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+The Project Gutenberg EBook of Pressure, Resistance, and Stability of Earth
+by J. C. Meem
+
+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: Pressure, Resistance, and Stability of Earth
+       American Society of Civil Engineers: Transactions, Paper No. 1174,
+         Volume LXX, December 1910
+
+Author: J. C. Meem
+
+Release Date: October 25, 2005 [EBook #16938]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK PRESSURE AND RESISTANCE ***
+
+
+
+
+Produced by Juliet Sutherland, Taavi Kalju and the Online
+Distributed Proofreading Team at https://www.pgdp.net
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+
+
+AMERICAN SOCIETY OF CIVIL ENGINEERS INSTITUTED 1852
+
+TRANSACTIONS
+
+Paper No. 1174
+
+PRESSURE, RESISTANCE, AND STABILITY OF EARTH.[A]
+
+BY J.C. MEEM, M. AM. SOC. C. E.
+
+WITH DISCUSSION BY MESSRS. T. KENNARD THOMSON, CHARLES E. GREGORY,
+FRANCIS W. PERRY, E.P. GOODRICH, FRANCIS L. PRUYN, FRANK H. CARTER, AND
+J.C. MEEM.
+
+
+In the final discussion of the writer's paper, "The Bracing of Trenches
+and Tunnels, With Practical Formulas for Earth Pressures,"[B] certain
+minor experiments were noted in connection with the arching properties
+of sand. In the present paper it is proposed to take up again the
+question of earth pressures, but in more detail, and to note some
+further experiments and deductions therefrom, and also to consider the
+resistance and stability of earth as applied to piling and foundations,
+and the pressure on and buoyancy of subaqueous structures in soft
+ground.
+
+In order to make this paper complete in itself, it will be necessary, in
+some instances, to include in substance some of the matter of the former
+paper, and indulgence is asked from those readers who may note this
+fact.
+
+[Illustration: FIG. 1. SECTIONS OF BOX-FRAME FOR SAND-ARCH
+EXPERIMENT]
+
+_Experiment No. 1._--As the sand-box experiments described in the former
+paper were on a small scale, exception might be taken to them, and
+therefore the writer has made this experiment on a scale sufficiently
+large to be much more conclusive. As shown in Fig. 1, wooden abutments,
+3 ft. wide, 3 ft. apart, and about 1 ft. high, were built and filled
+solidly with sand. Wooden walls, 3 ft. apart and 4 ft. high, were then
+built crossing the abutments, and solidly cleated and braced frames were
+placed across their ends about 2 ft. back of each abutment. A false
+bottom, made to slide freely up and down between the abutments, and
+projecting slightly beyond the walls on each side, was then blocked up
+snugly to the bottom edges of the sides, thus obtaining a box 3 by 4 by
+7 ft., the last dimension not being important. Bolts, 44 in. long, with
+long threads, were run up through the false bottom and through 6 by 15
+by 2-in. pine washers to nuts on the top. The box was filled with
+ordinary coarse sand from the trench, the sand being compacted as
+thoroughly as possible. The ends were tightened down on the washers,
+which in turn bore on the compacted sand. The blocking was then knocked
+out from under the false bottom, and the following was noted:
+
+As soon as the blocking was removed the bottom settled nearly 2 in., as
+noted in Fig. 1, Plate XXIV, due to the initial compacting of the sand
+under the arching stresses. A measurement was taken from the bottom of
+the washers to the top of the false bottom, and it was noted as 41 in.
+(Fig. 1). After some three or four hours, as the arch had not been
+broken, it was decided to test it under greater loading, and four men
+were placed on it, four others standing on the haunches, as shown in
+Fig. 2, Plate XXIV. Under this additional loading of about 600 lb. the
+bottom settled 2 in. more, or nearly 4 in. in all, due to the further
+compression of the sand arch. About an hour after the superimposed load
+had been removed, the writer jostled the box with his foot sufficiently
+to dislodge some of the exposed sand, when the arch at once collapsed
+and the bottom fell to the ground.
+
+Referring to Fig. 2, if, instead of being ordinary sand, the block
+comprised within the area, _A U J V X_, had been frozen sand, there can
+be no reason to suppose that it would not have sustained itself, forming
+a perfect arch, with all material removed below the line, _V E J_, in
+fact, the freezing process of tunneling in soft ground is based on this
+well-known principle.
+
+[Illustration: FIG. 2.]
+
+[Illustration: FIG. 3.]
+
+If, then, instead of removing the mass, _J E V_, it is allowed to remain
+and is supported from the mass above, one must concede to this mass in
+its normal state the same arching properties it would have had if
+frozen, excepting, of course, that a greater thickness of key should be
+allowed, to offset a greater tendency to compression in moist and dry as
+against frozen sand, where both are measured in a confined area.
+
+If, in Fig. 2, _E V J_ = [phi] = the angle of repose, and it be assumed
+that _A J_, the line bisecting the angle between that of repose and the
+perpendicular, measures at its intersection with the middle vertical
+(_A_, Fig. 2) the height which is necessary to give a sufficient
+thickness of key, it may be concluded that this sand arch will be
+self-sustaining. That is, it is assumed that the arching effect is taken
+up virtually within the limits of the area, _A N_{1} V E J N A_, thus
+relieving the structure below of the stresses due to the weight or
+thrust of any of the material above; and that the portion of the
+material below _V E J_ is probably dead weight on any structure
+underneath, and when sustained from below forms a natural "centering"
+for the natural arch above. It is also probably true that the material
+in the areas, _X N_{1} A_ and _A N U_, does not add to the arching
+strength, more especially in those materials where cohesion may not be
+counted on as a factor. This is borne out by the fact that, in the
+experiment noted, a well-defined crack developed on the surface of the
+sand at about the point _U_{1}_, and extended apparently a considerable
+depth, assumed to be at _N_, where the haunch line is intersected by the
+slope line from _A_.
+
+[Illustration: PLATE XXIV, FIG. 1.--INITIAL SETTLEMENT IN 3-FT. SAND
+ARCH, DUE TO COMPRESSION OF MATERIAL ON REMOVING SUPPORTS FROM
+BOTTOM.]
+
+[Illustration: PLATE XXIV, FIG. 2.--FINAL SETTLEMENT OF SAND ARCH,
+DUE TO COMPRESSION IN EXCESS LOADING.]
+
+In this experiment the sand was good and sharp, containing some gravel,
+and was taken directly from the adjoining excavation. When thrown
+loosely in a heap, it assumed an angle of repose of about 45 degrees. It
+should be noted that this material when tested was not compacted as
+much, nor did it possess the same cohesion, as sand in its normal
+undisturbed condition in a bank, and for this reason it is believed that
+the depth of key given here is absolutely safe for all except
+extraordinary conditions, such as non-homogeneous material and others
+which may require special consideration.
+
+Referring again to the area, _A N_{1} V J N A_, Fig. 2, it is probable
+that, while self-sustaining, some at least of the lower portion must
+derive its initial support from the "centering" below, and the writer
+has made the arbitrary assumption that the lower half of it is carried
+by the structure while the upper half is entirely independent of it,
+and, in making this assumption, he believes he is adding a factor of
+safety thereto. The area, then, which is assumed to be carried by an
+underground structure the depth of which is sufficient to allow the
+lines, _V A_ and _J A_, to intersect below the surface, is the lower
+half of _A N_{1} V E J N A_, or its equivalent, _A V E J A_, plus the
+area, _V E J_, or _A V J A_, the angle, _A V J_, being
+
+           1                                   [phi]
+[alpha] = --- ( 90° - [phi] ) + [phi] = 45° + -------.
+           2                                     2
+
+It is not probable that these lines of thrust or pressure transmission,
+_A N_, _D K_, etc., will be straight, but, for purposes of calculation,
+they will be assumed to be so; also, that they will act along and
+parallel to the lines of repose of their natural slope, and that the
+thrust of the earth will therefore be measured by the relation between
+the radius and the tangent of this angle multiplied by the weight of
+material affected. The dead weight on a plane, _V J_, due to the
+material above, is, therefore, where
+
+    _l_ = span or extreme width of opening = _V J_,
+    _W_ = weight per cubic foot of material, and
+    _W_{1}_ = weight per linear foot.
+
+           2 × (_l_ / 2) tan. [alpha] × _W_
+_W_{1}_ = ---------------------------------- =
+                        2
+     1            / 1                       \
+    --- _l_ tan. { --- (90° - [phi]) + [phi] } _W_ =
+     2            \ 2                       /
+
+     _l_                [phi]
+    ----- tan. ( 45° + ------- ) _W_.
+      2                   2
+
+The application of the above to flat-arched or circular tunnels is very
+simple, except that the question of side thrust should be considered
+also as a factor. The thrust against the side of a tunnel in dry sand
+having a flat angle of repose will necessarily be greater than in very
+moist sand or clay, which stands at a much steeper angle, and, for the
+same reason, the arch thrust is greater in dryer sand and therefore the
+load on a tunnel structure should not be as great, the material being
+compact and excluding cohesion as a factor. This can be illustrated by
+referring to Fig. 3 in which it is seen that the flatter the position of
+the "rakers" keying at _W_{1}_, _W_{2}_, and _W_, the greater will be
+the side thrust at _A_, _C_, and _F_. It can also be illustrated by
+assuming that the arching material is composed of cubes of polished
+marble set one vertically above the other in close columns. There would
+then be absolutely no side thrust, but, likewise, no arching properties
+would be developed, and an indefinite height would probably be reached
+above the tunnel roof before friction enough would be developed to cause
+it to relieve the structure of any part of its load. Conversely, if it
+be assumed that the superadjacent material is composed of large bowling
+balls, interlocking with some degree of regularity, it can be seen that
+those above will form themselves into an arch over the "centering" made
+up of those supported directly by the roof of the structure, thus
+relieving the structure of any load except that due to this "centering."
+
+If, now, the line, _A B_, in Fig. 4, be drawn so as to form with _A C_
+the angle, [beta], to be noted later, and it be assumed that it measures
+the area of pressure against _A C_, and if the line, _C F_, be drawn,
+forming with _C G_, the angle, [alpha], noted above, then _G F_ can be
+reduced in some measure by reason of the increase of _G C_ to _C B_,
+because the side thrust above the line, _B C_, has slightly diminished
+the loading above. The writer makes the arbitrary assumption that this
+decrease in _G F_ should equal 20% of _B C_ = _F D_{1}_. If, then, the
+line, _B D_{1}_ be drawn, it is conceded that all the material within
+the area, _A B D_{1} G C A_, causes direct pressure against or upon the
+structure, _G C A_, the vertical lines being the ordinates of pressure
+due to weight, and the horizontal lines (qualified by certain ratios)
+being the abscissas of pressure due to thrust. An extreme measurement of
+this area of pressure is doubtless approximately more nearly a curve
+than the straight lines given, and the curve, _A R T I D_{II}_, is
+therefore drawn in to give graphically and approximately the safe area
+of which any vertical ordinate, multiplied by the weight, gives the
+pressure on the roof at that point, and any horizontal line, or
+abscissa, divided by the tangent of the angle of repose and multiplied
+by the weight per foot, gives the pressure on the side at that point.
+
+[Illustration: FIG. 4.]
+
+The practical conclusion of this whole assumption is that the material
+in the area, _F E C B B_{1}_, forms with the equivalent opposite area an
+arch reacting against the face, _C B B_{1}_ and that, as heretofore
+noted, the lower half (or its equivalent, _B D_{1} G B_) of the weight
+of this is assumed to be carried by the structure, the upper half being
+self-sustaining, as shown by the line, _B_{III} D_{IV}_ (or, for
+absolute safety, the curved line), and therefore, if rods could be run
+from sheeting inside the tunnel area to a point outside the line, _F
+B_{1}_, as indicated by the lines, 5, 6, 7, 8, 11, 12, 13, etc., that
+the internal bracing of this tunnel could be omitted, or that the tunnel
+itself would be relieved of all loading, whereas these rods would be
+carrying some large portion at least of the weight within the area
+circumscribed by the curve, _D_{II} I T G_, and further, that a tunnel
+structure of the approximate dimensions shown would carry its maximum
+load with the surface of the ground between _D_{IV}_ and _F_, beyond
+which point the pressure would remain the same for all depths.
+
+In calculating pressures on circular arches, the arched area should
+first be graphically resolved into a rectangular equivalent, as in the
+right half of Fig. 4, proceeding subsequently as noted.
+
+The following instances are given as partial evidence that in ordinary
+ground, not submerged, the pressures do not exceed in any instance those
+found by the above methods, and it is very probable that similar
+instances or experiences have been met by every engineer engaged in
+soft-ground tunneling:
+
+In building the Bay Ridge tunnel sewer, in 62d and 64th Streets,
+Brooklyn, the arch timber bracing shown in Fig. 1, Plate XXVI, was used
+for more than 4,000 ft., or for two-thirds of the whole 5,800 ft. called
+for in the contract. The external width of opening, measured at the
+wall-plate, averaged about 19 ft. for the 14½-ft. circular sewer and 19½
+ft. for the 15-ft. sewer. The arch timber segments in the cross-section
+were 10 by 12-in. North Carolina pine of good grade, with 2 in. off the
+butt for a bearing to take up the thrust. They were set 5 ft. apart on
+centers, and rested on 6 by 12-in. wall-plates of the same material as
+noted above. The ultimate strength of this material, across the grain,
+when dry and in good condition, as given by the United States Forestry
+Department tests is about 1,000 lb. in compression. Some tests[C] made
+in 1907 by Mr. E.F. Sherman for the Charles River Dam in Boston, Mass.,
+show that in yellow pine, which had been water-soaked for two years,
+checks began to open at from 388 to 581 lb. per sq. in., and that yields
+of ¼ in. were noted at from 600 to 1,000 lb. As the tunnel wall-plates
+described in this paper were subject to occasional saturation, and
+always to a moist atmosphere, they could never have been considered as
+equal to dry material. Had the full loading shown by the foregoing
+come on these wall-plates, they would have been subjected to a stress of
+about 25 tons each, or nearly one-half of their ultimate strength. In
+only one or two instances, covering stretches of 100 ft. in one case and
+200 ft. in another, where there were large areas of quicksand sufficient
+to cause semi-aqueous pressure, or pockets of the same material causing
+eccentric loading, did these wall-plates show any signs of heavy
+pressure, and in many instances they were in such good condition that
+they could be taken out and used a second and a third time. Two
+especially interesting instances came under the writer's observation: In
+one case, due to a collapse of the internal bracing, the load of an
+entire section, 25 ft. long and 19 ft. wide, was carried for several
+hours on ribs spaced 5 ft. apart. The minimum cross-section of these
+ribs was 73 sq. in., and they were under a stress, as noted above, of
+50,000 lb., or nearly up to the actual limit of strength of the
+wall-plate where the rib bore on it. When these wall-plates were
+examined, after replacing the internal bracing, they did not appear to
+have been under any unusual stress.
+
+[Illustration: PLATE XXV, FIG. 1.--NORMAL SLOPES AND STRATA
+OF NEWLY EXCAVATED BANKS.]
+
+[Illustration: PLATE XXV, FIG. 2.--NORMAL SLOPES AND STRATA
+OF NEWLY EXCAVATED BANKS.]
+
+In another instance, for a distance of more than 700 ft., the sub-grade
+of the sewer was 4 ft. below the level of the water in sharp sand. In
+excavating for "bottoms" the water had to be pumped at the rate of more
+than 300 gal. per min., and it was necessary to close-sheet a trench
+between the wall-plates in which to place a section of "bottom." In
+spite of the utmost care, some ground was necessarily lost, and this was
+shown by the slight subsidence of the wall-plates and a loosening up of
+the wedges in the supports bearing on the arch timbers. During this
+operation of "bottoming," two men on each side were constantly employed
+in tightening up wedges and shims above the arch timbers. It is
+impossible to explain the fact that these timbers slackened (without
+proportionate roof settlement) by any other theory than that the arching
+was so nearly perfect that it relieved the bracing of a large part of
+the load, the ordinary loose material being held in place by the arching
+or wedging together of the 2-in. by 3-ft. sheeting boards in the roof,
+arranged in the form of a segmental arch. The material above this roof
+was coarse, sharp sand, through which it had been difficult to tunnel
+without losing ground, and it had admitted water freely after each rain
+until the drainage of a neighboring pond had been completed, the men
+never being willing to resume work until the influx of water had
+stopped.
+
+The foregoing applies only to material ordinarily found under ground not
+subaqueous, or which cannot be classed as aqueous or semi-aqueous
+material. These conditions will be noted later.
+
+[Illustration: FIG. 5.]
+
+[Illustration: FIG. 6.]
