the greatest
+works of civil engineering have been those concerned directly with
+transport, and hence are the product of the present era, beginning in
+the early 19th century. The development of the ancient arts of route
+location, bridge construction, and tunnel driving received a powerful
+stimulation after 1800 under the impetus of the modern canal, highway,
+and, especially, the railroad.
+
+The Hoosac Tunnel, driven through Hoosac Mountain in the very
+northwest corner of Massachusetts between 1851 and 1875, was the first
+major tunneling work in the United States. Its importance is due not
+so much to this as to its being literally the fountainhead of modern
+rock-tunneling technology. The remarkable thing is that the work was
+begun using methods of driving almost unchanged during centuries
+previous, and was completed twenty years later by techniques which
+were, for the day, almost totally mechanized. The basic pattern of
+operation set at Hoosac, using pneumatic rock drills and efficient
+explosives, remains practically unchanged today.
+
+The general history of the Hoosac project is so thoroughly recorded
+that the briefest outline of its political aspects will suffice here.
+Hoosac Mountain was the chief obstacle in the path of a railroad
+projected between Greenfield, Massachusetts, and Troy, New York.
+The line was launched by a group of Boston merchants to provide a
+direct route to the rapidly developing West, in competition with the
+coastal routes via New York. The only route economically reasonable
+included a tunnel of nearly five miles through the mountain--a
+length absolutely without precedent, and an immense undertaking in
+view of the relatively primitive rock-working methods then available.
+
+[Illustration: Figure 5.--BURLEIGH ROCK DRILL, improved model of about
+1870, mounted on frame for surface work. (Catalog and price list: The
+Burleigh Rock Drill Company, 1876.)]
+
+The bore's great length and the desire for rapid exploitation inspired
+innovation from the outset of the work. The earliest attempts at
+mechanization, although ineffectual and without influence on tunnel
+engineering until many years later, are of interest. These took the
+form of several experimental machines of the "full area" type,
+intended to excavate the entire face of the work in a single operation
+by cutting one or more concentric grooves in the rock. The rock
+remaining between the grooves was to be blasted out. The first such
+machine tested succeeded in boring a 24-foot diameter opening for 10
+feet before its total failure. Several later machines proved of equal
+merit.[2] It was the Baltimore and Ohio's eminent chief engineer,
+Benjamin H. Latrobe, who in his _Report on the Hoosac Tunnel_
+(Baltimore, Oct. 1, 1862, p. 125) stated that such apparatus contained
+in its own structure the elements of failure, "... as they require
+the machines to do too much and the powder too little of the work,
+thus contradicting the fundamental principles upon which all
+labor-saving machinery is framed ... I could only look upon it as a
+misapplication of mechanical genius."
+
+[Illustration: Figure 6.--HOOSAC TUNNEL. Flash-powder photograph of
+Burleigh drills at the working face. (_Photo courtesy of State
+Library, Commonwealth of Massachusetts._)]
+
+Latrobe stated the basic philosophy of rock-tunnel work. No mechanical
+agent has ever been able to improve upon the efficiency of explosives
+for the shattering of rock. For this reason, the logical application
+of machinery to tunneling was not in replacing or altering the
+fundamental process itself, but in enabling it to be conducted with
+greater speed by mechanically drilling the blasting holes to receive
+the explosive.
+
+Actual work on the Hoosac Tunnel began at both ends of the tunnel in
+about 1854, but without much useful effect until 1858 when a contract
+was let to the renowned civil engineer and railroad builder, Herman
+Haupt of Philadelphia. Haupt immediately resumed investigations of
+improved tunneling methods, both full-area machines and mechanical
+rock drills. At this time mechanical rock-drill technology was in a
+state beyond, but not far beyond, initial experimentation. There
+existed one workable American machine, the Fowle drill, invented in
+1851. It was steam-driven, and had been used in quarry work, although
+apparently not to any commercial extent. However, it was far too
+large and cumbersome to find any possible application in tunneling.
+Nevertheless, it contained in its operating principle, the seed of a
+practical rock drill in that the drill rod was attached directly to
+and reciprocated by a double-acting steam piston. A point of great
+importance was the independence of its operation on gravity,
+permitting drilling in any direction.
+
+While experimenting, Haupt drove the work onward by the classical
+methods, shown in the left-hand section of the model (fig. 2). At
+the far right an advance heading or adit is being formed by pick and
+hammer work; this is then deepened into a top heading with enough
+height to permit hammer drilling, actually the basic tunneling
+operation. A team is shown "double jacking," i.e., using two-handed
+hammers, the steel held by a third man. This was the most efficient of
+the several hand-drilling methods. The top-heading plan was followed
+so that the bulk of the rock could be removed in the form of a bottom
+bench, and the majority of drilling would be downward, obviously the
+most effective direction. Blasting was with black powder and its
+commercial variants. Some liberty was taken in depicting these steps
+so that both operations might be shown within the scope of the model:
+in practice the heading was kept between 400 and 600 feet in advance
+of the bench so that heading blasts would not interfere with the bench
+work. The bench carriage simply facilitated handling of the blasted
+rock. It was rolled back during blasts.
+
+[Illustration: Figure 7.--HOOSAC TUNNEL. GROUP OF MINERS descending
+the west shaft with a Burleigh drill. (_Photo courtesy of State
+Library, Commonwealth of Massachusetts._)]
+
+The experiments conducted by Haupt with machine drills produced no
+immediate useful results. A drill designed by Haupt and his associate,
+Stuart Gwynn, in 1858 bored hard granite at the rate of 5/8 inch per
+minute, but was not substantial enough to bear up in service. Haupt
+left the work in 1861, victim of intense political pressures and
+totally unjust accusations of corruption and mismanagement. The
+work was suspended until taken over by a state commission in 1862.
+Despite frightful ineptitude and very real corruption, this period
+was exceedingly important in the long history both of Hoosac Tunnel
+and of rock tunneling in general.
+
+The merely routine criticism of the project had by this time become
+violent due to the inordinate length of time already elapsed and the
+immense cost, compared to the small portion of work completed. This
+served to generate in the commission a strong sense of urgency to
+hurry the project along. Charles S. Storrow, a competent engineer, was
+sent to Europe to report on the progress of tunneling there, and in
+particular on mechanization at the Mont Cenis Tunnel then under
+construction between France and Italy. Germain Sommeiller, its chief
+engineer, had, after experimentation similar to Haupt's, invented a
+reasonably efficient drilling machine which had gone into service at
+Mont Cenis in March 1861. It was a distinct improvement over hand
+drilling, almost doubling the drilling rate, but was complex and
+highly unreliable. Two hundred drills were required to keep 16 drills
+at work. But the vital point in this was the fact that Sommeiller
+drove his drills not with steam, but air, compressed at the tunnel
+portals and piped to the work face. It was this single factor, one of
+application rather than invention, that made the mechanical drill
+feasible for tunneling.
+
+All previous effort in the field of machine drilling, on both sides
+of the Atlantic, had been directed toward steam as the motive power.
+In deep tunnels, with ventilation already an inherent problem, the
+exhaust of a steam drill into the atmosphere was inadmissible.
+Further, steam could not be piped over great distances due to serious
+losses of energy from radiation of heat, and condensation. Steam
+generation within the tunnel itself was obviously out of the question.
+It was the combination of a practical drill, and the parallel
+invention by Sommeiller of a practical air compressor that resulted in
+the first workable application of machine rock drilling to tunneling.
+
+[Illustration: Figures 8 & 9.--HOOSAC TUNNEL. CONTEMPORARY
+ENGRAVINGS. As such large general areas could not be sufficiently
+illuminated for photography, the Museum model was based primarily on
+artists' versions of the work. (_Science Record_, 1872; _Leslie's
+Weekly_, 1873.)]
+
+The Sommeiller drills greatly impressed Storrow, and his report of
+November 1862 strongly favored their adoption at Hoosac. It is curious
+however, that not a single one was brought to the U.S., even on trial.
