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diff --git a/39785.txt b/39785.txt new file mode 100644 index 0000000..9141fce --- /dev/null +++ b/39785.txt @@ -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. 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