+
+The writer will take up next the question of pressures against the faces
+of sheeted trenches or retaining walls, in material of the same
+character as noted above. Referring to Fig. 2, it is not reasonable to
+suppose that having passed the line, _R F J_, the character of the
+stresses due to the thrust of the material will change, if bracing
+should be substituted for the material in the area, _W V J R_, or if, as
+in Fig. 3, canvas is rolled down along the lines, _E G_ and _A O_, and
+if, as this section is excavated between the canvas faces, temporary
+struts are erected, there is no reason to believe that with properly
+adjusted weights at _W_ or _W_{2}_, an exact equilibrium of forces and
+conditions cannot be obtained. Or, again, if, as in Fig. 5, the face,
+_P Q_, is sheeted and rodded back to the surface, keying the rods taut,
+there is undoubtedly a stable condition and one which could not fail in
+theory or practice, nor can anyone, looking at Fig. 5, doubt that the
+top timbers are stressed more heavily than those at the bottom. The
+assumption is that the tendency of the material to slide toward the toe
+causes it to wedge itself between the face of the sheeting on the one
+hand and some plane between the sheeting and the plane of repose on the
+other, and that the resistance to this tendency will cause an arching
+thrust to be developed along or parallel to the lines, _A N_, _B M_,
+etc., Fig. 2, which are assumed to be the lines of repose, or curves
+approximating thereto. As the thrust is greatest in that material
+directly at the face, _A O_, Fig. 6, and is nothing at the plane of
+repose, _C O_, it may be assumed arbitrarily that the line, _B O_,
+bisecting this angle divides this area into two, in one of which the
+weight resolves itself wholly into thrust, the other being an area of no
+thrust, or wholly of weight bearing on the plane of repose. Calling this
+line, _B O_, the haunch line, the thrust in the area, _A O B_, is
+measured by its weight divided by the tangent of the angle,
+_P Q R_ = [phi], which is the angle of repose; that is, the thrust at
+any given point, _R_ = _R Q_ ÷ tan. [phi].
+
+The writer suggests that, in those materials which have steeper angles
+of repose than 45°, the area of pressure may be calculated as above, the
+thrust being computed, however, as for an angle of 45 degrees.
+
+In calculating the bending moment against a wall or bracing, there is
+the weight of the mass multiplied by the distance of its center of
+gravity vertically above the toe, or, approximately:
+
+                                   2
+Area, _A O B_ × weight per unit × --- height,
+                                   3
+
+where _h_ = height,
+
+_W_ = weight per cubic foot of material = 90 lb.,
+
+              90° - [phi]
+and [beta] = -------------
+                  2
+
+_P_ = pressure per linear foot (vertically),
+
+                  _h_                         2
+then _P_ = _h_ × ----- (tan. [beta]) × _W_ × --- _h_ =
+                   2                          3
+     1
+    --- _h^{3}_ _W_ tan. [beta].
+     3
+
+When the angle of repose, [phi], is less than 45°, this result must be
+reduced by dividing by tan. [phi]; that is,
+
+       1
+_h_ = --- _h^{3}_ tan. [beta] ÷ tan. [phi].
+       3
+
+Figs. 1 and 2, Plate XXV, show recently excavated banks of gravel and
+sand, which, standing at a general angle of 45°, were in process of
+"working," that is, there was continual slipping down of particles of
+the sand, and it may be well to note that in time, under exposure to
+weather conditions, these banks would finally assume a slope of about 33
+degrees. They are typical, however, as showing the normal slope of
+freshly excavated sandy material, and a slope which may be used in
+ordinary calculations. The steps seen in Plate XXV show the different
+characteristics of ground in close proximity. In Fig. 2, Plate XXVI,[D]
+may be seen a typical bank of gravel and sand; it shows the well-defined
+slope of sand adjacent to and in connection with the cohesive properties
+of gravel.
+
+The next points to be considered are the more difficult problems
+concerning subaqueous or saturated earths. The writer has made some
+experiments which appear to be conclusive, showing that, except in pure
+quicksand or wholly aqueous material, as described later, the earth and
+water pressures act independently of each other.
+
+For a better understanding of the scope and purpose of this paper, the
+writer divides supersaturated or subaqueous materials into three
+classes:
+
+_Class A._--Firm materials, such as coarse and fine gravels, gravel and
+sands mixed, coarse sands, and fine sands in which there is not a large
+proportion of fine material, such as loam, clay, or pure quicksand.
+
+_Class B._--Semi-aqueous materials, such as fine sands in which there is
+a large proportion of clay, etc., pure clays, silts, peats, etc.
+
+_Class C._--Aqueous materials, such as pure quicksands, in which the
+solid matter is so finely divided that it is amorphous and virtually
+held in suspension, oils, quicksilver, etc.
+
+Here it may be stated that the term, "quicksand," is so illusive that a
+true definition of it is badly needed. Many engineers call quicksand any
+sand which flows under the influence of water in motion. The writer
+believes the term should be applied only to material so "soupy" that its
+properties are practically the same as water under static conditions, it
+being understood that any material may be unstable under the influence
+of water at sufficiently high velocities, and that it is with a static
+condition, or one approximately so, that this paper deals.
+
+A clear understanding of the firm materials noted in Class A will lead
+to a better solution of problems dealing with those under Class B, as it
+is to this Class A that the experiments largely relate.
+
+The experiments noted below were made with varying material, though the
+principal type used was a fine sand, under the conditions in which it is
+ordinarily found in excavations, with less than 40% voids and less than
+10% of very fine material.
+
+[Illustration: FIG. 7.]
+
+_Experiment No. 2._--The first of these experiments, which in this
+series will be called No. 2, was simple, and was made in order to show
+that this material does not flow readily under ordinary conditions, when
+not coupled with the discharge of water under high velocity. A bucket 12
+in. in diameter, containing another bucket 9 in. in diameter, was used.
+A 6 by 6-in. hole was cut in the bottom of the inner bucket. About 3 in.
+of sand was first placed in the bottom of the larger bucket and it was
+partly filled with water. The inside bucket was then given a false
+bottom and partly filled with wet sand, resting on the sand in the
+larger bucket. Both were filled with water, and the weight, _W_, Fig. 7,
+on the arm was shifted until it balanced the weight of the inside bucket
+in the water, the distance of the weight, _W_, from the pivot being
+noted. The false bottom was then removed and the inside bucket, resting
+on the sand in the larger one, was partly filled with sand and both were
+filled with water, the conditions at the point of weighing being exactly
+the same, except that the false bottom was removed, leaving the sand in
+contact through the 6 by 6-in. opening. It is readily seen that, if the
+sand had possessed the aqueous properties sometimes attributed to sand
+under water, that in the inside bucket would have flowed out through the
+square hole in the bottom, allowing it to be lifted by any weight in
+excess of the actual weight of the bucket, less its buoyancy, as would
+be the case if it contained only water instead of sand and water. It was
+found, however, that the weight, resting at a distance of more than
+nine-tenths of the original distance from the pivot, would not raise the
+inside bucket. On lifting this inside bucket bodily, however, the water
+at once forced the sand out through the bottom, leaving a hole almost
+exactly the shape and size of the bottom orifice, as shown in Fig. 1,
+Plate XXVII. It should be stated that, in each case, the sand was put in
+in small handfuls and thoroughly mixed with water, but not packed, and
+allowed to stand for some time before the experiments were tried, to
+insure the compactness of ordinary conditions. It is seen from Fig. 1,
+Plate XXVII, that the sand was stable enough to allow the bucket to be
+put on its side for the moment of being photographed, although it had
+been pulled out of the water a little less than 3 min.
+
+[Illustration: PLATE XXVI, FIG. 1.--TYPES OF ARCH TIMBERS USED IN
+BAY RIDGE TUNNEL SEWER.]
+
+[Illustration: PLATE XXVI, FIG. 2.--NORMAL SLOPE OF LOOSE SAND,
+GRAVEL, AND CEMENTED GRAVEL, IN CLOSE PROXIMITY.]
+
+_Experiment No. 3._--In order to show that the arching properties of
+sand are not destroyed under subaqueous conditions, a small sand-box,
+having a capacity of about 1 cu. ft., and similar to that described in
+Experiment No. 1, was made. The bottom was cut out, with the exception
+of a ¾-in. projection on two sides, and a false bottom was placed below
+and outside of the original bottom, with bolts running through it,
+keying to washers on top of the sand, with which the box was partly
+filled. One side of the box contained a glass front, in order that
+conditions of saturation could be observed. The box of sand was then
+filled with water and, after saturation had been completed and the nuts
+and washers had been tightened down, the box was lifted off the floor.
+There was found to be no tendency whatever for the bottom to fall away,
+showing conclusively that the arching properties had not been destroyed
+by the saturation of the sand.
+
+The next three experiments were intended to show the relative pressure
+over any given area in contact with the water in the one case or sand
+and water in the other.
+
+[Illustration: FIG. 8.]
+
+_Experiment No. 4._--The apparatus for this experiment consisted of a
+3-in. pipe about 4-in. long and connected with a ¾-in. goose-neck pipe
+17 in. high above the top of the bowl shown in Fig. 8 and in Fig. 2,
+Plate XXVII. A loose rubber valve was intended to be seated on the upper
+face of the machined edge of the bowl and weighted down sufficiently to
+balance it against a head of water corresponding to the 17-in. head in
+the goose-neck. The bowl was then to be filled with sand and the
+difference, if any, noted between the weight required to hold the
+flap-valve down under the same head of water flowing through the sand.
+The results of this experiment were not conclusive, owing to the
+difficulty of making contact over the whole area of the sand and the rim
+of the bowl at the same time. At times, for instance, less than 1 lb.
+would hold back the water indefinitely, while, again, 2 or 3 lb. would
+be required as opposed to the 4½ lb. approximate pressure required to
+hold down the clear water. Again, at times the water would not flow
+through the neck at all, even after several hours, and after increasing
+the head by attaching a longer rubber tube thereto. In view of these
+conditions, this experiment would not be noted here, except that it
+unexpectedly developed one interesting fact. In order to insure against
+a stoppage of water, as above referred to, gravel was first put into the
+bottom of the bowl and the flap-valve was then rubbed down and held
+tightly while the pipe was filled. On being released, the pressure of
+water invariably forced out the whole body of sand, as shown in Fig. 2,
+Plate XXVII. Care was taken to see that the sand was saturated in each
+case, and the experiment was repeated numberless times, and invariably
+with the same result. The sand contained about 40% of voids. The
+deduction from this experiment is that the pressure of water is against
+rather than through sand and that any excess of voids occurring adjacent
+to a face against which there is pressure of water will be filled with
+sand, excepting in so far, of course, as the normal existing voids allow
+the pressure of the water to be transmitted through them.
+
+[Illustration: PLATE XXVII, FIG. 1.--EXPERIMENT SHOWING PROPERTIES
+OF SAND.]
+
+[Illustration: PLATE XXVII, FIG. 2.--SAND PUSHED UP FROM BOWL BY
+WATER PRESSURE THROUGH GOOSE-NECK.]
+
+If, then, the covering of sand over a structure is sufficiently heavy to
+allow arching action to be set up, the structure against which the
+pressure is applied must be relieved of much of the pressure of water
+against the area of sand not constituted as voids acting outside of the
+arching area. This is confirmed by the two following experiments:
+
+_Experiment No. 5._--The same apparatus was used here as in Experiment
+No. 2, Fig. 7, except that the inside bucket had a solid bottom. The
+inside and outside buckets were filled with water and the point was
+noted at which the weight would balance the inside bucket at a point
+some 3 in. off the bottom of the outside bucket. This point was
+measured, and the bottom of the larger bucket was covered over with sand
+so that in setting solidly in the sand the inside bucket would occupy
+the same relative position as it did in the water. The same weight was
+then applied and would not begin to lift the inner bucket. For instance,
+in the first part of the experiment the weight stood at 12 in. from the
+pivot, while in the next step the weight, standing at the end of the
+bar, had no effect, and considerable external pressure had to be exerted
+before the bucket could be lifted. Immediately after it was relieved,
+however, the weight at 12 in. would hold it clear of the sand. No
+attempt was made to work the bucket into the sand; the sand was leveled
+up and the bucket was seated on it, turned once or twice to insure
+contact, and then allowed to stand for some time before making the
+experiment. No attempt was made to establish the relationship between
+sands of varying voids, the general fact only being established, by a
+sufficient number of experiments, that the weight required to lift the
+bucket was more than double in sand having 40% of voids than that
+required to lift the bucket in water only.
+
+[Illustration: FIG. 9.]
+
+_Experiment No. 6._--The apparatus for this experiment consisted
+essentially of a hydraulic chamber about 8 in. in diameter and 1 ft.
+high, the top being removable and containing a collar with suitable
+packing, through which a 2½-in. piston moved freely up and down, the
+whole being similar to the cylinder and piston of a large hydraulic
+jack, as shown in Fig. 1, Plate XXVIII. Just below the collar and above
+the chamber there was a ½-in. inlet leading to a copper pipe and thence
+to a high-pressure pump. Attached to this there was a gauge to show the
+pressure obtained in the chamber, all as shown in Fig. 9. The purpose of
+the apparatus was to test the difference in pressure on any object
+submerged in clear water and on the same object buried in the sand under
+water. It is readily seen that, if pressure be applied to the water in
+this chamber, the amount of pressure (as measured by the gauge)
+necessary to lift the piston will be that due to the weight of the
+piston, less its displacement, plus the friction of the piston in the
+collar.
+
+[Illustration: PLATE XXVIII, FIG. 1.--APPARATUS FOR MEASURING LOSS
+OF PRESSURE IN SUBAQUEOUS MATERIALS.]
+
+[Illustration: PLATE XXVIII, FIG. 2.--RAISING ROOF OF BATTERY TUBES,
+IN BROOKLYN, BY "BLEEDING" SAND THROUGH DISPLACED PLATES.]
+
+Now, if for any reason the bottom area of the piston against which the
+water pressure acts be reduced, it will necessarily require a
+proportionate amount of increase in the pressure to lift this piston.
+If, therefore, it is found that 10 lb., for illustration, be required to
+lift the piston when plunged in clear water, and 20 lb. be required to
+lift it when buried in sand, it can be assumed at once that the area of
+the piston has been reduced 50% by being buried in the sand, eliminating
+the question of the friction of the sand itself around the piston. In
+order to determine what this friction might be, the writer arranged a
+table standing on legs above the bottom of the chamber, allowing the
+piston to move freely through a hole in its center. Through this table
+pipes were entered (as shown in part of Fig. 9). The whole was then
+placed in the chamber with the piston in place, and the area above was
+filled with sand and water. It is thus seen that, the end of the piston
+being free and in clear water, the difference, if any, between the
+pressure required to lift the piston when in clear water alone and in
+the case thus noted, where it was surrounded by sand, would measure the
+friction of the sand on the piston. After several trials of this,
+however, it was clearly seen that the friction was too slight to be
+noted accurately by a gauge registering single pounds, that is, with a
+piston in contact with 6 in. of sand vertically, a friction of 25 lb.
+per sq. ft. would only require an increase of 1.8 lb. on the gauge. It
+is therefore assumed that the friction on so small a piston in sand need
+not be considered as a material factor in the experiments made.
+
+The piston was plunged into clear water, and it was found that the
+pressure required to lift it was about 4 lb. The cap was then taken off,
+a depth of about 2 in. of sand was placed in the bottom of the chamber,
+and then the piston was set in place and surrounded by sand to a depth
+of some 6 in., water being added so that the sand was completely
+saturated. This was allowed to stand until it had regained the stability
+of ordinary sand in place, whereupon the cap with the collar bearing was
+set in place over the piston, the machine was coupled up, and the pump
+was started. A series of four experiments, extending over a period of
+two or three days, gave the following results:
+
+_Test 1._--The piston began to move at a pressure of 25 lb. The pressure
+gradually dropped to 7½ lb., at which point, apparently, it came out of
+the sand, and continued at 7½ lb. during the remainder of the test.
+
+_Test 2._--The piston was plunged back into the sand, without removing
+the cap, and allowed to stand for about 2 hours. No attempt was made to
+pack the sand or to see its condition around the piston, it being
+presumed, however, that it had reasonable time to get a fair amount of
+set. At slightly above 20 lb. the piston began to move, and as soon as a
+pocket of water accumulated behind the piston the pressure immediately
+dropped to 9 lb. and continued at this point until it came out of the
+sand.
+
+_Test 3._--The piston was plunged into the sand and hammered down
+without waiting for the sand to come to a definite set. In this case the
+initial pressure shown by the gauge was 17½ lb., which immediately
+dropped to 8 lb. as soon as the piston had moved sufficiently far to
+allow water to accumulate below it.
+
+_Test 4._--The cap was again removed, the piston set up in place, the
+sand compacted around it in approximately the same condition it would
+have had if the sand had been in place underground; the cap was then set
+in place and, after an hour, the pump was started. The pressure
+registered was 25 lb. and extended over a period of several seconds
+before there was any movement in the piston. The piston responded
+finally without any increase of pressure, and, after lifting an inch or
+two, the pressure gradually dropped to 10 lb., where it remained until
+the piston came out of the sand.
+
+The sum and average of these tests shows a relation of 22 lb. for the
+piston in sand to about 8½ lb. as soon as the volume of water had
+accumulated below it, which would correspond very closely to a sand
+containing 40% of voids, which was the characteristic of the sand used
+in this experiment.