+Storrow does speak of Sommeiller's intent to keep the details of the
+machine to himself until it had been further improved, with a view to
+its eventual exploitation. The fact is, that although workable, the
+Sommeiller drill proved to be a dead end in rock-drill development
+because of its many basic deficiencies. It did exert the indirect
+influence of inspiration which, coupled with a pressing need for
+haste, led to renewed trials of drilling machinery at Hoosac. Thomas
+Doane, chief engineer under the state commission, carried this program
+forth with intensity, seeking and encouraging inventors, and himself
+working on the problem. The pattern of the Sommeiller drill was
+generally followed; that is, the drill was designed as a separate,
+relatively light mechanical element, adapted for transportation by
+several miners, and attachable to a movable frame or carriage during
+operation. Air was of course the presumed power. To be effective, it
+was necessary that a drill automatically feed the drill rod as the
+hole deepened, and also rotate the rod automatically to maintain a
+round, smooth hole. Extreme durability was essential, and usually
+proved the source of a machine's failure. The combination of these
+characteristics into a machine capable of driving the drill rod into
+the rock with great force, perhaps five times per second, was a
+severe test of ingenuity and materials. Doane in 1864 had three
+different experimental drills in hand, as well as various steam
+and water-powered compressors.
+
+Success finally came in 1865 with the invention of a drill by Charles
+Burleigh, a mechanical engineer at the well-known Putnam Machine Works
+of Fitchburg, Massachusetts. The drills were first applied in the east
+heading in June of 1866. Although working well, their initial success
+was limited by lack of reliability and a resulting high expense for
+repairs. They were described as having "several weakest points." In
+November, these drills were replaced by an improved Burleigh drill
+which was used with total success to the end of the work. The era of
+modern rock tunneling was thus launched by Sommeiller's insight in
+initially applying pneumatic power to a machine drill, by Doane's
+persistence in searching for a thoroughly practical drill, and by
+Burleigh's mechanical talent in producing one. The desperate need to
+complete the Hoosac Tunnel may reasonably be considered the greatest
+single spur to the development of a successful drill.
+
+The significance of this invention was far reaching. Burleigh's was
+the first practical mechanical rock drill in America and, in view of
+its dependability, efficiency, and simplicity when compared to the
+Sommeiller drill, perhaps in the world. The Burleigh drill achieved
+success almost immediately. It was placed in production by Putnam for
+the Burleigh Rock Drill Company before completion of Hoosac in 1876,
+and its use spread throughout the western mining regions and other
+tunnel works. For a major invention, its adoption was, in relative
+terms, instantaneous. It was the prototype of all succeeding
+piston-type drills, which came to be known generically as "burleighs,"
+regardless of manufacture. Walter Shanley, the Canadian contractor who
+ultimately completed the Hoosac, reported in 1870, after the drills
+had been in service for a sufficient time that the techniques for
+their most efficient use were fully understood and effectively
+applied, that the Burleigh drills saved about half the drilling costs
+over hand drilling. The per-inch cost of machine drilling averaged 5.5
+cents, all inclusive, vs. 11.2 cents for handwork. The more important
+point, that of speed, is shown by the reports of average monthly
+progress of the tunnel itself, before and after use of the air drills.
+
+  _Year_     _Average monthly
+              progress in feet_
+
+  1865              55
+  1866              48
+  1867              99
+  1868              --
+  1869             138
+  1870             126
+  1871             145
+  1872             124
+
+[Illustration: Figure 10.--TRINITROGLYCERINE BLAST at Hoosac Tunnel.
+(_Leslie's Weekly_, 1873.)]
+
+The right portion of the model (fig. 3) represents the workings during
+the final period. The bottom heading system was generally used after
+the Burleigh drills had been introduced. Four to six drills were
+mounted on a carriage designed by Doane. These drove the holes for
+the first blast in the center of the heading in about six hours. The
+full width of the heading, the 24-foot width of the tunnel, was then
+drilled and blasted out in two more stages. As in the early section,
+the benches to the rear were later removed to the full-tunnel height
+of about 20 feet. This operation is shown by a single drill (fig. 4)
+mounted on a screw column. Three 8-hour shifts carried the work
+forward: drilling occupied half the time and half was spent in running
+the carriage back, blasting, and mucking (clearing the broken rock).
+
+[Illustration: Figure 11.--HOOSAC TUNNEL survey crew at engineering
+office. The highest accuracy of the aboveground and underground survey
+work was required to insure proper vertical and horizontal alignment
+and meeting of the several separately driven sections. (_Photo
+courtesy of State Library, Commonwealth of Massachusetts._)]
+
+The tunnel's 1028-foot central shaft, completed under the Shanley
+contract in 1870 to provide two additional work faces as well as a
+ventilation shaft is shown at the far right side of this half of the
+model. Completed so near the end of the project, only 15 percent of
+the tunnel was driven from the shaft.
+
+The enormous increase in rate of progress was not due entirely to
+machine drilling. From the outset of his jurisdiction, Doane undertook
+experiments with explosives as well as drills, seeking an agent more
+effective than black powder. In this case, the need for speed was not
+the sole stimulus. As the east and west headings advanced further and
+further from the portals, the problem of ventilation grew more acute,
+and it became increasingly difficult to exhaust the toxic fumes
+produced by the black powder blasts.
+
+In 1866, Doane imported from Europe a sample of trinitroglycerine,
+the liquid explosive newly introduced by Nobel, known in Europe as
+"glonoin oil" and in the United States as "nitroglycerine." It already
+had acquired a fearsome reputation from its tendency to decompose with
+heat and age and to explode with or without the slightest provocation.
+Nevertheless, its tremendous power and characteristic of almost
+complete smokelessness led Doane to employ the chemist George W.
+Mowbray, who had blasted for Drake in the Pennsylvania oil fields, to
+develop techniques for the bulk manufacture of the new agent and for
+its safe employment in the tunnel.
+
+Mowbray established a works on the mountain and shortly developed
+a completely new blasting practice based on the explosive. Its
+stability was greatly increased by maintaining absolute purity in the
+manufacturing process. Freezing the liquid to reduce its sensitivity
+during transport to the headings, and extreme caution in its handling
+further reduced the hazard of its use. At the heading, the liquid was
+poured into cylindrical cartridges for placement in the holes. As with
+the Burleigh drill, the general adoption of nitroglycerine was
+immediate once its qualities had been demonstrated. The effect on the
+work was notable. Its explosive characteristics permitted fewer blast
+holes over a given frontal area of working face, and at the same time
+it was capable of effectively blowing from a deeper drill hole, 42
+inches against 30 inches for black powder, so that under ideal
+conditions 40 percent more tunnel length was advanced per cycle of
+operations. A new fuse and a system of electric ignition were
+developed which permitted simultaneous detonation and resulted
+in a degree of effectiveness impossible with the powder train and
+cord fusing used with the black powder. Over a million pounds of
+nitroglycerine were produced by Mowbray between 1866 and completion
+of the tunnel.
+
+[Illustration: Figure 12.--WORKS AT THE CENTRAL SHAFT, HOOSAC TUNNEL,
+for hoisting, pumping and air compressing machinery, and general
+repair, 1871. (_Photo courtesy of State Library, Commonwealth of
+Massachusetts._)]
+
+[Illustration: Figure 13.--HOOSAC TUNNEL. AIR-COMPRESSOR BUILDING on
+Hoosac River near North Adams. The compressors were driven partially
+by waterpower, derived from the river. (_Photo courtesy of State
+Library, Commonwealth of Massachusetts._)]
+
+[Illustration: Figure 14.--WEST PORTAL OF HOOSAC TUNNEL before
+completion, 1868, showing six rings of lining brick. (_Photo
+courtesy of State Library, Commonwealth of Massachusetts._)]
+
+When the Shanleys took the work over in 1868, following political
+difficulties attending operation by the State, the period of
+experimentation was over. The tunnel was being advanced by totally
+modern methods, and to the present day the overall concepts have
+remained fundamentally unaltered: the Burleigh piston drill has been
+replaced by the lighter hammer drill; the Doane drill carriage by the
+more flexible "jumbo"; nitroglycerine by its more stable descendant
+dynamite and its alternatives; and static-electric blasting machines
+by more dependable magnetoelectric. But these are all in the nature of
+improvements, not innovations.