+
+The conclusions from this experiment appear to be absolutely final in
+illustrating the pressure due to water on a tunnel buried in sand,
+either on the arch above or on the sides or bottom, as well as the
+buoyant effect upon the tunnel bottom under the same conditions.
+
+While the apparatus would have to be designed and built on a much larger
+scale in order to measure accurately the pressures due to sands and
+earths of varying characteristics, it appears to be conclusive in
+showing the principle, and near enough to the theoretical value to be
+taken for practical purposes in designing structures against water
+pressures when buried in sand or earth.
+
+It should be carefully noted that the friction of the water through
+sand, which is always a large factor in subaqueous construction, is
+virtually eliminated here, as the water pressure has to be transmitted
+only some 6 or 8 in. to actuate the base of the piston, whereas in a
+tunnel only half submerged this distance might be as many feet, and
+would be a considerable factor.
+
+It should be noted also that although the area subject to pressure is
+diminished, the pressure on the area remaining corresponds to the full
+hydrostatic head, as would be shown by the pressure on an air gauge
+required to hold back the water, except, of course, as it may be
+diminished more or less by friction.
+
+The writer understands that experiments of a similar nature and with
+similar apparatus have been tried on clays and peats with results
+considerably higher; that is, in one case, there was a pressure of 40
+lb. before the piston started to move.
+
+The following is given, in part, as an analysis and explanation of the
+above experiments and notes:
+
+It is well known that if lead be placed in a hydraulic press and
+subjected to a sufficient pressure it will exhibit properties somewhat
+similar to soft clay or quicksand under pressure. It will flow out of an
+orifice or more than one orifice at the same pressure. This is due to
+the fact that practically voids do not exist and that the pressure is so
+great, compared with the molecular cohesion, that the latter is
+virtually nullified. It is also theoretically true that solid stone
+under infinitely high pressure may be liquefied. If in the cylinder of a
+hydraulic press there be put a certain quantity of cobblestones, leaving
+a clearance between the top of the stone and the piston, and if this
+space, together with the voids, be filled with water and subjected to a
+great pressure, the sides or the walls of the cylinder are acted on by
+two pressures, one almost negligible, where they are in contact with the
+stone, restraining the tendency of the stone to roll or slide outward,
+and the other due to the pressure of the water over the area against
+which there is no contact of stone. That this area of contact should be
+deducted from the pressure area can be clearly shown by assuming another
+cylinder with cross-sticks jammed into it, as shown in Fig. 10. A glance
+at this figure will show that there is no aqueous pressure on the walls
+of the cylinder with which the ends of the sticks come in contact and
+the loss of the pressure against the walls due to this is equal to the
+least sectional area of the stick or tube either at the point of contact
+or intermediate thereto.
+
+Following this reasoning, in Fig. 11 it is found that an equivalent area
+may be deducted covering the least area of continuous contact of the
+cobblestones, as shown along the dotted lines in the right half of the
+figure. Returning, if, when the pressure is applied, an orifice be made
+in the cylinder, the water will at once flow out under pressure,
+allowing the piston to come in contact with the cobblestones. If the
+flow of the water were controlled, so as to stop it at the point where
+the stone and water are both under direct pressure, it would be found
+that the pressures were totally independent of each other. The aqueous
+pressure, for instance, would be equal at every point, while the
+pressure on the stone would be through and along the lines of contact.
+If this contact was reasonably well made and covered 40% of the area,
+one would expect the stone, independently of the water, to stand 40% of
+the pressure which a full area of solid stone would stand. If this
+pressure should be enormously increased after excluding the water, it
+would finally result in crushing the stone into a solid mass; and if the
+pressure should be increased indefinitely, some theoretical point would
+be reached, as above noted, where the stone would eventually be
+liquefied and would assume liquid properties.
+
+[Illustration: FIG. 10.]
+
+[Illustration: FIG. 11.]
+
+The same general reasoning applies to pure sand, sand being in effect
+cobblestones in miniature. In pressing the piston down on dry sand it
+will be displaced into every existing abnormal void, but will be
+displaced into these voids rather than pressed into them, in the true
+definition of the word, and while it would flow out of an orifice in the
+sides or bottom, allowing the piston to be forced down as in a
+sand-jack, it would not flow out of an orifice in the top of the piston,
+except under pressures so abnormally high as to make the mass
+theoretically aqueous. If the positions of cylinder and piston be
+reversed, the piston pointing vertically upward and the sand "bled" into
+an orifice in or through it, the void caused by the outflow of this sand
+would be filled by sand displaced by the piston pressing upward rather
+than by sand from above.
+
+It was the knowledge of this principle which enabled the contractors to
+jack up successfully the roof of a long section of the cast-iron lined
+tubes under Joralemon Street in Brooklyn, in connection with the
+reconstruction of the Battery tubes at that point, the method of
+operation, as partly shown in Fig. 2, Plate XXVIII, being to cut through
+a section of the roof, 4 by 10 ft. in area, through which holes were
+drilled and through which again the sand was "bled," heavy pressure
+being applied from below through the medium of hydraulic jacks. By a
+careful manipulation of both these operations, sections of the roof of
+the above dimensions were eventually raised the required height of 30
+in. and permanently braced there in a single shift.
+
+If water in excess be put into a cylinder containing sand, and pressure
+be applied thereto, the water, if allowed to flow out of an orifice,
+will carry with it a certain quantity of sand, according to the
+velocity, and the observation of this might easily give rise to the
+erroneous impression that the sand, as well as the water, was flowing
+out under pressure, and, as heretofore stated, has caused many engineers
+and contractors to apply the term "quicksand" to any sand flowing
+through an orifice with water.
+
+Sand in its natural bed always contains some fine material, and where
+this is largely less than the percentage of voids, it has no material
+effect on the pressure exerted by the sand with or without water, as
+above noted. If, however, this fine material be largely in excess of the
+voids, it allows greater initial compression to take place when dry, and
+allows to be set up a certain amount of hydraulic action when saturated.
+If the base of the material be sand and the fill be so-called quicksand
+in excess of the voids, pressure will cause the quicksand to set up
+hydraulic action, and the action of the piston will appear to be similar
+to that of a piston acting on purely aqueous material.
+
+Just here the writer desires to protest against considering semi-aqueous
+masses, such as soupy sands, soft concrete, etc., as exerting
+hydrostatic pressure due to their weight in bulk, instead of to the
+specific gravity of the basic liquid. For instance, resorting again to
+the illustration of cubes and spheres, it may be assumed that a cubical
+receptacle has been partly filled with small cubes of polished marble,
+piled vertically in columns. When this receptacle is filled with liquid
+around the piles of cubes there will be no pressure on the sides except
+that due to the hydrostatic pressure of the water at 62½ lb. The bottom,
+however, will resist a combined pressure due to the water and the weight
+of the cubes. Again, assume that the receptacle is filled with small
+spheres, such as marbles, and that water is then poured in. The pressure
+due to the weight of the solids on the bottom is relieved by the loss in
+weight of the marbles due to the water, and also to the tendency of the
+marbles to arch over the bottom, and while the pressure on the sides is
+increased by this amount of thrust, the aqueous pressure is still that
+of a liquid at 62½ lb., and it is inconceivable that some engineers, in
+calculating the thrust of aqueous masses, speak of it as a liquid
+weighing, say, 120 or 150 lb. per cu. ft.; as well might they expect to
+anchor spherical copper floats in front of a bulkhead and expect the
+hydrostatic pressure against this bulkhead to be diminished because the
+actual volume and weight of the water directly in front of the bulkhead
+has been diminished. Those who have had experience in tying narrow deep
+forms for concrete with small wires or bolts and quickly filling them
+with liquid concrete, must realize that no such pressures are ever
+developed as would correspond to liquids of 150 lb. per cu. ft. If the
+solid material in any liquid is agitated, so that it is virtually in
+suspension, it cannot add to the pressure, and if allowed to subside it
+acts as a solid, independently of the water contained with it, although
+the water may change somewhat the properties of the material, by
+increasing or changing its cohesion, angle of repose, etc. That is, in
+substance, those particles which rest solidly on the bottom and are in
+contact to the top of the solid material, do not derive any buoyancy
+from the water, while those particles not in contact with the bottom
+directly or through other particles, lose just so much weight through
+buoyancy. If, then, the vertical depth of the earthy particles or sand
+above the bottom is so small that the arching effect against the sides
+is negligible, the full weight of the particles in contact, directly or
+vicariously, with the bottom acts as pressure on the bottom, while the
+full pressure of the water acts through the voids or on them, or is
+transmitted through material in contact with the bottom.
+
+Referring now to materials such as clays, peats, and other soft or
+plastic materials, it is idle to assume that these do not possess
+pressure-resisting and arching properties. For instance, a soft clay
+arch of larger dimensions, under the condition described early in this
+paper, would undoubtedly stand if the rods supporting the intrados of
+the arch were keyed back to washers covering a sufficiently large area.
+
+The fact that compressed air can be used at all in tunnel work is
+evidence that semi-aqueous materials have arching properties, and the
+fact that "blows" usually occur in light cover is further evidence of
+it.
+
+When air pressure is used to hold back the water in faces of large area,
+bracing has to be resorted to. This again shows that while full
+hydrostatic pressure is required to hold back the water, the pressure of
+the earth is in a measure independent of it.
+
+In a peaty or boggy material there is a condition somewhat different,
+but sufficiently allied to the soft clayey or soupy sands to place it
+under the same head in ordinary practice. It is undoubtedly true that
+piles can be driven to an indefinite depth in this material, and it is
+also true that the action of the pile is to displace rather than
+compress, as shown by the fact of driving portions of the tunnels under
+the North River for long distances without opening the doors of the
+shield or removing any of the material. The case of filling in bogs or
+marshes, causing them to sink at the point of filling and rise
+elsewhere, is readily explained by the fact that the water is confined
+in the interstices of the material, admitting of displacement but no
+compression.
+
+The application of the above to pressures over tunnels in materials of
+Class A is that the sand or solid matter is virtually assumed to be a
+series of columns with their bases in such intimate contact with the
+tunnel roof that water cannot exert pressure on the tunnel or buoyancy
+on the sand at the point of contact, and that if these columns are
+sufficiently deep to have their upper portions wholly or partly carried
+by the arching or wedging action, the pressure of any water on their
+surfaces is not transferred to the tunnel, and the only aqueous pressure
+is that which acts on the tunnel between the assumed columns or through
+the voids.
+
+Let _l_ = exterior width of tunnel,
+    _d_ = depth of cover, as:
+
+      _D_{W}_ = depth, water to roof,
+      _D_{E}_ =   "    earth to roof,
+      _D_{X}_ =   "    of cover of earth necessary to arching stability,
+
+that is:
+
+           _l_          / 90° - [phi] \
+_D_{X}_ = ----- ( tan. { ------------- } + [phi] ) =
+            2           \     2       /
+
+     _l_                [phi]
+    ----- tan.  (45° + ------- ),
+      2                   2
+
+   where [phi] = angle of repose,
+   and _D_{W}_ > _D_{E}_ > _D_{X}_.
+
+Then the pressure on any square foot of roof, as _V_{P}_ as at the base of
+any vertical ordinate, as 9 in Fig. 2, = _V_{O}_,
+
+   _W_{E}_ = weight per cubic foot of earth (90 lb.),
+   _W_{W}_ =   "     "    "     "   " water (62½ lb.), we have
+
+_V_{P}_ = _V_{O}_ × _W_{E}_ + _D_{W}_ × _W_{W}_ × 0.40 =
+
+                                1
+    _V_{O}_ × 90 + _D_{W}_ × 62--- × 0.4 = _V_{O}_ 90 + _D_{W}_ × 25.
+                                2
+
+And for horizontal pressure:
+
+_P_{h}_ = the horizontal pressure at any abscissa (10), Fig. 2, = _A_{10}_
+at depth of water _D_{W1}_ is
+
+             _A_{10}_ × 90                  1
+  _P_{h}_ = --------------- + _D_{W1}_ × 62--- × 0.4 =
+               tan. [phi]                   2
+
+     _A_{10}_ × 90
+    --------------- + _D_{W1}_ × 25.
+       tan. [phi]
+
+The only question of serious doubt is at just what depth the sand is
+incapable of arching itself, but, for purposes of safety, the writer has
+put this at the point, _F_, as noted above, = _D_{X}_, although he
+believes that experiments on a large scale would show it to be nearer
+0.67·_D_{X}_, above which the placing of additional back-fill will
+lighten the load on the structure.
+
+We have, then, for _D_{E}_ < _D_{X}_, the weight of the total prism of
+the earth plus the water in the voids, plus the added pressure of the
+water above the earth prism, that is:
+
+The pressure per square foot at the base of any vertical ordinate =
+_V_{P}_
+
+                                      1
+_V_{P}_ = _D_{E}_ × 90 + _D_{E}_ × 62--- × 0.40 +
+                                      2
+
+                               1
+    ( _D_{W}_ - _D_{E}_ ) × 62---.
+                               2
+
+To those who may contend that water acting through so shallow a prism of
+earth would exert full pressure over the full area of the tunnel, it may
+be stated that the water cannot maintain pressure over the whole area
+without likewise giving buoyancy to the sand previously assumed to be in
+columns, in which case there is the total weight of the water plus the
+weight of the prism of earth, less its buoyancy in water, that is
+
+                       1                        1
+_V_{P}_ = _D_{W}_ × 62--- + _D_{E}_ × ( 90 - 62--- ),
+                       2                        2
+
+which, by comparison with the former method, would appear to be less
+safe in its reasoning.
+
+[Illustration: COMBINED EARTH AND WATER PRESSURES. FIG. 12.]
+
+Next is the question of pressure against a wall or braced trench for
+materials under Class A. The pressure of sand is first calculated
+independently, as shown in Fig. 6. Reducing this to a basis of 100 lb.
+for each division of the scale measured horizontally, as shown, gives
+the line, _B O_, Fig. 12, measuring the outside limit of pressure due to
+the earth, the horizontal distance at any point between this line and
+the vertical face equalling the pressure against that face divided by
+the tangent of the angle of repose, which in this case is assumed to be
+45°, equalling unity. If the water pressure line, _C F_, is drawn, it
+shows the relative pressure of the water. In order to reduce this to the
+scale of 100 lb. horizontal measurement, the line, _C E_, is drawn,
+representing the water pressure to scale, that is, so that each
+horizontal measurement of the scale gives the pressure on the face at
+that point; and, allowing 50% for voids, halving this area gives the
+line, _C D_, between which and the vertical face any horizontal line
+measures the water pressure. Extending these pressure areas where they
+overlap gives the line, _B D_, which represents the total pressure
+against the face, measured horizontally.
+
+Next, as to the question of buoyancy in Class A materials. If a
+submerged structure rests firmly on a bottom of more or less firm sand,
+its buoyancy, as indicated by the experiments, will only be a percentage
+of its buoyancy in pure water, corresponding to the voids in the sand.
+In practice, however, an attempt to show this condition will fail, owing
+to the fact that in such a structure the water will almost immediately
+work under the edge and bottom, and cause the structure to rise, and the
+test can only be made by measuring the difference in uplift in a
+heavier-than-water structure, as shown in Experiment No. 5. For, if a
+structure lighter than the displaced water be buried in sand
+sufficiently deep to insure it against the influx of large volumes of
+water below, it will not rise. That this is not due entirely to the
+friction of the solid material on the sides has been demonstrated by the
+observation of subaqueous structures, which always tend to subside
+rather than to lift during or following disturbance of the surrounding
+earth.
+
+The following is quoted from the paper by Charles M. Jacobs, M. Am. Soc.
+C. E., on the North River Division of the Pennsylvania Railroad
+Tunnels:[E]
+
+     "There was considerable subsidence in the tunnels during
+     construction and lining, amounting to an average of 0.34 ft.
+     between the bulkhead lines. This settlement has been constantly
+     decreasing since construction, and appears to have been due almost
+     entirely to the disturbances of the surrounding materials during
+     construction. The silt weighs about 100 lb. per cu. ft. * * * and
+     contains about 38% of water. It was found that whenever this
+     material was disturbed outside the tunnels a displacement of the
+     tunnels followed."
+
+This in substance confirms observations made in the Battery tubes that
+subsidence of the structure followed disturbance of the outside
+material, although theoretically the tubes were buoyant in the aqueous
+material.
+
+The writer would urge, however, that, in all cases of submerged
+structures only partially buried in solid material, excess weighting be
+used to cover the contingencies of vibration, oscillation, etc., to
+which such structures may be subjected and which may ultimately allow
+leads of water to work their way underneath.
+
+On the other hand, he urges that, in cases of floor areas of deeply
+submerged structures, such as tunnels or cellars, the pressure to be
+resisted should be assumed to be only slightly in excess of that
+corresponding to the pressure due to the water through the voids.
+
+The question of pressure, etc., in Class B, or semi-aqueous materials
+will be considered next. Of these materials, as already shown, there are
+two types: (_a_) sand in which the so-called quicksand is largely in
+excess of any normal voids, and (_b_) plastic and viscous materials. The
+writer believes that these materials should be treated as mixtures of
+solid and watery particles, in the first of which the quicksand, or
+aqueous portion, being virtually in suspension, may be treated as water,
+and it must be concluded that the action here will be similar to that of
+sand and pure water, giving a larger value to the properties of water
+than actually exists. If, for instance, it should be found that such a
+mixture contained 40% of pure water, the writer would estimate its
+pressure on or against a structure as (_a_) that of a moist sand
+standing at a steep angle of repose, and (_b_) that of clear water, an
+allowance of 60% of the total volume being assumed, and the sum of these
+two results giving the total pressure. Until more definite data can be
+obtained by experiments on a larger scale, this assumed value of 60% of
+the total volume for the aqueous portion may be taken for all conditions
+of semi-aqueous materials, except, of course, where the solid and
+aqueous particles may be clearly defined, the pressures being computed
+as described in the preceding pages.
+
+As to the question of pure quicksand (if such there be) and other
+aqueous materials of Class C, such as water, oil, mercury, etc., it has
+already been shown that they are to be considered as liquids of their
+normal specific gravity; that is, in calculating the air pressure
+necessary to displace them, one should consider their specific gravity
+only, as a factor, and not the total weight per volume including any
+impurities which they might contain undissolved.
+
+In order to have a clearer conception of aqueous and semi-aqueous
+materials and their action, they must be viewed under conditions not
+ordinarily apparent. For instance, ideas of so-called quicksand are
+largely drawn from seeing structures sinking into it, or from observing
+it flowing through voids in the sheeting or casing. The action of sand
+and water under pressure is viewed during or after a slump, when the
+damage is being done, or has been done, whereas the correct view-point
+is under static conditions, before the slump takes place.
+
+The following is quoted from the report of Mr. C.M. Jacobs, Chief
+Engineer of the East River Gas Tunnel, built in 1892-93:
+
+     "We found that the material which had heretofore been firm or stiff
+     had, under erosion, obtained a soup-like consistency, and that a
+     huge cavity some 3 ft. wide and 26 ft. deep had been washed up
+     toward the river bed."
+
+This would probably be a fair description of much of the material of
+this class met with in such work, if compressed air had not been used.
+The writer believes that in soft material surrounding submerged
+structures the water actually contained in the voids is not
+infrequently, after a prolonged period of rest, cut off absolutely from
+its sources of pressure and that contact with these sources of pressure
+will not again be resumed until a leak takes place through the
+structure; and, even when there is a small flow or trickling of water
+through such material, it confines itself to certain paths or channels,
+and is largely excluded from the general mass.
+
+The broad principle of the bearing power of soil has been made the
+subject of too many experiments and too much controversy to be
+considered in a paper which is intended to be a description of
+experiments and observed data and notes therefrom. The writer is of the
+opinion, however, that entirely too little attention has been given to
+this bearing power of the soil; that while progress has been made in our
+knowledge of all classes of materials for structures, very little has
+been done which leads to any real knowledge of the material on which the
+foundation rests. For instance, it is inconceivable that 1 or 2 tons may
+sometimes be allowed on a square foot of soft clay, while the load on
+firm gravel is limited to from 4 to 6 tons. The writer's practical
+observations have convinced him that it is frequently much safer to put
+four times 6 tons on a square foot of gravel than it is to put
+one-fourth of 2 tons on a square foot of soft clay.
+
+In connection with the bearing power of soil, the writer also believes
+that too little study has been given to the questions of the lateral
+pressure of earth, and he desires to quote here from some experiments
+described in a book[F] published in England in 1876, to which his
+attention has recently been called. This book appears to have been
+intended for young people, but it is of interest to note the following
+quotations from a chapter entitled "Sand." This chapter begins by
+stating that:
+
+     "During the course of a lecture on the Suez Canal by Mr. John H.
+     Pepper, which was delivered nightly by him at the Polytechnic
+     Institute in London, he illustrated his lecture by some experiments
+     designed to exhibit certain properties of sand, which had reference
+     to the construction of the Suez Canal, and it is stated that though
+     the properties in question were by no means to be classed among
+     recent discoveries, the experiments were novel in form and served
+     to interest the public audience."
+
+Further quotation follows:
+
+     "When the Suez Canal was projected, many prophesied evil to the
+     undertaking, from the sand in the desert being drifted by the wind
+     into the canal, and others were apprehensive that where the canal
+     was cut through the sand the bottom would be pushed up by the
+     pressure on the banks * * *.
+
+     "The principle of lateral pressure may now be strikingly
+     illustrated by taking an American wooden pail and, having
+     previously cut a large circular hole in the bottom, this is now
+     covered with fine tissue paper, which should be carefully pasted on
+     to prevent the particles of sand from flowing through the small
+     openings between the paper and the wood * * * and being placed
+     upright and rapidly filled with sand, it may be carried about by
+     the handle without the slightest fear of the weight of the sand
+     breaking through the thin medium. * * *
+
+     "Probably one of the most convincing experiments is that which may
+     be performed with a cylindrical tube 18 in. long and 2 in. in
+     diameter, open at both ends. A piece of tissue paper is carefully
+     pasted on one end, so that when dry no cracks or interstices are
+     left. The tube is filled with dry sand to a height of say 12 in. In
+     the upper part is inserted a solid plug of wood 12 in. long and of
+     the same or very nearly the same diameter as the inside of the
+     tube, so that it will move freely up and down like the piston of an
+     air pump. The tube, sand, and piston being arranged as described,
+     may now be held by an assistant and the demonstrator, taking a
+     sledge hammer, may proceed to strike steadily on the end of the
+     piston and, although the paper will bulge out a little, the force
+     of the blow will not break it.
+
+     "If the assistant holding the tube allows it to jerk or rebound
+     after each blow of the hammer, the paper may break, because air and
+     sand are driven down by the succeeding blow, and therefore it must
+     be held steadily so that the piston bears fairly on the sand each
+     time.
+
+     "A still more conclusive and striking experiment may be shown with
+     a framework of metal constructed to represent a pail, the sides of
+     which are closed up by pasting sheets of tissue paper inside and
+     over the lower part. As before demonstrated, when a quantity of
+     sand is poured into the pail the tissue paper casing at the bottom
+     does not break, but if a sufficient quantity is used the sides
+     formed of tissue paper bulge out and usually give way in
+     consequence of the lateral pressure exerted by the particles of
+     sand."
+
+The writer has made the second experiment noted, with special apparatus,
+and finds that with tissue paper over the bottom of a 2-in. pipe, 15 in.
+long, about 12 in. of sand will stand the blow of a heavy sledge hammer,
+transmitted through a wooden piston, at least once and sometimes two or
+three times, while heavy blows given with a lighter hammer have no
+effect at all. That this is not due in any large measure to inertia can
+be shown by the fact that more than 200 lb. can safely be put on top of
+the wooden piston. It cannot be accounted for entirely by the friction,
+as the removal of the paper allows the sand to drop in a mass. The
+explanation is that the pressure is transmitted laterally to the sides,
+and as the friction is directly proportional to the pressure, the load
+or effect of the blow is carried by the proportional increase in the
+friction, and any diaphragm which will carry the direct bottom load will
+not have its stresses largely increased by any greater loading on top.
+
+The writer believes that experiments will show that in a sand-jack the
+tendency will be for the sides to burst rather than the bottom, and that
+the outflow from an orifice at or near the bottom is not either greatly
+retarded or accelerated by ordinary pressure on top. The occurrence of
+abnormal voids, however, causes the sand to be displaced into them.
+
+The important consideration of this paper is that all the experiments
+and observations noted point conclusively to the fact that pressure is
+transmitted laterally through ground, most probably along or nearly
+parallel to the angles of repose, or in cases of rock or stiff material,
+along a line which, until more conclusive experiments are made, may be
+taken as a mean between the horizontal and vertical, or approximately 45
+degrees. There is no reason to believe that this is not the case
+throughout the entire mass of the earth, that each cubic foot, or yard,
+or mile is supported or in turn supports its neighboring equivalent
+along such lines. The theory is not a new one, and its field is too
+large to encompass within the limits of a single paper, but, for
+practical purposes, and within the limited areas to which we must
+necessarily be confined, the writer believes it can be established
+beyond controversy as true. Certain it is that no one has yet found, in
+ground free from water pressure or abnormal conditions, any evidence of
+greater pressure at the bottom of a deep shaft or tunnel than that near
+the surface. Pressures due to the widening of mines beyond the limits of
+safety must not be taken as a controversion of this statement, as all
+arches have limits of safety, more especially if the useless material
+below the theoretical intrados is only partly supported, or is allowed
+to be suspended from the natural arch.
+
+The writer believes, also, that the question of confined foundations, in
+contradistinction to that of the spreading of foundations, may be worthy
+of full discussion, as it applies to safe and economical construction,
+and he offers, without special comment, the following observations:
+
+He has found that, in soft ground, results are often obtained with small
+open caissons sunk to a depth of a few feet and cleaned out and filled
+with concrete, which offer much better resistance than spreading the
+foundation over four or five times the equivalent area.
+
+He has found that small steel piles and coffer-dams, from 1-ft.
+cylinders to coffer-dams 4 or 5 ft. square, sunk to a depth of only 1 or
+2 ft. below adjacent excavations in ordinary sand, have safely resisted
+loads four or five times as great as those usually allowed.
+
+He believes that short cylinders, cleaned out and filled with concrete,
+or coffer-dams of short steel piling with the surface cleaned out to a
+reasonable depth and filled with concrete horizontally reinforced, will,
+in many instances, give as good results as, and, in most cases, very
+much better than, placing the foundation on an equivalent number of
+small long piles or a proportionately greater spread of foundation area,
+the idea being that the transmission of pressure to the sides of the
+coffer-dam will not only confine the side thrust, but will also transfer
+the loading in mass to a greater depth where the resistance to lateral
+pressure in the ground will be more stable; that is, the greater depth
+of foundation is gained without the increased excessive loading, or
+necessity for deep excavation.
+
+As to the question of the bearing value and friction on piles, the
+writer believes that while the literature on engineering is full of
+experimental data relating to friction on caissons, there is little to
+show the real value of friction on piles. The assumption generally made
+of an assumed bearing value, and the deduction therefrom of a value for
+the skin friction is fallacious. Distinction, also, is not made, but
+should be clearly drawn between skin friction, pure and simple, on
+smooth surfaces, and the friction due to pressure. Too often the bearing
+value on irregular surfaces as well as the bearing due to taper in
+piles, and lastly the resistance offered by binding, enter into the
+determination of so-called skin friction formulas. The essential
+condition of sinking a caisson is keeping it plumb; and binding, which
+is another way of writing increased bearing value, will oftentimes be
+fatal to success.
+
+The writer believes that a series of observations on caissons sunk plumb
+under homogeneous conditions of ground and superficial smoothness will
+show a proportional increase of skin friction per square foot average
+for each increase in the size of caissons, as well as for increase of
+depth in the sinking up to certain points, where it may finally become
+constant, as will be shown later. The determination of the actual
+friction or coefficient of friction between the surfaces of the pile and
+the material it encounters, is not difficult to determine. In sand it is
+approximately 40% of the pressure for reasonably smooth iron or steel,
+and 45% of the pressure for ordinary wood surfaces. If, for instance, a
+long shaft be withdrawn vertically from moulding sand, the hole may
+remain indefinitely as long as water does not get into it or it does not
+dry out. This is due to the tendency of the sand to arch itself
+horizontally over small areas. The same operation cannot be performed on
+dry sand, as the arching properties, while protecting the pile from
+excessive pressure due to excessive length, will not prevent the loose
+sand immediately surrounding the pile from exerting a constant pressure
+against the pile, and it is of this pressure that 45% may be taken as
+the real value of skin friction on piles in dry sand.
+
+In soft clays or peats which are displaced by driving, the tendency of
+this material to flow back into the original space causes pressure, of
+which the friction will be a measured percentage. In this case, however,
+the friction itself between the material and the clays or peat is
+usually very much less than 40%, and it is for this reason that piles of
+almost indefinite length may be driven in materials of this character
+without offering sufficient resistance to be depended on, as long as no
+good bearing ground is found at the point.
+
+If this material is under water, and is so soft as to be considered
+semi-aqueous, the pressure per square foot will increase in diminishing
+proportion to the depth, and the pressure per area will soon approach
+and become a constant, due to the resistance offered by the lateral
+arching of the solid material; whereas, in large circular caissons, or
+caisson shafts, where the horizontal arching effect is virtually
+destroyed, or at least rendered non-effective until a great depth is
+reached, the pressure must necessarily vary under these conditions
+proportionately to the depth and size of the caisson in semi-aqueous
+material. On the other hand, in large caisson shafts, especially those
+which are square, the pressure at the top due to the solid material will
+also increase proportionately to the depth, as already explained in
+connection with the pressures of earth against sheeting and retaining
+walls.
+
+The writer believes that the pressure on these surfaces may be
+determined with reasonable accuracy by the formulas already given in
+this paper, and with these pressures, multiplied by the coefficient of
+friction determined by the simplest experiment on the ground, results
+may be obtained which will closely approximate the actual friction on
+caissons at given depths. The friction on caissons, which is usually
+given at from 200 to 600 lb. per sq. ft., is frequently assumed to be
+the same on piles 12 in. or less in diameter, whereas the pressures on
+these surfaces, as shown, are in no way comparable.
+
+The following notes and observations are given in connection with the
+skin friction and the bearing value of piles:
+
+The writer has in his possession a copy of an official print which was
+recently furnished to bidders in connection with the foundation for a
+large public building in New York City. The experiments were made on
+good sand at a depth of approximately 43 ft. below water and 47 ft.
+below an adjacent excavation. In this instance a 16-in. pipe was sunk to
+the depth stated, cleaned out, and a 14-in. piston connected to a 10-in.
+pipe was inserted and the ground at the bottom of the 16-in. pipe
+subjected to a loading approximating 28 tons per sq. ft. After an
+initial settlement of nearly 3 in., there was no further settlement over
+an extended period, although the load of 28 tons per sq. ft. was
+continued.
+
+In connection with some recent underpinning work, 14-in. hollow
+cylindrical piles 6 ft. long were sunk to a depth of 6 ft. with an
+ordinary hand-hammer, being excavated as driven. These piles were then
+filled with concrete and subjected to a loading in some cases
+approximating 60 tons. After a settlement ranging from 9 to 13 in., no
+further settlement took place, although the loading was maintained for a
+considerable period.
+
+In connection with some other pile work, the writer has seen a 10-in.
+pipe, 3/8 in. thick, 4 ft. below the bottom of an open cylinder, at a
+depth of about 20 ft., sustain in gravel and sand a load approximating
+50 tons when cleaned out to within 2 ft. of the bottom.
+
+He has seen other cylindrical piles with a bearing ring of not more than
+¾ in. resting on gravel at a depth of from 20 to 30 ft., cleaned out
+practically to the bottom, sustain a measured load of 60 tons without
+settlement.
+
+As to skin friction in sand, a case came under his observation wherein a
+14-in. hollow cylindrical pile which had stood for 28 days at a depth of
+about 30 ft. in the sand, was cleaned out to its bottom and subjected to
+hydraulic pressure, measured by a gauge, and sunk 2 ft. into the sand
+without any pressure being registered on the gauge. It should be
+explained, however, that the gauge could be subjected to a pressure of
+250 lb., equal to a total pressure of 7,000 lb. on the piston of the
+jack without registering, which corresponded, assuming it all as skin
+friction, to a maximum of not more than 78 lb. per sq. ft., but it
+should be noted that this included bearing value as well, and that the
+pressure was very far from 7,000 lb., in all probability, at the
+beginning of the test.
+
+In the case of the California stove-pipe wells driven by the Board of
+Water Supply on Long Island, the writer is informed that one of these
+tubes, 12 in. in diameter, was sunk to a depth of 850 ft. In doing this
+work the pile was excavated below the footing with a sand pump and was
+then sunk by hydraulic pressure. Assuming the maximum capacity of the
+jacks at 100 tons, which is not probable, the skin friction could not
+have amounted to more than 75 lb. per sq. ft. It cannot be assumed in
+this case that the excavation of the material below the pile relieved
+the skin itself of some of its friction, as the operation consumed more
+than 6 weeks, and, even if excess material was removed, it is certain
+that a large percentage of it would have had time to adjust itself
+before the operation was completed.
+
+[Illustration: PLATE XXIX, FIG. 1.--A 14-GAUGE, 14-IN., HOLLOW
+(NON-TELESCOPIC), CALIFORNIA STOVE-PIPE PILE WHICH MET IMPENETRABLE
+MATERIAL.]
+
+[Illustration: PLATE XXIX, FIG. 2.--CHENOWETH PILE, PENETRATING HARD
+MATERIAL.]