+
+Unlike the preceding model, there was good documentation for this one.
+Also, the Hoosac was apparently the first American tunnel to be well
+recorded photographically. Early flashlight views exist of the drills
+working at the heading (fig. 6) as well as of the portals, the winding
+and pumping works at the central shaft, and much of the machinery and
+associated aspects of the project. These and copies of drawings of
+much of Doane's experimental apparatus, a rare technological record,
+are preserved at the Massachusetts State Library.
+
+
+
+
+Soft-Ground Tunneling
+
+
+So great is the difference between hard-rock and soft-ground tunneling
+that they constitute two almost separate branches of the field. In
+penetrating ground lacking the firmness or cohesion to support itself
+above an opening, the miner's chief concern is not that of removing
+the material, but of preventing its collapse into his excavation. The
+primitive methods depending upon brute strength and direct application
+of fire and human force were suitable for assault on rock, but lacked
+the artifice needed for delving into less stable material. Roman
+engineers were accomplished in spanning subterranean ways with masonry
+arches, but apparently most of their work was done by cut-and-cover
+methods rather than by actual mining.
+
+Not until the Middle Ages did the skill of effectively working
+openings in soft ground develop, and not until the Renaissance
+was this development so consistently successful that it could
+be considered a science.
+
+
+RENAISSANCE MINING
+
+From the earliest periods of rock working, the quest for minerals
+and metals was the primary force that drove men underground. It was
+the technology of mining, the product of slow evolution over the
+centuries, that became the technology of the early tunnel, with no
+significant modification except in size of workings.
+
+Every aspect of 16th century mining is definitively detailed in
+Georgius Agricola's remarkable _De re Metallica_, first published in
+Basel in 1556. During its time of active influence, which extended
+for two centuries, it served as the authoritative work on the subject.
+It remains today an unparalleled early record of an entire branch
+of technology. The superb woodcuts of mine workings and tools in
+themselves constitute a precise description of the techniques of the
+period, and provided an ideal source of information upon which to base
+the first model in the soft-ground series.
+
+[Illustration: Figure 15.--CENTERING FOR PLACEMENT OF FINISHED
+STONEWORK at west portal, 1874. At top-right are the sheds where
+the lining brick was produced. (_Photo courtesy of State Library,
+Commonwealth of Massachusetts._)]
+
+The model, representing a typical European mine, demonstrates the
+early use of timber frames or "sets" to support the soft material of
+the walls and roof. In areas of only moderate instability, the sets
+alone were sufficient to counteract the earth pressure, and were
+spaced according to the degree of support required. In more extreme
+conditions, a solid lagging of small poles or boards was set outside
+the frames, as shown in the model, to provide absolute support of the
+ground. Details of the framing, the windlass, and all tools and
+appliances were supplied by Agricola, with no need for interpretation
+or interpolation.
+
+The basic framing pattern of sill, side posts and cap piece, all
+morticed together, with lagging used where needed, was translated
+unaltered into tunneling practice, particularly in small exploratory
+drifts. It remained in this application until well into the 20th
+century.
+
+The pressure exerted upon tunnels of large area was countered during
+construction by timbering systems of greater elaboration, evolved
+from the basic one. By the time that tunnels of section large
+enough to accommodate canals and railways were being undertaken as
+matter-of-course civil engineering works, a series of nationally
+distinguishable systems had emerged, each possessing characteristic
+points of favor and fault. As might be suspected, the English system
+of tunnel timbering, for instance, was rarely applied on the
+Continent, nor were the German, Austrian or Belgian systems normally
+seen in Great Britain. All were used at one time or another in this
+country, until the American system was introduced in about 1855.
+While the timbering commonly remained in place in mines, it would be
+followed up by permanent masonry arching and lining in tunnel work.
+
+Overhead in the museum Hall of Civil Engineering are frames
+representing the English, Austrian and American systems. Nearby, a
+series of small relief models (fig. 19) is used to show the sequence
+of enlargement in a soft-ground railroad tunnel of about 1855, using
+the Austrian system. Temporary timber support of tunnels fell from use
+gradually after the advent of shield tunneling in conjunction with
+cast-iron lining. This formed a perfect support immediately behind the
+shield, as well as the permanent lining of the tunnel.
+
+
+BRUNEL'S THAMES TUNNEL
+
+The interior surfaces of tunnels through ground merely unstable are
+amenable to support by various systems of timbering and arching. This
+becomes less true as the fluidity of the ground increases. The soft
+material which normally comprises the beds of rivers can approach an
+almost liquid condition resulting in a hydraulic head from the
+overbearing water sufficient to prevent the driving of even the most
+carefully worked drift, supported by simple timbering. The basic
+defect of the timbering systems used in mining and tunneling was
+that there was inevitably a certain amount of the face or ceiling
+unsupported just previous to setting a frame, or placing over it the
+necessary section of lagging. In mine work, runny soil could, and did,
+break through such gaps, filling the working. For this reason, there
+were no serious attempts made before 1825 to drive subaqueous tunnels.
+
+In that year, work was started on a tunnel under the Thames between
+the Rotherhithe and Wapping sections of London, under guidance of the
+already famous engineer Marc Isambard Brunel (1769-1849), father of
+I. K. Brunel. The undertaking is of great interest in that Brunel
+employed an entirely novel apparatus of his own invention to provide
+continuous and reliable support of the soft water-bearing clay which
+formed the riverbed. By means of this "shield," Brunel was able to
+drive the world's first subaqueous tunnel.[3]
+
+[Illustration: Figure 16.--WEST PORTAL UPON COMPLETION, 1876.
+(_Photo courtesy of New-York Historical Society._)]
+
+The shield was of cast-iron, rectangular in elevation, and was
+propelled forward by jackscrews. Shelves at top, bottom, and sides
+supported the tunnel roof, floor, and walls until the permanent brick
+lining was placed. The working face, the critical area, was supported
+by a large number of small "breasting boards," held against the ground
+by small individual screws bearing against the shield framework. The
+shield itself was formed of 12 separate frames, each of which could be
+advanced independently of the others. The height was 22 feet 3 inches:
+the width 37 feet 6 inches.
+
+The progress was piecemeal. In operation the miners would remove one
+breasting board at a time, excavate in front of it, and then replace
+it in the advanced position--about 6 inches forward. This was repeated
+with the next board above or below, and the sequence continued until
+the ground for the entire height of one of the 12 sections had been
+removed. The board screws for that section were shifted to bear on the
+adjacent frames, relieving the frame of longitudinal pressure. It
+could then be screwed forward by the amount of advance, the screws
+bearing to the rear on the completed masonry. Thus, step by step the
+tunnel progressed slowly, the greatest weekly advance being 14 feet.
+
+In the left-hand portion of the model is the shaft sunk to begin
+operations; here also is shown the bucket hoist for removing the
+spoil. The V-type steam engine powering the hoist was designed by
+Brunel. At the right of the main model is an enlarged detail of the
+shield, actually an improved version built in 1835.
+
+The work continued despite setbacks of every sort. The financial ones
+need no recounting here. Technically, although the shield principle
+proved workable, the support afforded was not infallible. Four or five
+times the river broke through the thin cover of silt and flooded
+the workings, despite the utmost caution in excavating. When this
+occurred, masses of clay, sandbags, and mats were dumped over the
+opening in the riverbed to seal it, and the tunnel pumped out. I. K.
+Brunel acted as superintendent and nearly lost his life on a number of
+occasions. After several suspensions of work resulting from withdrawal
+or exhaustion of support, one lasting seven years, the work was
+completed in 1843.
+
+Despite the fact that Brunel had, for the first time, demonstrated a
+practical method for tunneling in firm and water-bearing ground, the
+enormous cost of the work and the almost overwhelming problems
+encountered had a discouraging effect rather than otherwise. Not for
+another quarter of a century was a similar project undertaken.