+
+In connection with this, the writer may call attention to the fact that
+piles driven in silt along the North River, and in soft material at
+other places, are sometimes 90 ft. in length, and even then do not offer
+sufficient resistance to be depended on for loading. This is due to the
+fact that the end of the pile does not bear in good material.
+
+The relation between bearing value and skin friction on a pile, where
+the end bearing is in good material, is well shown by a case where a
+wooden pile[G] struck solid material, was distorted under the continual
+blows of the hammer, and was afterward exposed. It is also shown in the
+case of a 14-in. California stove-pipe pile, No. 14 gauge, the point of
+which met firm material. The result, as shown by Fig. 1, Plate XXIX,
+speaks for itself. Fig. 2, Plate XXIX, shows a Chenoweth pile which was
+an experimental one driven by its designer. This pile, after getting
+into hard material, was subjected to the blow of a 4,000-lb. hammer
+falling the full length of the pile-driver, and the only result was to
+shatter the head of the pile, and not cause further penetration. Mr.
+Chenoweth has stated to the writer that he has found material so compact
+that it could not be penetrated with a solid pile--either with or
+without jetting--which is in line with the writer's experience.
+
+The writer believes that the foregoing notes will show conclusively that
+the factor to be sought in pile work is bearing value rather than depth
+or skin friction, and, however valuable skin friction may be in the
+larger caissons, it cannot be depended on in the case of small piles,
+except in values ranging from 25 to 100 lb. per sq. ft.
+
+In conclusion, he desires to thank the following gentlemen, who have
+contributed to the success of the experiments noted herein: Mr. James W.
+Nelson, of Richard Dudgeon, New York; Mr. George Noble, of John Simmons
+and Company, New York; and Mr. Pendleton, of Hindley and Pendleton,
+Brooklyn, N.Y.; all of whom have furnished apparatus for the experiments
+and have taken an interest in the results. And lastly, he desires
+especially to thank Mr. F.L. Cranford, of the Cranford Company, for men
+and material with which to make the experiments and without whose
+co-operation it would have been impracticable for the writer to have
+made them.
+
+Throughout this paper the writer has endeavored, as far as possible, to
+deduce from his observations and from the observations of others, as far
+as he has been able to obtain them, practical data and formulas which
+may be of use in establishing the relationship between the pressure,
+resistance, and stability of earths; and, while he does not wish to
+dictate the character of the discussion, he does ask that those who have
+made observations of a similar character or who have available data,
+will, as far as possible, contribute the same to this discussion. It is
+only by such observations and experiments, and deductions therefrom,
+that engineers may obtain a better knowledge of the handling of such
+materials.
+
+The writer believes that too much has been taken for granted in
+connection with earth pressures and resistance; and that, far too often,
+observations of the results of natural laws have been set down as
+phenomena. He believes that, both in experimenting and observing, the
+engineer will frequently find what is being looked for or expected and
+will fail to see the obvious alternative. He may add that his own
+experiments and observations may be criticized for the same reason, and
+he asks, therefore, that all possible light be thrown on this subject. A
+comparative study of much of our expert testimony or of the plans of
+almost any of the structures designed in connection with their bearing
+upon earth, or resistance to earth pressure, will show that under the
+present methods of interpretation of the underlying principles governing
+the calculations and designs relating to such structures, the results
+vary far too widely. Too much is left to the judgment of the engineer,
+and too frequently no fixed standards can be found for some of the most
+essential conditions.
+
+Until the engineer can say with certainty that his calculations are
+reasonably based on facts, he is forced to admit that his design must be
+lacking, either in the elements of safety, on the one hand, or of
+economy, on the other, and, until he can give to his client a full
+measure of both these factors in fair proportion, he cannot justly claim
+that his profession has reached its full development.
+
+Table 1 gives approximate calculations of pressures on two types of
+tunnels and on two heights of sheeted faces or walls, due to four
+varying classes of materials.
+
+TABLE 1.--PRESSURES ON TYPICAL STRUCTURES UNDER VARYING ASSUMED
+CONDITIONS.
+
+[Illustration: Key to Table of Pressures, etc.]
+
+_h_ = exterior height, _l_ = exterior width,
+
+    { [delta] = depth of cover, that is,
+    { _D_{E}_ = earth, and _D_{W}_ = water depth,
+
+[phi] = angle of repose, and, for tunnels _D_{W}_ > _D_{E}_ a depth
+
+       _l_           [phi]
+    = ----- ( 45° + ------- )
+        2              2
+
+_W_{E}_ = weight of 1 cu. ft. of earth = 90 lb.; _W_{W}_ = weight of 1
+cu. ft. of water = 62½ lb.
+
+Conditions: 1 = normal sand, 2 = dry sand, 3 = supersaturated firm sand
+with 40% of voids, 4 = supersaturated semi-aqueous material, 60%
+aqueous, that is, 60% water and aqueous material.
+
+
+_______________________________________________________
+              |        |        |        |            |
+   Combined   |        |        |        |            |
+    assumed   |  _h_   |  _l_   | [phi]  |   _D_{E}_  |
+  conditions. |        |        |        |            |
+______________|________|________|________|____________|
+              |        |        |        |            |
+     I_{1}    |   20   |   30   |   45°  |     40     |
+     I_{2}    |   20   |   30   |   30°  |     40     |
+    II_{1}    |   15   |   15   |   45°  |     40     |
+    II_{2}    |   15   |   15   |   30°  |     40     |
+   III_{1}    |   15   |        |   45°  |     15     |
+   III_{2}    |   15   |        |   30°  |     15     |
+    IV_{1}    |   30   |        |   45°  |     30     |
+    IV_{2}    |   30   |        |   30°  |     30     |
+______________|________|________|________|____________|
+
+____________________________________________________________________
+              |        |        |        |            |            |
+   Combined   |        |        |        |            |            |
+    assumed   |  _h_   |  _l_   | [phi]  |  _D_{E}_   |   _D_{W}_  |
+  conditions. |        |        |        |            |            |
+______________|________|________|________|____________|____________|
+              |        |        |        |            |            |
+     I_{3}    |   20   |   30   |   50°  |     40     |     60     |
+     I_{4}    |   20   |   30   |   40°  |     40     |     60     |
+    II_{3}    |   15   |   15   |   50°  |     40     |     60     |
+    II_{4}    |   15   |   15   |   40°  |     40     |     60     |
+   III_{3}    |   15   |        |   50°  |     15     |     15     |
+   III_{4}    |   15   |        |   40°  |     15     |     15     |
+    IV_{3}    |   30   |        |   50°  |     30     |     30     |
+    IV_{4}    |   30   |        |   40°  |     30     |     30     |
+______________|________|________|________|____________|____________|
+
+
+APPROXIMATE PRESSURES ON TUNNELS, PER SQUARE FOOT.
+
+_________________________________________________________________________
+          |      |      |      |         ||      |      |      |
+Pressure  | I_{1}| I_{3}| I_{3}| I_{3}   || I_{2}| I_{4}| I_{4}| I_{4}
+per square|Earth.|Earth.|Water.|Combined.||Earth.|Earth.|Water.|Combined.
+foot, at  |      |      |      |         ||      |      |      |
+__________|______|______|______|_________||______|______|______|_________
+          |      |      |      |         ||      |      |      |
+ A        | 3,240| 3,690| 1,500|   5,190 || 2,325| 2,880| 2,250| 5,130
+ B        | 2,745| 3,105| 1,500|   4,605 || 1,845| 2,385| 2,250| 4,635
+ C        | 2,160| 2,475| 1,500|   3,975 || 1,350| 1,800| 2,250| 4,050
+ D        |   450|   540| 1,500|   2,040 ||   450|   450| 2,250| 2,700
+ E        |   360|   360| 1,625|   1,985 ||   450|   450| 2,438| 2,888
+ F        |   270|   270| 1,750|   2,025 ||   450|   360| 2,626| 2,986
+ G        |   225|   225| 1,875|   2,100 ||   360|   270| 2,814| 3,084
+__________|______|______|______|_________||______|______|______|_________
+_________________________________________________________________________
+          |      |      |      |         ||      |      |      |
+Pressure  |II_{1}|II_{3}|II_{3}|II_{3}   ||II_{2}|II_{4}|II_{4}|II_{4}
+per square|Earth.|Earth.|Water.|Combined.||Earth.|Earth.|Water.|Combined.
+foot at   |      |      |      |         ||      |      |      |
+__________|______|______|______|_________||______|______|______|_________
+          |      |      |      |         ||      |      |      |
+ A        | 1,485| 1,755| 1,500|   3,255 || 1,035| 1,305| 2,250| 3,555
+ B        | 1,305| 1,485| 1,500|   2,985 ||   945| 1,170| 2,250| 3,420
+ C        | 1,125| 1,215| 1,500|   2,715 ||   810|   990| 2,250| 3,240
+ D        |   405|   405| 1,500|   1,905 ||   540|   450| 2,250| 2,700
+ E        |   405|   405| 1,625|   2,030 ||   540|   450| 2,438| 2,888
+ F        |   360|   360| 1,750|   2,110 ||   540|   450| 2,626| 3,076
+ G        |   315|   315| 1,875|   2,190 ||   360|   360| 2,814| 3,174
+ H        |   180|   225| 2,000|   2,225 ||   180|   180| 3,000| 3,180
+ I        |    90|   110| 2,175|   2,285 ||   135|   135| 3,188| 3,323
+__________|______|______|______|_________||______|______|______|_________
+
+
+APPROXIMATE PRESSURES ON SHEETED TRENCH FACES OR WALLS
+
+___________________________________________________________________________
+          |       |       |       |       ||       |       |       |
+Pressure  |III_{1}|III_{3}|III_{3}|III_{3}||III_{2}|III_{4}|III_{4}|III_{4}
+per square|Earth. |Earth. |Water. | Total ||Earth. |Earth. |Water. | Total
+foot at   |       |       |       | earth ||       |       |       | earth
+          |       |       |       |  and  ||       |       |       |  and
+          |       |       |       | water.||       |       |       | water.
+__________|_______|_______|_______|_______||_______|_______|_______|_______
+          |       |       |       |       ||       |       |       |
+ A        |  575  |  510  |  100  |  610  || 1,350 |  810  |  140  |  950
+ B        |  400  |  350  |  190  |  540  ||   900 |  540  |  260  |  800
+ C        |  200  |  175  |  280  |  455  ||   450 |  270  |  380  |  650
+__________|_______|_______|_______|_______||_______|_______|_______|_______
+___________________________________________________________________
+          |      |      |      |      ||      |      |      |
+Pressure  |IV_{1}|IV_{3}|IV_{3}|IV_{3}||IV_{2}|IV_{4}|IV_{4}|IV_{4}
+per square|Earth.|Earth.|Water.|Total ||Earth.|Earth.|Water.|Total
+foot at   |      |      |      |earth ||      |      |      |earth
+          |      |      |      | and  ||      |      |      | and
+          |      |      |      |water.||      |      |      |water.
+__________|______|______|______|______||______|______|______|______
+          |      |      |      |      ||      |      |      |
+ A        | 1,370| 1,210|  100 | 1,310|| 3,175| 1,910|   150| 2,060
+ B        | 1,170| 1,030|  200 | 1,230|| 2,700| 1,610|   290| 1,900
+ C        |   970|   855|  290 | 1,145|| 2,250| 1,355|   430| 1,785
+ D        |   775|   680|  370 | 1,050|| 1,800| 1,100|   570| 1,670
+ E        |   590|   515|  460 |   975|| 1,350|   820|   710| 1,530
+ F        |   400|   350|  560 |   910||   900|   540|   860| 1,400
+ G        |   190|   170|  650 |   820||   450|   275| 1,000| 1,275
+__________|______|______|______|______||______|______|______|______
+
+
+FOOTNOTES:
+
+[Footnote A: Presented at the meeting of May 18th, 1910.]
+
+[Footnote B: _Transactions_, Am. Soc. C. E., Vol. LX, p. 1.]
+
+[Footnote C: _Engineering News_, July 1st, 1909.]
+
+[Footnote D: From "Gravel for Good Roads."]
+
+[Footnote E: _Transactions_, Am. Soc. C. E., Vol. LXVIII, pp. 58-60.]
+
+[Footnote F: "Discoveries and Inventions of the Nineteenth Century," by
+Robert Routledge, Assistant Examiner in Chemistry and in Natural
+Philosophy to the University of London.]
+
+[Footnote G: _Engineering News_, January 15th, 1909.]
+
+
+
+
+DISCUSSION
+
+
+T. KENNARD THOMSON, M. AM. SOC. C. E.--Although the author
+deserves great credit for the careful and thorough manner in which he
+has handled this subject, his paper should be labeled "Dangerous for
+Beginners," especially as he is an engineer of great practical
+experience; if he were not, comparatively little attention would be paid
+to his statements. The paper is dangerous because many will read only
+portions of it, or will not read it thoroughly. For instance, at the
+beginning, the author cites several experiments in which considerable
+force is required to start the lifting of a weight or plunger in sand
+and water and much less after the start. This reminds the speaker of the
+time when, as a schoolboy, he tried to pick up stones from the bottom of
+the river and was told that the "suction" was caused by atmospheric
+pressure.
+
+The inference is that tunnels, etc., in sand, etc., are not in any
+danger of rising, even though they are lighter than water. Toward the
+end of the paper, however, the author states that tunnels should be
+weighted, but he rather spoils this by stating that they should be
+weighted only enough to overcome the actual water pressure, that is,
+between the voids of the sand. It seems to the speaker that the only
+really safe way is to make the tunnel at least as heavy as the water
+displaced in order to prevent it from coming up, and to take other
+measures to prevent it from going down. The City of Toronto, Canada,
+formerly pumped its water supply through a 6-ft. iron pipe, buried in
+the sand under Toronto Bay and then under Toronto Island, with an intake
+in the deep water of the lake. During a storm a mass of seaweed, etc.,
+was washed against the intake, completely blocking it, and although the
+man at the pumping station knew that something was wrong, he continued
+to pump until the water was drawn out of the pipe, with the result that
+about half a mile of the conduit started to rise and then broke at
+several places, thus allowing it to fill with water. Eventually, the
+city went down to bed-rock under the Bay for its water tunnel.
+
+Another reason for calling this paper dangerous for beginners is that it
+is improbable that experienced engineers or contractors will omit the
+bracing at the bottom, although, since the paper was printed, a glaring
+instance has occurred where comparatively little bracing was put in the
+bottom of a 40-ft. cut, the result being a bad cave-in from the bottom,
+although all the top braces remained in place. Most engineers will agree
+that nearly every crib which has failed slipped out from the bottom, and
+did not turn over.
+
+The objection to the angle of repose is that it is not possible to
+ascertain it for any material deposited by Nature. It could probably be
+ascertained for a sand bank deposited by Man, but not for an excavation
+to be made in the ground, for it is known that nearly all earth, etc.,
+has been deposited under great pressure, and is likely to be cemented
+together by clay, loam, roots, trees, boulders, etc., and differs in
+character every few feet.
+
+A deep vertical cut can often be made, even in New York quicksand, from
+which the water has been drawn, and, if not subjected to jars, water,
+etc., this material will stand for considerable time and then come down
+like an avalanche, killing any one in its way. In such cases very little
+bracing would prevent the slide from starting, provided rain, etc., did
+not loosen the material.
+
+The author, of course, treats dry and wet materials differently, but
+there are very few places where dry material is not likely to become wet
+before the excavation is completed.
+
+In caisson work, if the caisson can be kept absolutely plumb, it can be
+sunk without having to overcome much friction, while, on the other hand,
+if it is not kept plumb, the material is more or less disturbed and
+begins to bind, causing considerable friction. The author claims that
+the pressure does not increase with the depth, but all caisson men will
+probably remember that the friction to be overcome per square foot of
+surface increases with the depth.
+
+In calculating retaining walls, many engineers add the weight of the
+soil to the water, and calculate for from 90 to 100 lb. per cu. ft. The
+speaker is satisfied that in the so-called New York quicksand it is
+sufficient to use the weight of the water only. If the sand increased
+the side pressure above the water pressure, engineers would expect to
+use more compressed air to hold it back, while, as a matter of fact, the
+air pressure used seldom varies much from that called for by the
+hydrostatic head.
+
+Although allowance for water pressure is sufficient for designing
+retaining walls in New York quicksand, it is far from sufficient in
+certain silty materials. For instance, in Maryland, a coffer-dam,
+excavated to a depth of 30 ft. in silt and water, had the bottom shoved
+in 2 ft., in spite of the fact that the waling pieces were 5 ft. apart
+vertically at the top and 3 ft. at the bottom, and were braced with 12
+by 12-in. timbers, every 7 ft. horizontally. The walings split, and the
+cross-braces cut into the waling pieces from 1 to 2 in.; in other words,
+the pressure seemed to be almost irresistible. This is quite a contrast
+to certain excavations in Brooklyn, which, without any bracing whatever,
+were safely carried down 15 ft.
+
+Any engineer who tries to guess at the angle of repose, and, from the
+resulting calculations, economizes on his bottom struts, will find that
+sooner or later an accident on one job will cause enough loss of life
+and money to pay for conservative timbers for the rest of his life. So
+much for side pressures. As to the pressure in the roof of a tunnel,
+probably every engineer will agree that almost any material except
+unfrozen water will tend to arch more or less, but how much it is
+impossible to say. It is doubtful whether any experienced engineer would
+ever try to carry all the weight over the roof, except in the case of
+back-fill, and even then he would have to make his own assumption (which
+sounds more polite than "guess").
+
+The author has stated, however, that when the tunnel roof and sides are
+in place, no further trouble need be feared. On the contrary, in 1885,
+the Canadian Pacific Railroad built a tunnel through clayey material and
+lined it with ordinary 12 by 12-in. timber framing, about 2 or 3 ft.
+apart. After the tunnel was completed, it collapsed. It was re-excavated
+and lined with 12 by 12-in. timbers side by side, and it collapsed
+again; then the tunnel was abandoned, and, for some 20 years, the track,
+carried around on a 23° curve, was used until a new tunnel was built
+farther in. This trouble could have been caused either by the sliding or
+swelling of the material, and the speaker is inclined to believe that it
+was caused by swelling, for it is known, of course, that most material
+has been deposited by Nature under great pressure, and, by excavating in
+certain materials, the air and moisture would cause those materials to
+swell and become an irresistible force.
+
+To carry the load, Mr. Meem prefers to rely on the points of the piles
+rather than the side friction. In such cases the pile would act as a
+post, and would probably fail when ordinarily loaded, unless firmly
+supported at the sides. The speaker has seen piles driven from 80 to 90
+ft. in 10 min., which offered almost no resistance, and yet, a few days
+later, they would sustain 40 tons each. No one would dream of putting 40
+tons on a 90-ft. pile resting on rock, if it were not adequately
+supported.
+
+It is the speaker's opinion that bracing should not be omitted for
+either piles or coffer-dams.
+
+
+CHARLES E. GREGORY, ASSOC. M. AM. SOC. C. E.--In describing his
+last experiment with the hydraulic chambers and plunger, Mr. Meem states
+that, after letting the pressure stand at 25 lb., etc., the piston came
+up. This suggests that the piston might have been raised at a much lower
+pressure, if it had been allowed to stand long enough.
+
+The depth and coarseness of the sand were not varied to ascertain
+whether any relation exists between them and the pressure required to
+lift the piston. If the pressure varied with the depth of sand, it would
+indicate that the reduction was due to the resistance of the water when
+finely divided by the sand; if it varied with the coarseness of the
+sand, as it undoubtedly would, especially if the sand grains were
+increased to spheres 1 in. in diameter, it would show that it was
+independent of the voids in the sand, but dependent on dividing the
+water into thin films.
+
+The speaker believes that the greater part of the reduction of pressure
+on the bottom of the piston might be better explained by the viscosity
+of the water, than to assume that a considerable part of the plunger is
+not in contact with it. The water, being divided by fine sand into very
+thin films, has a tensile strength which is capable of resisting the
+pressure for at least a limited time.
+
+If the water is capable of exerting its full hydrostatic pressure
+through the sand, the total pressure would be the full hydrostatic
+pressure on the bottom of the piston where in contact, and, where
+separated from it by a grain of sand, the pressure would be decreased
+only by the weight of the grain. If a large proportion of the top area
+of a grain is in contact, as assumed by the author, this reduction of
+pressure would be very small. A correct interpretation can be obtained
+only after more complete experiments have been made.
+
+For horizontal pressures exerted by saturated sands on vertical walls,
+it has not been demonstrated that anything should be deducted from full
+water pressure. No matter how much of the area is in direct contact with
+the sand rather than the water, the full water pressure would be
+transmitted through each sand grain from its other side and, if
+necessary, from and through many other grains which may be in turn in
+contact with it. The pressure on such a wall will be water pressure over
+its entire surface, and, in addition, the thrust of the sand after
+correcting for its loss of weight in the water.
+
+The fact that small cavities may be excavated from the sides of trenches
+or tunnels back of the sheeting proves only that there is a local
+temporary arching of the material, or that the cohesion of the particles
+is sufficient to withstand the stress temporarily, or that there is a
+combination of cohesion and arching. The possibility of making such
+excavations does not prove that pressure does not exist at such points.
+That sand or earth will arch under certain conditions has long been an
+accepted fact. The sand arches experimented with developed their
+strength only after considerable yielding and, therefore, give no index
+of the distribution or intensity of stress before such yielding.
+Furthermore, sand and earth in Nature are not constrained by forms and
+reinforcing rods.
+
+Mr. Meem's paper is very valuable in that it presents some unusual
+phenomena, but many of the conclusions drawn therefrom cannot be
+accepted without further demonstration.
+
+
+FRANCIS W. PERRY, ASSOC. M. AM. SOC. C. E.--Pressure-gauge
+observations on a number of pneumatic caissons recently sunk, through
+various grades of sand, to rock at depths of from 85 to 105 ft. below
+ground-water, invariably showed working-chamber air-pressures equal, as
+closely as could be observed, to the hydrostatic pressures computed, for
+corresponding depths of cutting-edge, as given in Table 2.
+
+These observations and computations were made by the speaker in
+connection with the caisson foundations for the Municipal Building, New
+York City.
+
+
+TABLE 2.--EQUIVALENT FEET OF DEPTH BELOW WATER PER POUND
+PRESSURE.
+
+Pressure, |Equivalent |Equivalent   |Observed      |
+in        |feet of    |elevation    |pressure.     |
+pounds.   |depth.     |for water    |              |
+          |           |at--6.85.    |              |
+          |___________|_____________|              |
+          |           |             |              |
+          |M.H.W.     |Ground-water.|              |
+__________|___________|_____________|______________|
+          |           |             |              |
+  1       |  2.31     |  9.06       |Practically   |
+  2       |  4.63     | 11.48       |the same as   |
+  3       |  6.94     | 13.79       |computed      |
+  4       |  9.25     | 16.10       |for           |
+  5       | 11.57     | 18.42       |ground-water. |
+  6       | 13.88     | 20.73       |              |
+  7       | 16.19     | 23.04       |              |
+  8       | 18.50     | 25.35       |              |
+  9       | 20.82     | 27.67       |              |
+ 10       | 23.13     | 29.98       |              |
+ 11       | 25.44     | 32.29       |              |
+ 12       | 27.76     | 34.61       |              |
+ 13       | 30.07     | 36.92       |              |
+ 14       | 32.38     | 39.23       |              |
+ 15       | 34.70     | 41.55       |              |
+ 16       | 37.01     | 43.86       |              |
+ 17       | 39.32     | 46.17       |              |
+ 18       | 41.63     | 48.48       |              |
+ 19       | 43.95     | 50.80       |              |
+ 20       | 46.26     | 53.11       |              |
+ 21       | 48.57     | 55.42       |              |
+ 22       | 50.89     | 57.74       |              |
+ 23       | 53.20     | 60.05       |              |
+ 24       | 55.51     | 62.36       |              |
+ 25       | 57.82     | 64.67       |              |
+ 26       | 60.14     | 66.99       |              |
+ 27       | 62.45     | 69.30       |              |
+ 28       | 64.76     | 71.61       |              |
+ 29       | 67.08     | 73.93       |              |
+ 30       | 69.39     | 76.24       |              |
+ 31       | 71.70     | 78.55       |              |
+ 32       | 74.01     | 80.86       |              |
+ 33       | 76.33     | 83.18       |              |
+ 34       | 78.64     | 85.49       |              |
+ 35       | 80.95     | 87.80       |              |
+ 36       | 83.27     | 90.12       |              |
+ 37       | 85.58     | 92.43       |              |
+ 38       | 87.89     | 94.74       |              |
+ 39       | 90.20     | 97.05       |              |
+ 40       | 92.52     | 99.37       |              |
+ 41       | 94.83     |101.68       |              |
+ 42       | 97.14     |103.99       |              |
+ 43       | 99.46     |106.31       |              |
+ 44       |101.77     |108.62       |              |
+ 45       |104.08     |110.93       |              |
+ 46       |106.39     |113.24       |              |
+__________|___________|_____________|______________|
+
+                                    34
+NOTE.--Equivalent depth in feet = ------ × pressure.
+                                   14.7
+
+
+E.P. GOODRICH, M. AM. SOC. C. E. (by letter).--This paper is to
+be characterized by superlatives. Parts of it are believed to be
+exceptionally good, while other parts are considered equally dangerous.
+The author's experimental work is extremely interesting, and the writer
+believes the results obtained to be of great value; but the analytical
+work, both mathematical and logical, is emphatically questioned.
+
+The writer believes that, in the design of permanent structures,
+consideration of arch action should not be included, at least, not until
+much more information has been obtained. He also believes that the
+design of temporary structures with this inclusion is actually dangerous
+in some instances, and takes the liberty of citing the following
+statement by the author, with regard to his first experiment:
+
+     "About an hour after the superimposed load had been removed, the
+     writer jostled the box with his foot sufficiently to dislodge some
+     of the exposed sand, when the arch at once collapsed and the bottom
+     fell to the ground."
+
+The writer emphatically questions the author's ideas as to "the
+thickness of key" which "should be allowed" over tunnels, believing that
+conditions within an earth mass, except in very rare instances, are
+such that true arch action will seldom take place to any definite
+extent, through any considerable depths. Furthermore, the author's
+reason for bisecting the angle between the vertical and the angle of
+repose of the material, when he undertakes to determine the thickness of
+key, is not obvious. This assumption is shown to be absurd when carried
+to either limit, for when the angle of repose equals zero, as is the
+case with water, this, method would give a definite thickness of key,
+while there can be absolutely no arch action possible in such a case;
+and, when the angle of repose is 90°, as may be assumed in the case of
+rock, this method would give an infinite thickness of key, which is
+again seen to be absurd. It would seem as if altogether too many
+unknowable conditions had been assumed. In any case, no arch action can
+be brought into play until a certain amount of settlement has taken
+place so as to bring the particles into closer contact, and in such a
+way that the internal stresses are practically those only of
+compression, and the shearing stresses are within the limits possible
+for the material in question.
+
+The author has repeatedly made assumptions which are not borne out by
+the application of his mathematical formulas to actual extreme
+conditions. This method of application to limiting conditions is
+concededly sometimes faulty; but the writer believes that no earth
+pressure theory, or one concerning arch action, can be considered as
+satisfactory which does not apply equally well to hydraulic pressure
+problems when the proper assumptions are made as to the factors for
+friction, cohesion, etc. For example, when the angle of repose is
+considered as zero, in the author's first formula for _W_{1}_, the value
+becomes ½ _W_{1}_, whereas it should depend solely on the depth, which
+does not enter the formula, and not at all on the width of opening, _l_,
+which is thus included.
+
+The author has given no experiments to prove his statement that "the
+arch thrust is greater in dryer sand," and the accuracy of the statement
+is questioned. Again, no reason is apparent for assuming the direction
+of the "rakers" in Fig. 3 as that of the angle of repose. The writer
+cannot see why that particular angle is repeatedly used, when almost any
+other would give results of a similar kind. The author has made no
+experiments which show any connection between the angle of repose, as he
+interprets it, and the lines of arch action which he assumes to exist.
+
+With regard to the illustration of the condition which is thought to
+exist when the "material is composed of large bowling balls," supposedly
+all of the same size, the writer believes the conclusion to be
+erroneous, and that this can be readily seen by inspection of a diagram
+in which such balls are represented as forming a pile similar to the
+well-known "pile of shells" of the algebras, in the diagram of which a
+pile of three shells, resting on the base, has been omitted. It is then
+seen that unless the pressures at an angle of 60° with the horizontal
+are sufficient to produce frictional resistance of a very large amount,
+the balls will roll and instantly break the arch action suggested by the
+author. Consequently, an almost infinitesimal settlement of the
+"centering" may cause the complete destruction of an arch of earth.
+
+The author's logic is believed to be entirely faulty in many cases
+because he repeatedly makes assumptions which are not in accordance with
+demonstrated fact, and finally sums up the results by the statement: "It
+is conceded" (line 2, p. 357, for example), when the writer, for one,
+has not even conceded the accuracy of the assumptions. For instance, the
+author's well-known theory that pressures against retaining walls are a
+maximum at the top and decrease to zero at the bottom, is in absolute
+contradiction to the results of experiments conducted on a large scale
+by the writer on the new reinforced concrete retaining wall near the St.
+George Ferry, on Staten Island, New York City, which will soon be
+published, and in which the usual law of increase of lateral pressure
+with depth is believed to be demonstrated beyond question. It must be
+conceded that a considerable arch action (so-called) actually exists in
+many cases; but it should be equally conceded by the advocates of the
+existence of such action that changes in humidity, due to moving water,
+vibration, and appreciable viscosity, etc., will invariably destroy this
+action in time. In consequence, the author's reasoning in regard to the
+pressures against the faces of retaining walls is believed to be open to
+grave question as to accuracy of assumption, method, and conclusion.
+
+The author is correct in so far as he assumes that "the character of the
+stresses due to the thrust of the material will" not "change if bracing
+should be substituted for the material in the area" designated by him,
+etc., provided he makes the further assumption that absolutely no
+motion, however infinitesimal, has taken place meantime; but, unless
+such motion has actually taken place, no arch action can have developed.
+An arch thrust can result only with true arch action, that is, with
+stable abutments, and the mass stressed wholly in compression, with
+corresponding shortening of the arch line. The arch thrust must be
+proportional to the elastic deformation (shortening) of the arch line.
+If any such arch as is shown in Fig. 5 is assumed to carry the whole of
+the weight of material above it, that assumed arch must relieve all the
+assumed arches below. Therefore each of the assumed arches can carry
+nothing more than its own mass. Otherwise the resulting thrust would
+increase with the depth, which is opposed to the author's theory.
+
+Turning again to the condition that each arch can carry only its own
+weight: if these arches are assumed of thicknesses proportional to the
+distance upward from the bottom of the wall, they will be similar
+figures, and it is easily demonstrated that the thrust will then be
+uniform in amount throughout the whole height of the wall, except,
+perhaps, at the very top. This condition is contrary to the author's
+ideas and also to the facts as demonstrated by the writer's experiment
+on the 40-ft. retaining wall at St. George. Consequently, the author's
+statement: "nor can anyone * * * doubt that the top timbers are stressed
+more heavily than those at the bottom," is emphatically doubted and
+earnestly denied by the writer. Furthermore, "the assumption" made by
+the author as to "the tendency of the material to slide" so as to cause
+it "to wedge * * * between the face of the sheeting * * * and some plane
+between the sheeting and the plane of repose," is considered as
+absolutely unwarranted, and consequently the whole conclusion is
+believed to be unjustified. Nor is the author's assumption (line 5, p.
+361), that "the thrust * * * is measured by its weight divided by the
+tangent of the * * * angle of repose" at all obvious.
+
+The author presents some very interesting photographs showing the
+natural surface slopes of various materials; but it is interesting to
+note that he describes these slopes as having been produced by the
+"continual slipping down of particles." The vast difference between
+angles of repose produced in this manner by the rolling friction of
+particles and the internal angles of friction, which must be used in all
+earth-pressure investigations, has been repeatedly called to the
+attention of engineers by the writer.[H]
+
+The writer's experiments are entirely in accord with those of the author
+in which the latter claims to demonstrate that "earth and water
+pressures act independently of each other," and the writer is much
+delighted that his own experiments have been thus confirmed.
+
+In Experiment No. 3, the query is naturally suggested: "What would have
+been the result if the nuts and washers had first been tightened and
+water then added?" Although the writer has not tried the experiment, he
+is rather inclined to the idea that the arch would have collapsed. With
+regard to Experiment No. 5, there is to be noted an interesting
+possibility of its application to the theoretical discussion of masonry
+dams, in which films of water are assumed to exist beneath the structure
+or in crevices or cracks of capillary dimensions. The writer has always
+considered the assumptions made by many designing engineers as
+unnecessarily conservative. In regard to the author's conclusions from
+Experiment No. 6, it should be noted that no friction can exist between
+particles of sand and surrounding water unless there is a tendency of
+the latter to move; and that water in motion does not exert pressures
+equal to those produced when in a static condition, the reduction being
+proportional to the velocity of flow.
+
+The author's conclusion (p. 371), that "pressure will cause the
+quicksand to set up hydraulic action," does not seem to have been
+demonstrated by his experiments, but to be only his theory. In this
+instance, the results of the writer's experiments are contrary to the
+author's theory and conclusion.
+
+The writer will heartily add his protest to that of the author "against
+considering semi-aqueous masses, such as soupy sands, soft concrete,
+etc., as exerting hydrostatic pressure due to their weight in bulk,
+instead of to the specific gravity of the basic liquid." Again,
+similarly hearty concurrence is given to the author's statement:
+
+     "If the solid material in any liquid is agitated, so that it is
+     virtually in suspension, it cannot add to the pressure, and if
+     allowed to subside it acts as a solid, independently of the water
+     contained with it, although the water may change somewhat the
+     properties of the material, by increasing or changing its cohesion,
+     angle of repose, etc."