+
+The Thames Tunnel was used for foot and light highway traffic until
+about 1870 when it was incorporated into the London Underground
+railway system, which it continues to serve today. The roofed-over
+top sections of the two shafts may still be seen from the river.
+
+A number of contemporary popular accounts of the tunnel exist, but one
+of the most thorough and interesting expositions on a single tunnel
+work of any period is Henry Law's _A Memoir of the Thames Tunnel_,
+published in 1845-1846 by John Weale. Law, an eminent civil engineer,
+covers the work in incredible detail from its inception until the
+major suspension in late 1828 when slightly more than half completed.
+The most valuable aspect of his record is a series of plates of
+engineering drawings of the shield and its components, which, so
+far as is known, exist nowhere else. These formed the basis of the
+enlarged section of the shield, shown to the right of the model of the
+tunnel itself. A vertical section through the shield is reproduced
+here from Law for comparison with the model (figs. 21 and 23).
+
+[Illustration: Figure 17.--SOFT-GROUND TUNNELING. The support of
+walls and roof of mine shaft by simple timbering; 16th century.
+MHT model--3/4" scale. (Smithsonian photo 49260-J.)]
+
+[Illustration: Figure 18.--SOFT-GROUND TUNNELING. The model of a 16th
+century mine in the Museum of History and Technology was constructed
+from illustrations in such works as G. E. von Loehneyss' _Bericht vom
+Bergwerck_, 1690, as well as the better known ones from _De re
+Metallica_.]
+
+[Illustration: Figure 19.--THE SUCCESSIVE STAGES in the enlargement
+of a mid-19th century railroad tunnel, using the Austrian system of
+timbering. MHT model.]
+
+[Illustration: Figure 20.--M. I. BRUNEL'S THAMES TUNNEL, 1825-1843,
+the first driven beneath a body of water. MHT model--1/4" scale.
+(Smithsonian photo 49260-F.)]
+
+
+THE TOWER SUBWAY
+
+Various inventors attempted to improve upon the Brunel shield, aware
+of the fundamental soundness of the shield principle. Almost all
+bypassed the rectangular sectional construction used in the Thames
+Tunnel, and took as a starting point a sectional shield of circular
+cross section, advanced by Brunel in his original patent of 1818.
+James Henry Greathead (1844-1896), rightfully called the father of
+modern subaqueous tunneling, surmised in later years that Brunel had
+chosen a rectangular configuration for actual use, as one better
+adapted to the sectional type of shield. The English civil engineer,
+Peter W. Barlow, in 1864 and 1868 patented a circular shield, of one
+piece, which was the basis of one used by him in constructing a small
+subway of 1350 feet beneath the Thames in 1869, the first work to
+follow the lead of Brunel. Greathead, acting as Barlow's contractor,
+was the designer of the shield actually used in the work, but it was
+obviously inspired by Barlow's patents.
+
+The reduction of the multiplicity of parts in the Brunel shield to
+a single rigid unit was of immense advantage and an advance perhaps
+equal to the shield concept of tunneling itself. The Barlow-Greathead
+shield was like the cap of a telescope with a sharpened circular ring
+on the front to assist in penetrating the ground. The diaphragm
+functioned, as did Brunel's breasting boards, to resist the
+longitudinal earth pressure of the face, and the cylindrical portion
+behind the diaphragm bore the radial pressure of roof and walls. Here
+also for the first time, a permanent lining formed of cast-iron
+segments was used, a second major advancement in soft-ground tunneling
+practice. Not only could the segments be placed and bolted together
+far more rapidly than masonry lining could be laid up, but unlike the
+green masonry, they could immediately bear the full force of the
+shield-propelling screws.
+
+Barlow, capitalizing on Brunel's error in burrowing so close to the
+riverbed, maintained an average cover of 30 feet over the tunnel,
+driving through a solid stratum of firm London clay which was
+virtually impervious to water. As the result of this, combined with
+the advantages of the solid shield and the rapidly placed iron lining,
+the work moved forward at a pace and with a facility in startling
+contrast to that of the Thames Tunnel, although in fairness it must be
+recalled that the face area was far less.
+
+The clay was found sufficiently sound that it could be readily
+excavated without the support of the diaphragm, and normally three
+miners worked in front of the shield, digging out the clay and passing
+it back through a doorway in the plate. This could be closed in case
+of a sudden settlement or break in. Following excavation, the shield
+was advanced 18 inches into the excavated area by means of 6 screws,
+and a ring of lining segments 18 inches in length bolted to the
+previous ring under cover of the overlapping rear skirt of the shield.
+The small annular space left between the outside of the lining and the
+clay by the thickness and clearance of the skirt--about an inch--was
+filled with thin cement grout. The tunnel was advanced 18 inches
+during each 8-hour shift. The work continued around the clock, and the
+900-foot river section was completed in only 14 weeks.[4] The entire
+work was completed almost without incident in just under a year, a
+remarkable performance for the world's second subaqueous tunnel.
+
+[Illustration: Figure 21.--ENLARGED DETAIL of Brunel's tunneling
+shield, vertical section. The first two and part of the third of the
+twelve frames are shown. To the left is the tunnel's completed brick
+lining and to the right, the individual breasting boards and screws
+for supporting the face. The propelling screws are seen at top and
+bottom, bearing against the lining. Three miners worked in each frame,
+one above the other. MHT model--3/4" scale. (Smithsonian photo
+49260-G.)]
+
+[Illustration: Figure 22.--BROADSIDE PUBLISHED AFTER COMMENCEMENT
+OF WORK on the Thames Tunnel, 1827. (MHT collections.)
+
+  OPEN TO THE PUBLIC EVERY DAY (_Sundays excepted_) _from Seven in the
+  Morning, until Eight in the Evening_,
+
+  THE THAMES TUNNEL.
+
+  Fig. 1 shows a transverse section of the Thames, and beneath it a
+  longitudinal section of the Tunnel, as it will be when completed;
+  with the ascents in the inclinations in which they will be finished.
+
+  Fig. 2 shows the two arched entrances of the Tunnel from the shaft.
+
+  Fig. 3 is a representation of the iron shield, and shows a workman
+  in each of the compartments.
+
+  The Entrance to the Tunnel is near to Rotherhithe Church, and nearly
+  opposite to the London-Docks. The nearest landing place from the river
+  is Church Stairs. The Greenwich and Deptford coaches which go the
+  lower road, start hourly from Charing-cross, and Gracechurch-street,
+  and pass close by the works at Rotherhithe.
+
+  Books relative to the Tunnel may be had at the works.
+
+  The Public may view the Tunnel every day (Sundays excepted) from
+  Seven in the morning until Eight in the Evening, upon payment of
+  One Shilling each Person.
+
+  The extreme northern end of the Tunnel is for the present secured
+  by a strong wall; but visitors will find a dry, warm, and gravelled
+  promenade, as far as to almost the centre of the river, and
+  brilliantly lighted with oil gas.
+
+  The entrance is from Rotherhithe Street, and by a safe, commodious,
+  and easy stair case.
+
+    H. Teape & Son, Printers, Tower-hill, London.]
+
+[Illustration: Figure 23.--VERTICAL SECTION THROUGH BRUNEL'S SHIELD.
+The long lever, x, supported the wood centering for turning the
+masonry arches of the lining. (LAW, _A Memoir of the Thames
+Tunnel._)]
+
+[Illustration: Figure 24.--THAMES TUNNEL. SECTION THROUGH riverbed and
+tunnel following one of the break-throughs of the river. Inspection of
+the damage with a diving bell. (BEAMISH, _A Memoir of the Life of Sir
+Marc Isambard Brunel_.)]
+
+The Tower Subway at first operated with cylindrical cars that nearly
+filled the 7-foot bore; the cars were drawn by cables powered by small
+steam engines in the shafts. This mode of power had previously been
+used in passenger service only on the Greenwich Street elevated
+railway in New York. Later the cars were abandoned as unprofitable and
+the tunnel turned into a footway (fig. 32). This small tunnel, the
+successful driving due entirely to Greathead's skill, was the
+forerunner of the modern subaqueous tunnel. In it, two of the three
+elements essential to such work thereafter were first applied: the
+one-piece movable shield of circular section, and the segmental
+cast-iron lining.