+
+On the other hand, it is believed that the author's statement, as to
+"the tendency of marbles to arch," a few lines above the one last
+quoted, should be qualified by the addition of the words, "only when a
+certain amount of deflection has taken place so as to bring the arch
+into action." Again, on the following page, a somewhat similar
+qualification should be added to the sentence referring to the soft clay
+arch, that it would "stand if the rods supporting the intrados of the
+arch were keyed back to washers covering a sufficiently large area," by
+inserting the words, "unless creeping pressures (such as those
+encountered by the writer in his experiments) were exceeded."
+
+The writer considers as very doubtful the formula for _D_{x}_, which is
+the same as that for _W_{1}_, already discussed. The author's statement
+that "additional back-fill will [under certain circumstances] lighten
+the load on the structure," is considered subject to modification by
+some such clause as the following, "the word 'lighten' here being
+understood to mean the reduction to some extent of what would be the
+total pressure due to the combined original and added back-fill,
+provided no arch action occurred."
+
+The writer is in entire agreement with the author as to the probability
+that water is often "cut off absolutely from its source of pressure,"
+with the attendant results described by the author (p. 378); and again,
+that too little attention has been given to the bearing power of soil,
+with the author's accompanying criticism.
+
+The writer cannot see, however, where the author's experiments
+demonstrate his statement "that pressure is transmitted laterally
+through ground, most probably along or nearly parallel to the angles of
+repose," or any of the conclusions drawn by him in the paragraph (p.
+381), which contains this questionable statement. Again the writer is at
+a loss as to how to interpret the statement that the author has found
+that "better resistance" has been offered by "small open caissons sunk
+to a depth of a few feet and cleaned out and filled with concrete" than
+by "spreading the foundation over four or five times the equivalent
+area." The writer agrees with the author in the majority of his
+statements as to the "bearing value and friction on piles," but believes
+that he is indulging in pure theory in some of his succeeding remarks,
+wherein he ascribes to arch action the results which he believes would
+be observed if "a long shaft be withdrawn vertically from moulding
+sand." These phenomena would be due rather to capillary action and the
+resulting cohesion.
+
+Naturally, the writer doubts the author's conclusions as to the pressure
+at the top of large square caisson shafts when he states that "the
+pressure at the top * * * will * * * increase proportionately to the
+depth." Again, the author is apparently not conversant with experiments
+made by the Dock Department of New York City, concerning piles driven in
+the Hudson River silt, which showed that a single heavily loaded pile
+carried downward with it other unloaded piles, driven considerable
+distances away, showing that it was not the pile which lacked in
+resistance, as much as the surrounding earth.
+
+In conclusion, the writer heartily concurs with the statement that "too
+much has been taken for granted in connection with earth pressures and
+resistance," and he is sorry to be forced to add that he believes the
+author to be open to the criticism which he himself suggests, that "both
+in experimenting and observing, the engineer [and in this case the
+author] will frequently find what is being looked for or expected and
+will fail to see the obvious alternative."
+
+
+FRANCIS L. PRUYN, M. AM. SOC. C. E. (by letter).--Mr. Meem
+should be congratulated, both in regard to the highly interesting
+theories which he advances on the subject of sand pressures--the
+pressures of subaqueous material--and on his interesting experiments in
+connection therewith.
+
+The experiment in which the plunger on the hydraulic ram is immersed in
+sand and covered with water does not seem to be conclusive. By this
+experiment the author attempts to demonstrate that the pressure of the
+water transmitted through the sand is only about 40% as great as when
+the sand is not there. The travel of ground-water through the earth is
+at times very slow, and occasionally only at the rate of from 2 to 3 ft.
+per hour. In the writer's opinion, Mr. Meem's experiment did not cover
+sufficient time during which the pressure was maintained at any given
+point. It is quite probable that it may take 15 or 20 min. for the full
+pressure to be transmitted through the sand to the bottom of the
+plunger, and it is hoped, therefore, that he will make further
+experiments lasting long enough to demonstrate this point.
+
+In regard to the question of skin friction on caissons and piles, it may
+be of interest to mention an experiment which the writer made during the
+sinking of the large caissons for the Williamsburg Bridge. These
+caissons were about 70 ft. long and 50 ft. wide. The river bottom was
+about 50 ft. below mean high water, and the caissons penetrated sand of
+good quality to a depth of from 90 to 100 ft. below that level. On two
+occasions calculations were made to determine the skin friction while
+the caissons were being settled. With the cutting edge from 20 to 30 ft.
+below the river bottom, the calculations showed that the skin friction
+was between 500 and 600 lb. per sq. ft. The writer agrees with Mr. Meem
+that, in the sinking of caissons, the arch action of sand is, in a great
+measure, destroyed by the compressed air which escapes under the cutting
+edge and percolates up through the material close to the sides of the
+caissons.
+
+With reference to the skin friction on piles, the writer agrees with Mr.
+Meem that in certain classes of material this is almost a negligible
+quantity. The writer has jacked down 9-in. pipes in various parts of New
+York City, and by placing a recording gauge on the hydraulic jack, the
+skin friction on the pile could be obtained very accurately. In several
+instances the gauge readings did not vary materially from the surface
+down to a penetration of 50 ft. In these instances the material inside
+the pipe was cleaned out to within 1 ft. of the bottom of the pile, so
+that the gauge reading indicated only the friction on the outside of the
+pipe plus the bearing value developed by its lower edge. For a 9-in.
+pipe, the skin friction on the pile plus the bearing area of the bottom
+of the pipe seems to be about 20 tons, irrespective of the depth. After
+the pipe had reached sufficient depth, it was concreted, and, after the
+concrete had set, the jack was again placed on it and gauge readings
+were taken. It was found that in ordinary sands the concreted steel pile
+would go down from 3 to 6 in., after which it would bring up to the full
+capacity of a 60-ton jack, showing, by gauge reading, a reaction of from
+70 to 80 tons.
+
+It is the writer's opinion that, in reasonably compact sands situated at
+a depth below the surface which will not allow of much lateral movement,
+a reaction of 100 tons per sq. ft. of area can be obtained without any
+difficulty whatever.
+
+
+FRANK H. CARTER, ASSOC. M. AM. SOC. C. E. (by letter).--Mr.
+Meem has contributed much that is of value, particularly on water
+pressures in sand; just what result would be obtained if coarse crushed
+stone or similar material were substituted for sand in Experiment No. 6,
+is not obvious.
+
+It has been the practice lately, among some engineers in Boston, as well
+as in New York City, to assume that water pressures on the underside of
+inverts is exerted on one-half the area only. The writer, however, has
+made it a practice first to lay a few inches of cracked stone on the
+bottom of wet excavations in order to keep water from concrete which is
+to be placed in the invert. In addition to the cracked stone under the
+inverts, shallow trenches dug laterally across the excavation to insure
+more perfect drainage, have been observed. Both these factors no doubt
+assist the free course of water in exerting pressure on the finished
+invert after the underdrains have been closed up on completion of the
+work. The writer, therefore, awaits with interest the repetition of
+Experiment No. 6, with water on the bottom of a piston buried in coarse
+gravel or cracked stone.
+
+As for the arching effect of sand, the writer believes that Mr. Meem has
+demonstrated an important principle, on a small scale. It must be
+regretted, however, that the box was not made larger, for, to the
+writer, it appears unsafe to draw such sweeping conclusions from small
+experiments. As small models of sailboats fail to develop completely
+laws for the design and control of large racing yachts, so experiments
+in small sand boxes may fail to demonstrate the laws governing actual
+pressures on full-sized structures.
+
+For some time the writer has been using a process of reasoning similar
+to that of the author for assumptions of earth pressure on the roofs of
+tunnel arches, except that the vertical forces assumed to hold up the
+weight of the earth have been ascribed to cohesion and friction, along
+what might be termed the sides of the "trench excavation."
+
+The writer fails to find proof in this paper of the author's statement
+that earth pressures on the sides of a structure buried in earth are
+greater at the top than at the bottom of a trench. That some banks are
+"top-heavy," is, no doubt, a fact, the writer having often heard similar
+expressions used by experienced trench foremen, but, in every case
+called to his attention, local circumstances have caused the
+top-heaviness, either undermining at the bottom of the trench, too much
+banked earth on top, or the earth excavated from the trench being too
+near the edge of the cut.
+
+For some years the writer has been making extended observations on deep
+trenches, and, thus far, has failed to find evidence, except in aqueous
+material, of earth pressures which might be expected from the known
+natural slope of the material after exposure to the elements; and this
+latter feature may explain why sheeted trenches stand so much better
+than expected. If air had free access to the material, cohesion would be
+destroyed, and theoretical pressures would be more easily developed.
+With closely-sheeted trenches, weathering is practically excluded, and
+the bracing, which seemingly is far too light, holds up the trench with
+scarcely a mark of pressure. As an instance, in 1893, the writer was
+successfully digging sewer trenches from 10 to 14 ft. deep, through
+gravel, in the central part of Connecticut, without bracing; because of
+demands of the work in another part of the city, a length of several
+hundred feet of trench was left open for three days, resulting in the
+caving-in of the sides. The elements had destroyed the cohesion, and the
+sides of the trenches no longer stood vertically.
+
+Recently, in the vicinity of Boston, trenches, 32 ft. wide, and from 25
+to 35 ft. deep, with heavy buildings on one side, have been braced with
+8 by 10-in. stringers, and bracers at 10-ft. centers longitudinally, and
+from 3 to 5 ft. apart vertically; this timbering apparently was too
+slight for pressures which, theoretically, might be expected from the
+natural slope of the material. Just what pressures develop on the sides
+of the structures in these deep trenches after pulling the top sheeting
+(the bottom sheeting being left in place) is, of course, a matter of
+conjecture. There can be no doubt that there is an arching of the
+material, as suggested by the author. How much this may be assisted by
+the practical non-disturbance of the virgin material is, of course,
+indeterminate. That substructures and retaining walls designed according
+to the Rankine or similar theories have an additional factor of safety
+from too generous an assumption in regard to earth pressure is
+practically admitted everywhere. It is almost an engineering axiom that
+retaining walls generally fail because of insufficient foundation only.
+
+For the foregoing reasons, and particularly from observations on the
+effect of earth pressures on wooden timbers used as bracing, the writer
+believes that, ordinarily, the theoretical earth pressures computed by
+Rankine and Coulomb are not realized by one-half, and sometimes not even
+by one-third or one-quarter in trenches well under-drained, rapidly
+excavated, and thoroughly braced.
+
+
+J.C. MEEM, M. AM. SOC. C. E. (by letter).--The writer has been
+much interested in this discussion, and believes that it will be of
+general value to the profession. It is unfortunate, however, that
+several of the points raised have been due to a careless reading of, or
+failure to understand, the paper.
+
+Taking up the discussion in detail, the writer will first answer the
+criticisms of Mr. Goodrich. He says:
+
+     "The writer believes that, in the design of permanent structures,
+     consideration of arch action should not be included, at least, not
+     until more information has been obtained. He also believes that the
+     design of temporary structures with this inclusion is actually
+     dangerous in some instances."
+
+If the arching action of earth exists, why should it not be recognized
+and considered? The design of timbering for a structure to rest, for
+instance, at a depth of from 200 to 300 ft. in normal dry earth, without
+considering this action, would be virtually prohibitive.
+
+Mr. Goodrich proceeds to show one of the dangers of considering such
+action by quoting the writer, as follows:
+
+     "About an hour after the superimposed load had been removed, the
+     writer jostled the box with his foot sufficiently to dislodge some
+     of the exposed sand, when the arch at once collapsed and the bottom
+     fell to the ground."
+
+He fails, as do so many other critics of this theory, to distinguish the
+difference between that portion of the sand which acts as so-called
+"centering" and that which goes to make up the sustaining arch. The
+dislodgment of any large portion of this "centering" naturally causes
+collapse, unless it is caught, in which case the void in the "centering"
+is filled from the material in the sustaining arch, and this, in turn,
+is filled from that above, and so on, until the stability of each arch
+is in turn finally established. This, however, does not mean that,
+during the process of establishing this equilibrium of the arch
+stresses, there is no arching action of any of the material above, but
+only that some of the so-called arches are temporarily sustained by
+those below. That is, in effect, each area of the material above
+becomes, in turn, a dependent, an independent, and finally an
+interdependent arch.
+
+If Mr. Goodrich's experience has led him to examine any large number of
+tunnel arches or brick sewers, he will have noted in many of them
+longitudinal cracks at the soffits of the arches and perhaps elsewhere.
+These result from three causes:
+
+_First._--In tunneling, there is more or less loss of material, while,
+in back-filling, the material does not at first reach its final
+compactness. Therefore, in adjusting itself to normal conditions, this
+material causes impact loads to come upon the green arch, and these tend
+to crack it.
+
+_Second._--No matter how tightly a brick or other arch is keyed in,
+there must always be some slight subsidence when the "centers" are
+struck. This, again, results in a shock, or impact loading, to the
+detriment of the arch.
+
+_Third._--The most prolific cause, however, is that in tunneling, as
+well as in back-filling open cuts, the material backing up the haunches
+is more or less loosened and therefore is not at first compact enough to
+prevent the spreading of the haunches when the load comes on the arch.
+This causes cracking, but, as soon as the haunches have been pressed out
+against the solid material, the cracking usually ceases, unless the
+pressure has been sufficiently heavy to cause collapse.
+
+An interesting example of this was noted in the Joralemon Street branch
+of the Rapid Transit Tunnel, in Brooklyn, in which a great many of the
+cast-iron rings were cracked under the crown of the arch, during
+construction; but, in spite of this, they sustained, for more than two
+years, a loading which, according to Mr. Goodrich, was continually
+increasing. In other words, the cracked arch sustained a greater loading
+than that which cracked the plates during construction, according to his
+theory, as noted in the following quotation:
+
+     "But it should be equally conceded by the advocates of the
+     existence of such action that changes in humidity, due to moving
+     water, vibration, and appreciable viscosity, etc., will invariably
+     destroy this action in time."
+
+As to the correctness of this theory Mr. Goodrich would probably have
+great difficulty in convincing naturalists, who are aware that many
+animals live in enlarged burrows the stability of which is dependent on
+the arching action of the earth; in fact, many of these burrows have
+entrances under water. He would also have some difficulty in convincing
+those experienced miners who, after a cave-in, always wait until the
+ground has settled and compacted itself before tunneling, usually with
+apparent safety, over the scene of the cave-in.
+
+The writer quotes as follows from Mr. Goodrich's discussion:
+
+     "In any case, no arch action can be brought into play until a
+     certain amount of settlement has taken place so as to bring the
+     particles into closer contact, and in such a way that the internal
+     stresses are practically those only of compression, and the
+     shearing stresses are within the limits possible for the material
+     in question."
+
+Further:
+
+     "Consequently, an almost infinitesimal settlement of the
+     'centering' may cause the complete destruction of an arch of
+     earth."
+
+And further:
+
+     "On the other hand, it is believed that the author's statement, as
+     to the 'tendency of marbles to arch,' * * * should be qualified by
+     the addition of the words, 'only when a certain amount of
+     deflection has taken place so as to bring the arch into action.'"
+
+In a large measure the writer agrees with the first and last quotations,
+but sees no reason to endorse the second, as it is impossible to
+consider any arch being built which does not settle slightly, at least,
+when the "centers" are struck.
+
+Regarding his criticism of the lack of arching action in balls or
+marbles, he seems to reason that the movement of the marbles would
+destroy the arch action. It is very difficult for the writer to conceive
+how it would be possible for balls or marbles to move when confined as
+they would be confined if the earth were composed of them instead of its
+present ingredients, and under the same conditions otherwise. Mr.
+Goodrich can demonstrate the correctness of the writer's theories,
+however, if he will repeat the writer's Experiment No. 3, with marbles,
+with buckshot, and with dry sand. He is also advised to make the
+experiment with sand and water, described by the writer, and is assured
+that, if he will see that the washers are absolutely tight before
+putting the water into the box, he can do this without bringing about
+the collapse of the arch; the only essential condition is that the
+bottom shall be keyed up tightly, so as not to allow the escape of any
+sand. He is also referred to the two photographs, Plate XXIV,
+illustrating the writer's first experiment, showing how increases in the
+loading resulted in compacting the material of the arch and in the
+consequent lowering of the false bottom. As long as the exposed sand
+above this false bottom had cohesion enough to prevent the collapse of
+the "centering," this arch could have been loaded with safety up to the
+limits of the compressive strength of the sand.