+
+The documentation of this work is far thinner than for the Thames
+Tunnel. The most accurate source of technical information is a brief
+historical account in Copperthwaite's classic _Tunnel Shields and the
+Use of Compressed Air in Subaqueous Works_, published in 1906.
+Copperthwaite, a successful tunnel engineer, laments the fact that he
+was able to turn up no drawing or original data on this first shield
+of Greathead's, but he presents a sketch of it prepared in the
+Greathead office in 1895, which is presumably a fair representation
+(fig. 33). The Tower Subway model was built on the basis of this and
+several woodcuts of the working area that appeared contemporaneously
+in the illustrated press. In this and the adjacent model of Beach's
+Broadway Subway, the tunnel axis has been placed on an angle to the
+viewer, projecting the bore into the case so that the complete circle
+of the working face is included for a more suggestive effect. This was
+possible because of the short length of the work included.
+
+Henry S. Drinker, also a tunnel engineer and author of the most
+comprehensive work on tunneling ever published, treats rock tunneling
+in exhaustive detail up to 1878. His notice of what he terms
+"submarine tunneling" is extremely brief. He does, however, draw a
+most interesting comparison between the first Thames Tunnel, built by
+Brunel, and the second, built by Greathead 26 years later:
+
+    FIRST THAMES TUNNEL             SECOND THAMES TUNNEL
+                                     (TOWER SUBWAY)
+
+  Brickwork lining, 38 feet       Cast-iron lining of 8 feet
+  wide by 22-1/2 feet high.       outside diameter.
+
+  120-ton cast-iron shield,       2-1/2-ton, wrought-iron shield,
+  accommodating 36 miners.        accommodating at most 3 men.
+
+  Workings filled by irruption   "Water encountered at almost
+  of river five times.            any time could have been
+                                  gathered in a stable pail."
+
+  Eighteen years elapsed between  Work completed in about
+  start and finish of work.       eleven months.
+
+  Cost: $3,000,000.               Cost: $100,000.
+
+[Illustration: Figure 25.--TRANSVERSE SECTION THROUGH SHIELD, after
+inundation. Such disasters, as well as the inconsistency of the
+riverbed's composition, seriously disturbed the alignment of the
+shield's individual sections. (LAW, _A Memoir of the Thames Tunnel_.)]
+
+[Illustration: Figure 26.--LONGITUDINAL SECTION THROUGH THAMES TUNNEL
+after sandbagging to close a break in the riverbed. The tunnel is
+filled with silt and water. (LAW, _A Memoir of the Thames Tunnel_.)]
+
+[Illustration: Figure 27.--INTERIOR OF THE THAMES TUNNEL shortly after
+completion in 1843. (_Photo courtesy of New York Public Library
+Picture Collection._)]
+
+[Illustration: Figure 28.--THAMES TUNNEL in use by London Underground
+railway. (_Illustrated London News_, 1869?)]
+
+[Illustration: Figure 29.--PLACING A segment of cast-iron lining in
+Greathead's Tower Subway, 1869. To the rear is the shield's diaphragm
+or bulkhead. MHT model--1-1/2" scale. (Smithsonian photo 49260-B.)]
+
+
+BEACH'S BROADWAY SUBWAY
+
+Almost simultaneously with the construction of the Tower Subway,
+the first American shield tunnel was driven by Alfred Ely Beach
+(1826-1896). Beach, as editor of the _Scientific American_ and
+inventor of, among other things, a successful typewriter as early as
+1856, was well known and respected in technical circles. He was not
+a civil engineer, but had become concerned with New York's pressing
+traffic problem (even then) and as a solution, developed plans for a
+rapid-transit subway to extend the length of Broadway. He invented a
+shield as an adjunct to this system, solely to permit driving of the
+tunnel without disturbing the overlying streets.
+
+An active patent attorney as well, Beach must certainly have known of
+and studied the existing patents for tunneling shields, which were,
+without exception, British. In certain aspects his shield resembled
+the one patented by Barlow in 1864, but never built. However, work on
+the Beach tunnel started in 1869, so close in time to that on the
+Tower Subway, that it is unlikely that there was any influence from
+that source. Beach had himself patented a shield, in June 1869, a
+two-piece, sectional design that bore no resemblance to the one used.
+His subway plan had been first introduced at the 1867 fair of the
+American Institute in the form of a short plywood tube through which
+a small, close-fitting car was blown by a fan. The car carried 12
+passengers. Sensing opposition to the subway scheme from Tammany, in
+1868 Beach obtained a charter to place a small tube beneath Broadway
+for transporting mail and small packages pneumatically, a plan he
+advocated independently of the passenger subway.
+
+[Illustration: Figure 30.--CONTEMPORARY ILLUSTRATIONS of Tower
+Subway works used as basis of the model in the Museum of History
+and Technology. (_Illustrated London News_, 1869.)
+
+  ADVANCING THE SHIELD. FITTING THE CASTINGS.]
+
+[Illustration: Figure 31.--EXCAVATION IN FRONT OF SHIELD, Tower
+Subway. This was possible because of the stiffness of the clay
+encountered. MHT model--front of model shown in fig. 29.
+(Smithsonian photo 49260-A.)]
+
+Under this thin pretense of legal authorization, the sub-rosa
+excavation began from the basement of a clothing store on Warren
+Street near Broadway. The 8-foot-diameter tunnel ran eastward a short
+distance, made a 90-degree turn, and thence southward under Broadway
+to stop a block away under the south side of Murray Street. The total
+distance was about 312 feet. Work was carried on at night in total
+secrecy, the actual tunneling taking 58 nights. At the Warren Street
+terminal, a waiting room was excavated and a large Roots blower
+installed for propulsion of the single passenger car. The plan was
+similar to that used with the model in 1867: the cylindrical car
+fitted the circular tunnel with only slight circumferential clearance.
+The blower created a plenum within the waiting room and tunnel area
+behind the car of about 0.25 pounds per square inch, resulting in a
+thrust on the car of almost a ton, not accounting for blowby. The car
+was thus blown along its course, and was returned by reversing the
+blower's suction and discharge ducts to produce an equivalent vacuum
+within the tunnel.
+
+[Illustration: Figure 32.--INTERIOR OF COMPLETED TOWER SUBWAY.
+(THORNBURY, _Old and New London, 1887, vol. 1, p. 126_.)]
+
+The system opened in February of 1870 and remained in operation for
+about a year. Beach was ultimately subdued by the hostile influences
+of Boss Tweed, and the project was completely abandoned. Within a very
+few more years the first commercially operated elevated line was
+built, but the subway did not achieve legitimate status in New York
+until the opening of the Interborough line in 1904. Ironically, its
+route traversed Broadway for almost the length of the island.
+
+[Illustration: Figure 33.--VERTICAL SECTION through the Greathead
+shield used at the Tower Subway, 1869. The first one-piece shield of
+circular section. (COPPERTHWAITE, _Tunnel Shields and the Use of
+Compressed Air in Subaqueous Works_.)]
+
+The Beach shield operated with perfect success in this brief trial,
+although the loose sandy soil encountered was admittedly not a severe
+test of its qualities. No diaphragm was used; instead a series of 8
+horizontal shelves with sharpened leading edges extended across the
+front opening of the shield. The outstanding feature of the machine
+was the substitution for the propelling screws used by Brunel and
+Greathead of 18 hydraulic rams, set around its circumference. These
+were fed by a single hand-operated pump, seen in the center of figure
+34. By this means the course of the shield's forward movement could be
+controlled with a convenience and precision not attainable with
+screws. Vertical and horizontal deflection was achieved by throttling
+the supply of water to certain of the rams, which could be
+individually controlled, causing greater pressure on one portion of
+the shield than another. This system has not changed in the ensuing
+time, except, of course, in the substitution of mechanically produced
+hydraulic pressure for hand.
+
+[Illustration: Figure 34.--BEACH'S Broadway Subway. Advancing the
+shield by hydraulic rams, 1869. MHT model--1-1/2" scale. (Smithsonian
+photo 49260-E.)]
+
+[Illustration: Figure 35.--VERTICAL SECTION through the Beach shield
+used on the Broadway Subway, showing the horizontal shelves (C), iron
+cutting ring (B), hydraulic rams (D), hydraulic pump (F), and rear
+protective skirt (H). (_Scientific American_, March 5, 1870.)]
+
+Unlike the driving of the Tower Subway, no excavation was done in
+front of the shield. Rather, the shield was forced by the rams into
+the soil for the length of their stroke, the material which entered
+being supported by the shelves. This was removed from the shelves and
+hauled off. The ram plungers then were withdrawn and a 16-inch length
+of the permanent lining built up within the shelter of the shield's
+tail ring. Against this, the rams bore for the next advance. Masonry
+lining was used in the straight section; cast-iron in the curved. The
+juncture is shown in the model.
+
+[Illustration: Figure 36.--INTERIOR of Beach Subway showing iron
+lining on curved section and the pneumatically powered passenger car.
+View from waiting room. (_Scientific American_, March 5, 1870.)]
+
+Enlarged versions of the Beach shield were used in a few tunnels in
+the Midwest in the early 1870's, but from then until 1886 the shield
+method, for no clear reason, again entered a period of disuse finding
+no application on either side of the Atlantic despite its virtually
+unqualified proof at the hands of Greathead and Beach. Little precise
+information remains on this work. The Beach system of pneumatic
+transit is described fully in a well-illustrated booklet published
+by him in January 1868, in which the American Institute model is
+shown, and many projected systems of pneumatic propulsion as well
+as of subterranean and subaqueous tunneling described. Beach again
+(presumably) is author of the sole contemporary account of the
+Broadway Subway, which appeared in _Scientific American_ following its
+opening early in 1870. Included are good views of the tunnel and car,
+of the shield in operation, and, most important, a vertical sectional
+view through the shield (fig. 35).
+
+It is interesting to note that optical surveys for maintenance of the
+course apparently were not used. The article illustrated and described
+the driving each night of a jointed iron rod up through the tunnel
+roof to the street, twenty or so feet above, for "testing the
+position."
+
+
+THE FIRST HUDSON RIVER TUNNEL
+
+Despite the ultimate success of Brunel's Thames Tunnel in 1843, the
+shield in that case afforded only moderately reliable protection
+because of the fluidity of the soil driven through, and its tendency
+to enter the works through the smallest opening in the shield's
+defense. An English doctor who had made physiological studies of the
+effects on workmen of the high air pressure within diving bells is
+said to have recommended to Brunel in 1828 that he introduce an
+atmosphere of compressed air into the tunnel to exclude the water
+and support the work face.
+
+This plan was first formally described by Sir Thomas Cochrane
+(1775-1860) in a British patent of 1830. Conscious of Brunel's
+problems, he proposed a system of shaft sinking, mining, and tunneling
+in water-bearing materials by filling the excavated area with air
+sufficiently above atmospheric pressure to prevent the water from
+entering and to support the earth. In this, and his description of air
+locks for passage of men and materials between the atmosphere and the
+pressurized area, Cochrane fully outlined the essential features of
+pneumatic excavation as developed since.
+
+[Illustration: Figure 37.--THE GIANT ROOTS LOBE-TYPE BLOWER used for
+propelling the car.]
+
+In 1839, a French engineer first used the system in sinking a mine
+shaft through a watery stratum. From then on, the sinking of shafts,
+and somewhat later the construction of bridge pier foundations, by the
+pneumatic method became almost commonplace engineering practice in
+Europe and America. Not until 1879 however, was the system tried in
+tunneling work, and then, as with the shield ten years earlier, almost
+simultaneously here and abroad. The first application was in a small
+river tunnel in Antwerp, only 5 feet in height. This project was
+successfully completed relying on compressed air alone to support the
+earth, no shield being used. The importance of the work cannot be
+considered great due to its lack of scope.
+
+[Illustration: Figure 38.--TESTING ALIGNMENT of the Broadway Subway at
+night by driving a jointed rod up to street level. (_Scientific
+American_, March 5, 1870.)]
+
+In 1871 Dewitt C. Haskin (1822-1900), a west coast mine and railroad
+builder, became interested in the pneumatic caissons then being used
+to found the river piers of Eads' Mississippi River bridge at St.
+Louis. In apparent total ignorance of the Cochrane patent, he evolved
+a similar system for tunneling water-bearing media, and in 1873
+proposed construction of a tunnel through the silt beneath the Hudson
+to provide rail connection between New Jersey and New York City.
+
+[Illustration: Figure 39.--HASKIN'S pneumatically driven tunnel
+under the Hudson River, 1880. In the engine room at top left was the
+machinery for hoisting, generating electricity for lighting, and air
+compressing. The air lock is seen in the wall of the brick shaft.
+MHT model--0.3" scale. (Smithsonian photo 49260.)]
+
+[Illustration: Figure 40.--ARTIST'S CONCEPTION OF MINERS escaping
+into the air lock during the blowout in Haskin's tunnel.]
+
+It would be difficult to imagine a site more in need of such
+communication. All lines from the south terminated along the west
+shore of the river and the immense traffic--cars, freight and
+passengers--was carried across to Manhattan Island by ferry and barge
+with staggering inconvenience and at enormous cost. A bridge would
+have been, and still is, almost out of the question due not only to
+the width of the crossing, but to the flatness of both banks. To
+provide sufficient navigational clearance (without a drawspan),
+impracticably long approaches would have been necessary to obtain
+a permissibly gentle grade.
+
+Haskin formed a tunneling company and began work with the sinking of
+a shaft in Hoboken on the New Jersey side. In a month it was halted
+because of an injunction by, curiously, the D L & W Railroad, who
+feared for their vast investment in terminal and marine facilities.
+Not until November of 1879 was the injunction lifted and work again
+commenced. The shaft was completed and an air lock located in one wall
+from which the tunnel proper was to be carried forward. It was
+Haskin's plan to use no shield, relying solely on the pressure of
+compressed air to maintain the work faces and prevent the entry of
+water. The air was admitted in late December, and the first
+large-scale pneumatic tunneling operation launched. A single 26-foot,
+double-track bore was at first undertaken, but a work face of such
+diameter proved unmanageable and two oval tubes 18 feet high by 16
+feet wide were substituted, each to carry a single track. Work went
+forward with reasonable facility, considering the lack of precedent.
+A temporary entrance was formed of sheet-iron rings from the air lock
+down to the tunnel grade, at which point the permanent work of the
+north tube was started. Immediately behind the excavation at the face,
+a lining of thin wrought-iron plates was built up, to provide form for
+the 2-foot, permanent brick lining that followed. The three stages are
+shown in the model in about their proper relationship of progress. The
+work is shown passing beneath an old timber-crib bulkhead, used for
+stabilizing the shoreline.
+
+The silt of the riverbed was about the consistency of putty and under
+good conditions formed a secure barrier between the excavation and the
+river above. It was easily excavated, and for removal was mixed with
+water and blown out through a pipe into the shaft by the higher
+pressure in the tunnel. About half was left in the bore for removal
+later. The basic scheme was workable, but in operation an extreme
+precision was required in regulating the air pressure in the work
+area.[5] It was soon found that there existed an 11-psi difference
+between the pressure of water on the top and the bottom of the working
+face, due to the 22-foot height of the unlined opening. Thus, it was
+impossible to maintain perfect pneumatic balance of the external
+pressure over the entire face. It was necessary to strike an average
+with the result that some water entered at the bottom of the face
+where the water pressure was greatest, and some air leaked out at the
+top where the water pressure was below the air pressure. Constant
+attention was essential: several men did nothing but watch the
+behavior of the leaks and adjusted the pressure as the ground density
+changed with advance. Air was supplied by several steam-driven
+compressors at the surface.
+
+The air lock permitted passage back and forth of men and supplies
+between the atmosphere and the work area, without disturbing the
+pressure differential. This principle is demonstrated by an animated
+model set into the main model, to the left of the shaft (fig. 39). The
+variation of pressure within the lock chamber to match the atmosphere
+or the pressurized area, depending on the direction of passage, is
+clearly shown by simplified valves and gauges, and by the use of light
+in varying color density. In the Haskin tunnel, 5 to 10 minutes were
+taken to pass the miners through the lock so as to avoid too abrupt a
+physiological change.
+
+Despite caution, a blowout occurred in July 1880 due to air leakage
+not at the face, but around the temporary entrance. One door of the
+air lock jammed and twenty men drowned, resulting in an inquiry which
+brought forth much of the distrust with which Haskin was regarded by
+the engineering profession. His ability and qualifications were
+subjected to the bitterest attack in and by the technical press. There
+is some indication that, although the project began with a staff of
+competent engineers, they were alienated by Haskin in the course of
+work and at least one withdrew. Haskin's remarks in his own defense
+indicate that some of the denunciation was undoubtedly justified. And
+yet, despite this reaction, the fundamental merit of the pneumatic
+tunneling method had been demonstrated by Haskin and was immediately
+recognized and freely acknowledged. It was apparent at the same time,
+however, that air by itself did not provide a sufficiently reliable
+support for large-area tunnel works in unstable ground, and this
+remains the only major subaqueous tunnel work driven with air alone.
+
+[Illustration: Figure 41.--LOCATION OF HUDSON RIVER TUNNEL. (_Leslie's
+Weekly_, 1879.)]
+
+After the accident, work continued under Haskin until 1882 when funds
+ran out. About 1600 feet of the north tube and 600 feet of the south
+tube had been completed. Greathead resumed operations with a shield
+for a British company in 1889, but exhaustion of funds again caused
+stoppage in 1891. The tunnel was finally completed in 1904, and is
+now in use as part of the Hudson and Manhattan rapid-transit system,
+never providing the sought-after rail link. A splendid document of the
+Haskin portion of the work is S. D. V. Burr's _Tunneling Under the
+Hudson River_ published in 1885. It is based entirely upon firsthand
+material and contains drawings of most of the work, including the
+auxiliary apparatus. It is interesting to note that electric
+illumination (arc, not incandescent, lights) and telephones were used,
+unquestionably the first employment of either in tunnel work.
+
+[Illustration: Figure 42.--ST. CLAIR TUNNEL. View of front of shield
+showing method of excavation in firm strata. Incandescent electric
+illumination was used. 1889-90. MHT model--1" scale. (Smithsonian
+photo 49260-D.)]
+
+
+THE ST. CLAIR TUNNEL
+
+The final model of the soft-ground series reflects, as did the Hoosac
+Tunnel model for hard-rock tunneling, final emergence into the modern
+period. Although the St. Clair Tunnel was completed over 70 years ago,
+it typifies in its method of construction, the basic procedures of
+subaqueous work in the present day. The Thames Tunnel of Brunel, and
+Haskin's efforts beneath the Hudson, had clearly shown that by
+themselves, both the shield and pneumatic systems of driving through
+fluid ground were defective in practice for tunnels of large area.
+Note that the earliest successful works by each method had been of
+very small area, so that the influence of adverse conditions was
+greatly diminished.
+
+The first man to perceive and seize upon the benefits to be gained by
+combining the two systems was, most fittingly, Greathead. Although he
+had projected the technique earlier, in driving the underground City
+and South London Railway in 1886, he brought together for the first
+time the three fundamental elements essential for the practical
+tunneling of soft, water-bearing ground: compressed-air support of the
+work during construction, the movable shield, and cast-iron, permanent
+lining. The marriage was a happy one indeed; the limitations of each
+system were almost perfectly overcome by the qualities of the others.
+
+The conditions prevailing in 1882 at the Sarnia, Ontario, terminal of
+the Grand Trunk Railway, both operational and physical, were almost
+precisely the same as those which inspired the undertaking of the
+Hudson River Tunnel. The heavy traffic at this vital U.S.--Canada rail
+interchange was ferried inconveniently across the wide St. Clair
+River, and the bank and river conditions precluded construction of a
+bridge. A tunnel was projected by the railway in that year, the time
+when Haskin's tribulations were at their height. Perhaps because of
+this lack of precedent for a work of such size, nothing was done
+immediately. In 1884 the railway organized a tunnel company; in 1886
+test borings were made in the riverbed and small exploratory drifts
+were started across from both banks by normal methods of mine
+timbering. The natural gas, quicksand, and water encountered soon
+stopped the work.
+
+[Illustration: Figure 43.--REAR VIEW OF ST. CLAIR SHIELD showing the
+erector arm placing a cast-iron lining segment. The three motions of
+the arm--axial, radial, and rotational, were manually powered.
+(Smithsonian photo 49260-C.)]
+
+It was at this time that the railway's president visited Greathead's
+City and South London workings. The obvious answer to the St. Clair
+problem lay in the successful conduct of this subway. Joseph Hobson,
+chief engineer of the Grand Trunk and of the tunnel project, in
+designing a shield, is said to have searched for drawings of the
+shields used in the Broadway and Tower Subways of 1868-9, but unable
+to locate any, he relied to a limited extent on the small drawings of
+those in Drinker's volume. There is no explanation as to why he did
+not have drawings of the City and South London shield at that moment
+in use, unless one considers the rather unlikely possibility that
+Greathead maintained its design in secrecy.
+
+[Illustration: Figure 44.--OPENING OF THE ST. CLAIR TUNNEL, 1891.
+(_Photo courtesy of Detroit Library, Burton Historical Collection._)]
+
+The Hobson shield followed Greathead's as closely as any other, in
+having a diaphragm with closable doors, but a modification of Beach's
+sharpened horizontal shelves was also used. However, these functioned
+more as working platforms than supports for the earth. The machine was
+21-1/2 feet in diameter, an unprecedented size and almost twice that
+of Greathead's current one. It was driven by 24 hydraulic rams.
+Throughout the entire preliminary consideration of the project there
+was a marked sense of caution that amounted to what seems an almost
+total lack of confidence in success. Commencement of the work from
+vertical shafts was planned so that if the tunnel itself failed, no
+expenditure would have been made for approach work. In April 1888,
+the shafts were started near both riverbanks, but before reaching
+proper depth the almost fluid clay and silt flowed up faster than it
+could be excavated and this plan was abandoned. After this second
+inauspicious start, long open approach cuts were made and the work
+finally began. The portals were established in the cuts, several
+thousand feet back from each bank and there the tunneling itself
+began. The portions under the shore were driven without air. When the
+banks were reached, brick bulkheads containing air locks were built
+across the opening and the section beneath the river, about 3,710 feet
+long, driven under air pressure of 10 to 28 pounds above atmosphere.
+For most of the way, the clay was firm and there was little air
+leakage. It was found that horses could not survive in the compressed
+air, and so mules were used under the river.
+
+In the firm clay, excavation was carried on several feet in front of
+the shield, as shown in the model (fig. 42). About twelve miners
+worked at the face. However, in certain strata the clay encountered
+was so fluid that the shield could be simply driven forward by the
+rams, causing the muck to flow in at the door openings without
+excavation. After each advance, the rams were retracted and a ring of
+iron lining segments built up, as in the Tower Subway. Here, for the
+first time, an "erector arm" was used for placing the segments, which
+weighed about half a ton. In all respects, the work advanced with
+wonderful facility and lack of operational difficulty. Considering
+the large area, no subaqueous tunnel had ever been driven with such
+speed. The average monthly progress for the American and Canadian
+headings totaled 455 feet, and at top efficiency 10 rings or a length
+of 15.3 feet could be set in a 24-hour day in each heading. The 6,000
+feet of tunnel was driven in just a year; the two shields met
+vis-a-vis in August of 1890.
+
+The transition was complete. The work had been closely followed by the
+technical journals and the reports of its successful accomplishment
+thus were brought to the attention of the entire civil engineering
+profession. As the first major subaqueous tunnel completed in America
+and the first in the world of a size able to accommodate full-scale
+rail traffic, the St. Clair Tunnel served to dispel the doubts
+surrounding such work, and established the pattern for a mode of
+tunneling which has since changed only in matters of detail.
+
+Of the eight models, only this one was built under the positive
+guidance of original documents. In the possession of the Canadian
+National Railways are drawings not only of all elements of the shield
+and lining, but of much of the auxiliary apparatus used in
+construction. Such materials rarely survive, and do so in this case
+only because of the foresight of the railway which, to avoid paying a
+high profit margin to a private contractor as compensation for the
+risk and uncertainty involved, carried the contract itself and,
+therefore, preserved all original drawing records.
+
+While the engineering of tunnels has been comprehensively treated in
+this paper from the historical standpoint, it is well to still reflect
+that the advances made in tunneling have not perceptibly removed the
+elements of uncertainty but have only provided more positive and
+effective means of countering their forces. Still to be faced are the
+surprises of hidden streams, geologic faults, shifts of strata,
+unstable materials, and areas of extreme pressure and temperature.
+
+
+
+
+BIBLIOGRAPHY
+
+
+  AGRICOLA, GEORGIUS. _De re Metallica._ [English transl. H. C. and L.
+     H. Hoover (_The Mining Magazine_, London, 1912).] Basel: Froben,
+     1556.
+
+  BEACH, ALFRED ELY. _The pneumatic dispatch._ New York: The American
+     News Company, 1868.
+
+  BEAMISH, RICHARD. _A memoir of the life of Sir Marc Isambard
+     Brunel._ London: Longmans, Green, Longmans and Roberts, 1862.
+
+  BURR, S. D. V. _Tunneling under the Hudson River._ New York: John
+     Wiley and Sons, 1885.
+
+  COPPERTHWAITE, WILLIAM CHARLES. _Tunnel shields and the use of
+     compressed air in subaqueous works._ New York: D. Van Nostrand
+     Company, 1906.
+
+  DRINKER, HENRY STURGESS. _Tunneling, explosive compounds and rock
+     drills._ New York: John Wiley and Sons, 1878.
+
+  LATROBE, BENJAMIN H. Report on the Hoosac Tunnel (Baltimore, October
+     1, 1862). Pp. 125-139, app. 2, in _Report of the commissioners upon
+     the Troy and Greenfield Railroad and Hoosac Tunnel_. Boston, 1863.
+
+  LAW, HENRY. A memoir of the Thames Tunnel. _Weale's Quarterly Papers
+     on Engineering_ (London, 1845-46), vol. 3, pp. 1-25 and vol. 5,
+     pp. 1-86.
+
+  The pneumatic tunnel under Broadway, N.Y. _Scientific American_
+     (March 5, 1870), pp. 154-156.
+
+  _Report of the commissioners upon the Troy and Greenfield Railroad
+     and Hoosac Tunnel to his excellency the governor and the honorable
+     the executive council of the state of Massachusetts, February 28,
+     1863._ Boston, 1863.
+
+  STORROW, CHARLES S. Report on European tunnels (Boston, November 28,
+     1862). Pp. 5-122, app. 1, in _Report of the commissioners upon the
+     Troy and Greenfield Railroad and Hoosac Tunnel...._ Boston, 1863.
+
+  The St. Clair Tunnel. _Engineering News_ (in series running October
+     4 to December 27, 1890).
+
+
+
+
+FOOTNOTES
+
+  [1] There are two important secondary techniques for opening
+      subterranean and subaqueous ways, neither a method truly of
+      tunneling. One of these, of ancient origin, used mainly in the
+      construction of shallow subways and utility ways, is the "cut and
+      cover" system, whereby an open trench is excavated and then roofed
+      over. The result is, in effect, a tunnel. The concept of the other
+      method was propounded in the early 19th century but only used
+      practically in recent years. This is the "trench" method, a sort
+      of subaqueous equivalent of cut and cover. A trench is dredged in
+      the bed of a body of water, into which prefabricated sections of
+      large diameter tube are lowered, in a continuous line. The joints
+      are then sealed by divers, the trench is backfilled over the tube,
+      the ends are brought up to dryland portals, the water is pumped
+      out, and a subterranean passage results. The Chesapeake Bay Bridge
+      Tunnel (1960-1964) is a recent major work of this character.
+
+  [2] In 1952 a successful machine was developed on this plan, with
+      hardened rollers on a revolving cutting head for disintegrating
+      the rock. The idea is basically sound, possessing advantages in
+      certain situations over conventional drilling and blasting
+      systems.
+
+  [3] In 1807 the noted Cornish engineer Trevithick commenced a small
+      timbered drift beneath the Thames, 5 feet by 3 feet, as an
+      exploratory passage for a larger vehicular tunnel. Due to the
+      small frontal area, he was able to successfully probe about 1000
+      feet, but the river then broke in and halted the work. Mine
+      tunnels had also reached beneath the Irish Sea and various rivers
+      in the coal regions of Newcastle, but these were so far below the
+      surface as to be in perfectly solid ground and can hardly be
+      considered subaqueous workings.
+
+  [4] Unlike the Brunel tunnel, this was driven from both ends
+      simultaneously, the total overall progress thus being 3 feet per
+      shift rather than 18 inches. A top speed of 9 feet per day could
+      be advanced by each shield under ideal conditions.
+
+  [5] Ideally, the pressure of air within the work area of a
+      pneumatically driven tunnel should just balance the hydrostatic
+      head of the water without, which is a function of its total height
+      above the opening. If the air pressure is not high enough, water
+      will, of course, enter, and if very low, there is danger of
+      complete collapse of the unsupported ground areas. If too high,
+      the air pressure will overcome that due to the water and the air
+      will force its way out through the ground, through increasingly
+      larger openings, until it all rushes out suddenly in a "blowout."
+      The pressurized atmosphere gone, the water then is able to pour
+      in through the same opening, flooding the workings.
+
+
+
+
+INDEX
+
+
+  Agricola, Georgius, 215, 216
+
+
+  Barlow, Peter W., 221, 227
+  Beach, Alfred Ely, 224, 227-229, 231, 237
+  Brunel, Marc Isambard (the elder), 204, 205, 217, 218, 221,
+    224, 229, 231, 236
+  Burleigh, Charles, 212, 213
+  Burleigh Rock Drill Company, 212
+  Burr, S. D. V., 236
+
+  Cochrane, Sir Thomas, 231, 232
+  Copperthwaite, William Charles, 224
+
+  Doane, Thomas, 210, 212, 213, 215
+  Drinker, Henry S., 224, 237
+
+  Greathead, James Henry, 204, 218, 221, 224, 229, 231, 235-237
+  Gwynn, Stuart, 210
+
+  Haskin, DeWitt C., 204, 232, 234-236
+  Haupt, Herman, 204, 209, 210
+  Hobson, Joseph, 237
+
+  Latrobe, Benjamin H., 208, 209
+  Law, Henry, 218
+
+  Mowbray, George W., 213, 215
+
+  Nobel, Alfred B., 213
+
+  Putnam Machine Works, 212
+
+  Shanley, Walter, 212
+  Shanley Bros., 215
+  Sommeiller, Germain, 210
+  Storrow, Charles S., 210
+
+  Tweed, William Marcy (Boss), 229
+
+  Weale, John, 218
+
+
+       *       *       *       *       *
+
+
+Transcriber's Notes
+
+All obvious typographical errors corrected. Formatting inconsistancies
+and spelling were standardized. Paragraphs split by illustrations were
+rejoined. The text in the reproduced handbill for the Thames Tunnel
+was transcribed with a slight modification to the figure description
+portion. The Index was extracted from the full publication Index.
+
+
+
+
+
+
+
+End of the Project Gutenberg EBook of Tunnel Engineering. A Museum Treatment, by
+Robert M. Vogel
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