+
+To quote again from Mr. Goodrich:
+
+     "Furthermore, the author's reason for bisecting the angle between
+     the vertical and the angle of repose of the material, when he
+     undertakes to determine the thickness of key, is not obvious. This
+     assumption is shown to be absurd when carried to either limit, for
+     when the angle of repose equals zero, as is the case with water,
+     this method would give a definite thickness of key, while there can
+     be absolutely no arch action possible in such a case; and, when the
+     angle of repose is 90°, as may be assumed in the case of rock, this
+     method would give an infinite thickness of key, which is again seen
+     to be absurd."
+
+Mr. Goodrich assumes that water or liquid has an angle of repose equal
+to zero, which is true, but the writer's assumptions applied only to
+solid material, and the liquid gives an essentially different condition
+of pressure, as shown by a careful reading of the paper. In solid rock
+Mr. Goodrich assumes an angle of repose equal to 90°, for which there is
+no authority; that is, solid rock has no known angle of repose. In order
+to carry these assumptions to a definite conclusion, we must assume for
+that material with an angle of repose of 90° some solid material which
+has weight but no thrust, such as blocks of ice piled vertically. In
+this case Mr. Goodrich can readily see that there will be no arching
+action over the structure, and that the required thickness of key would
+be infinite. As to the other case, it is somewhat difficult to conceive
+of a solid with an angle of repose of zero; aqueous material does not
+fulfill this condition, as it is either a liquid or a combination of
+water and solid material. The best illustration, perhaps, would be to
+assume a material composed of iron filings, into which had been driven a
+powerful magnet, so that the iron filings would be drawn horizontally in
+one direction. It is easy to conceive, then, that in tunneling through
+this material there would be no necessity for holding up the roof; the
+definite thickness of key given, as being at the point of intersection
+of two 45° angles, would be merely a precautionary measure, and would
+not be required in practice.
+
+It is thus seen that both these conditions can be fulfilled with
+practical illustrations; that is, for an angle of repose of 90°, that
+material which has weight and no thrust, and for an angle of repose of
+zero, that solid material which has thrust but no weight.
+
+Mr. Goodrich says the author has given no experiments to prove his
+statement that the arch thrust is greater in dryer sand. If Mr. Goodrich
+will make the experiment partially described as Experiment No. 3, with
+absolutely dry sand, and with moist sand, and on a scale large enough
+to eliminate cohesion, he will probably find enough to convince him that
+in this assumption the writer is correct. At the same time, the writer
+has based his theory in this regard on facts which are not entirely
+conclusive, and his mind is open as to what future experiments on a
+large scale may develop. It is very probable, however, that an
+analytical and practical examination of the English experiments noted on
+pages 379 and 380, will be sufficient to develop this fact conclusively.
+
+The writer is forced to conclude that some of the criticisms by Mr.
+Goodrich result from a not too careful reading of the paper. For
+instance, he states:
+
+     "'It is conceded' (line 2, p. 357, for example) when the writer,
+     for one, has not even conceded the accuracy of the assumptions."
+
+A more careful reading would have shown Mr. Goodrich that this
+concession was one of the writer's as to certain pressures against or on
+tunnels, and, if Mr. Goodrich does not concede this, he is even more
+radical than the writer.
+
+And again:
+
+     "'Nor can anyone * * * doubt that the top timbers are stressed more
+     heavily than those at the bottom' is emphatically doubted and
+     earnestly denied by the writer."
+
+It is unfortunate that Mr. Goodrich failed to make the complete
+quotation, which reads:
+
+     "Nor can anyone, looking at Fig. 5, doubt," etc.
+
+A glance at Fig. 5 will demonstrate that, under conditions there set
+forth, the writer is probably correct in his assertion as relating to
+that particular instance. Further:
+
+     "For instance, the author's well-known theory that the pressures
+     against retaining walls are a maximum at the top and decrease to
+     zero at the bottom, is in absolute contradiction to the results of
+     experiments conducted on a large scale by the writer on the new
+     reinforced concrete retaining wall near the St. George Ferry, on
+     Staten Island."
+
+The writer's "well-known theory that pressures against retaining walls
+are a maximum at the top and decrease to zero at the bottom" applies
+only to pressures exerted by absolutely dry and normally dry material,
+and it seems to him that this so-called theory is capable of such easy
+demonstration, by the simple observation of any bracing in a deep trench
+in material of this class, that it ought to be accepted as at least
+safer than the old theory which it reverses. As to this "well-known
+theory" in material subject to water pressure, a careful reading of the
+paper, or an examination of Fig. 12 and its accompanying text, or an
+examination of Table 1, will convince Mr. Goodrich that, under the
+writer's analysis, this pressure does not decrease to zero at the
+bottom, but that in soft materials it may be approximately constant all
+the way down, while, in exceptionally soft material, conditions may
+arise where it may increase toward the bottom. The determination should
+be made by taking the solid material and drying it sufficiently so that
+water does not flow or seep from it. When this material is then
+compacted to the condition in which it would be in its natural state,
+its angle of repose may be measured, and may be found to be as high as
+60 degrees. The very fine matter should then be separated from the
+coarser material, and the latter weighed, to determine its proportion.
+Subtracting this from the total, the remainder could be credited to
+"aqueous matter." It is thus seen that with a material when partially
+dried in which the natural angle of repose might be 60°, and in which
+the percentage of water or aqueous matter when submerged might be 60%,
+there would be an increase of pressure toward the bottom.
+
+The writer does not know the exact nature of the experiments made at St.
+George's Ferry by Mr. Goodrich, but he supposes they were measurements
+of pressures on pistons through holes in the sheeting. He desires to
+state again that he cannot regard such experiments as conclusive, and
+believes that they are of comparative value only, as such experiments do
+not measure in any large degree the pressure of the solid material but
+only all or a portion of the so-called aqueous matter, that is, the
+liquid and very fine material which flows with it. Thus it is well known
+that, during the construction of the recent Hudson and North River
+Tunnels, pressures were tested in the silt, some of which showed that
+the silt exerted full hydrostatic pressure. At the same time, W.I. Aims,
+M. Am. Soc. C. E., stated in a public lecture, and recently also to the
+writer, that in 1890 he made some tests of the pressure of this silt in
+normal air for the late W.R. Hutton, M. Am. Soc. C. E. A hole, 12 in.
+square, was cut through the brickwork and the iron lining, just back of
+the lock in the north tube (in normal air), and about 1000 ft. from the
+New Jersey shore. It was found that the silt had become so firm that it
+did not flow into the opening. Later, a 4-in. collar and piston were
+built into the opening, and, during a period covering at least 3 months,
+constant observations showed that no pressure came upon it; in fact, it
+was stated that the piston was frequently worked back and forth to
+induce pressure, but no response was obtained during all this period.
+The conclusion must then be drawn that when construction, with its
+attendant disturbance, has stopped, the solid material surrounding
+structures tends to compact itself more or less, and solidify, according
+as it is more or less porous, forming in many instances what may be
+virtually a compact arch shutting off a large percentage of the normal,
+and some percentage even of the aqueous, pressure.
+
+That the pressure of normally dry material cannot be measured through
+small openings can be verified by any one who will examine such material
+back of bracing showing evidences of heavy pressure. The investigator
+will find that, if this material is free from water pressure, paper
+stuffed lightly into small openings will hold back indefinitely material
+which in large masses has frequently caused bracing to buckle and
+sheeting planks to bend and break; and the writer reiterates that such
+experiments should be made in trenches sheeted with horizontal sheeting
+bearing against short vertical rangers and braces giving horizontal
+sections absolutely detached and independent of each other. In no other
+way can such experiments be of real value (and even then only when made
+on a large scale) to determine conclusively the pressure of earth on
+trenches.
+
+As to the questions of the relative thrust of materials under various
+angles of repose, and of the necessity of dividing by the tangent, etc.;
+these, to the writer, seem to be merely the solution of problems in
+simple graphics.
+
+The writer believes that if Mr. Goodrich will make, even on a small
+scale, some of the experiments noted by the writer, he will be convinced
+that many of the assumptions which he cannot at present endorse are
+based on fact, and his co-operation will be welcomed with the greatest
+interest. Among the experiments which he is asked to make is the one in
+dry sand, noted as Experiment No. 3, whereby it can be shown very
+conclusively that additional back-fill will result in increased arching
+stability, on an arch which would collapse under lighter loading.
+
+The writer is indebted to Mr. Goodrich for pointing out some errors in
+omission and in typography (now corrected), and for his hearty
+concurrence in some of the assumptions which the writer believed would
+meet with greatest disapproval.
+
+In reply to Mr. Pruyn and Mr. Gregory, the writer assumed that the
+piston area in Experiment No. 6 should be reduced only by the actual
+contact of material with it. If this material in contact should be
+composed of theoretical spheres, resulting in a contact with points
+only, then the theoretical area reduced should be in proportion to this
+amount only. The writer does not believe, however, that this condition
+exists in practice, but thinks that the area is reduced very much more
+than by the actual theoretical contact of the material. He sees no
+reason, as far as he has gone, to doubt the accuracy of the deductions
+from this experiment.
+
+Regarding the question of the length of time required to raise the
+piston, he does not believe that the position of his critics is entirely
+correct in this matter; that is, it must either be conceded that the
+piston area is cut off from the source of pressure, or that it is in
+contact with it through more or less minute channels of water. If it is
+cut off, then the writer's contention is proved without the need of the
+experiment, and it is therefore conclusive that a submerged tunnel is
+not under aqueous pressure or the buoyant action of water. If, on the
+other hand, the water is in contact through channels bearing directly
+upon the piston and leading to the clear water chamber, any increase in
+pressure in the water chamber must necessarily result in a virtually
+instantaneous increase of the pressure against the piston, and therefore
+the action on the latter should follow almost immediately. In all cases
+during the experiments the piston did not respond until the pressure was
+approximately twice as great as required in clear water, therefore the
+writer must conclude either that the experiments proved it conclusively
+or that his assumption is proved without the necessity of the
+experiments. That is, the pressure is virtually not in evidence until
+the piston has commenced to move.
+
+Mr. Pruyn has added valuable information in his presentation of data
+obtained from specific tests of the bearing value of, and friction on,
+hollow steel piles. These data largely corroborate tests and
+observations by the writer, and are commended to general attention.
+
+Mr. Carter's information is also of special interest to the writer, as
+much of it is in the line of confirming his views. Mr. Carter does not
+yet accept the theory of increased pressure toward the top, but if he
+will examine or experiment with heavy bracing in deep trenches in clear
+sand, or material with well-defined angles of repose, he will probably
+find much to help him toward the acceptance of this view.
+
+The writer regrets that he has not now the means or appliances for
+further experiments with the piston chamber, but he does not believe
+that reliable results could be obtained in broken stone with so small a
+piston, as it is possible that the point of one stone only might be in
+contact with the piston. This would naturally leave the base exposed
+almost wholly to a clear water area. He does not believe, however, that
+in practice the laying of broken stone under inverts will materially
+change the ultimate pressure unless its cross-section represents a large
+area.
+
+Mr. Perry will find the following on page 369:
+
+     "It should be noted also that although the area subject to pressure
+     is diminished, the pressure on the area remaining corresponds to
+     the full hydrostatic head, as would be shown by the pressure on an
+     air gauge."
+
+This, of course, depends on the porosity of the material and the
+friction the water meets in passing through it.
+
+As to Mr. Thomson's discussion, the writer notes with regret two points:
+(_a_) that specific data are not given in many of the interesting cases
+of failures of certain structures or bracing; and (_b_), that he has
+not in all cases a clear understanding of the paper. For instance, the
+writer has not advocated the omission of bottom bracing or sheeting. He
+has seen many instances where it has been, or could have been, safely
+omitted, but he desires to make it clear that he does not under any
+circumstances advocate its omission in good work; but only that, in
+well-designed bracing, its strength may be decreased as it approaches
+the bottom.
+
+Reference is again made to the diagram, Fig. 12, which shows that, in
+most cases of coffer-dams in combined aqueous and earth pressure, there
+may be nearly equal, and in some cases even greater, loading toward the
+bottom.
+
+The writer also specifically states that in air the difference between
+aqueous and earth pressure is plainly noted by the fact that bracing is
+needed so frequently to hold back the earth while the air is keeping out
+the water.
+
+The lack of specific data is especially noticeable in the account of the
+rise of the 6-ft. conduit at Toronto. It would be of great interest to
+know with certainly the weight of the pipe per foot, and whether it was
+properly bedded and properly back-filled. In all probability the
+back-filling over certain areas was not properly done, and as the pipe
+was exposed to an upward pressure of nearly 1600 lb. per ft., with
+probably only 500 or 600 lb. of weight to counterbalance it, it can
+readily be seen that it did not conform with the writer's general
+suggestion, that structures not compactly, or only partially, buried,
+should have a large factor of safety against the upward pressure.
+Opposed to Mr. Thomson's experience in this instance is the fact that
+oftentimes the tunnels under the East River approached very close to the
+surface, with the material above them so soupy (owing to the escape of
+compressed air) that their upper surfaces were temporarily in water, yet
+there was no instance in which they rose, although some of them were
+under excessive buoyant pressure.
+
+It is also of interest to note, from the papers descriptive of the North
+River Tunnel, that, with shield doors closed, the shield tended to rise,
+while by opening the doors to take in muck the shield could be brought
+down or kept down. The writer concurs with those who believe that the
+rising of the shield with closed doors was due to the slightly greater
+density of the material below, and was not in any way due to buoyancy.
+
+Concerning the collapse of the bracing in the tunnel built under a
+side-hill, the writer believes it was due to the fact that it was under
+a sliding side-hill, and that, if it had been possible to have
+back-filled over and above this tunnel to a very large extent, this
+back-fill would have resulted in checking the sliding of material
+against the tunnel, and the work would thereafter have been done with
+safety. This is corroborated by Mr. Thomson's statement that the tunnel
+was subsequently carried through safely by going farther into the hill.
+
+As to the angle of repose, Mr. Thomson seems to feel that its
+determination is so often impracticable that it is not to be relied on;
+and yet all calculations pertaining to earth pressure must be based on
+this factor. The writer believes that the angle of repose is not
+difficult to determine, and that observations of, and experiments on,
+exposed banks in similar material, and general experience in relation
+thereto, will enable one to determine it in nearly all cases within such
+reasonably accurate limits that only a small margin of safety need be
+added.
+
+Engineers are sent to Europe to study sewage disposal, water
+purification, transit problems, etc., but are rarely sent to an
+adjoining county or State to look at an exposed bank, which would
+perhaps solve a vexed problem in bracing and result in great economy in
+the design of permanent structures.
+
+Mr. Thomson's general views seem to indicate that much of the subject
+matter noted in the paper relates to unsolvable problems, for it appears
+that in many cases he believes the Engineer to be dependent on his
+educated guess, backed perhaps by the experienced guess of the foreman
+or practical man. The writer, on the contrary, believes that every
+problem relating to work of this class is capable of being solved,
+within reasonably accurate limits, and that the time is not far distant
+when the engineer, with his study of conditions, and samples of material
+before him, will be able to solve his earth pressure and earth
+resistance problems as accurately as the bridge engineer, with his
+knowledge of structural materials, solves bridge problems.
+
+The writer, in the course of his experience, has met with or been
+interested in the solution of many problems similar to the following:
+
+What difference in timbering should be made for a tunnel in ordinary,
+normally dry ground at a depth of 20 ft. to the roof, as compared with
+one at a depth of 90 ft.?
+
+What difference in timbering or in permanent design should be made for a
+horizontally-sheeted shaft, 5 ft. square, going to a depth of 45 ft. and
+one 25 by 70 ft., for instance, going to the same depth, assuming each
+to be braced and sheeted horizontally with independent bracing?
+
+What allowance should be made for the strength of interlock, assuming
+that a circular bulkhead of sand, 30 ft. in diameter, is to be carried
+by steel sheet-piling exposed around the outside for a depth of 40 ft.?
+
+What average pressure per square foot of area should be required to
+drive a section of a 3 by 15-ft. roof shield, as compared with the
+pressure needed to drive the whole roof shield with an area four times
+as great?
+
+To what depth could a 12 by 12-in. timber be driven, under gradually
+added pressure, up to 60 tons, for instance, in normal sand?
+
+What frictional resistance should be assumed on a hollow, steel,
+smooth-bore pile which had been driven through sharp sand and had
+penetrated soft, marshy material the bearing resistance of which was
+practically valueless?
+
+What allowance should be made for the buoyancy of a tunnel 20 ft. in
+diameter, the top of which was buried to a depth of 20 ft. in sand above
+which there was 40 ft. of water?
+
+It is believed by the writer that most of the authorities are silent as
+to the solution of problems similar to the above, and it is because of
+this lack of available data that he has directed his studies to them.
+The belief that the results of these studies, together with such
+observations and experiments as relate thereto, may be of interest, has
+caused him to set them forth in this paper.
+
+He desires to state his belief that if problems similar to the above
+were given for definite solution, not based on ordinary safe practice,
+and without conference, to a number of engineers prominently interested
+in such matters, the results would vary so widely as to convince some of
+the critics of this paper that the greater danger lies rather in the
+non-exploration of such fields than in the setting forth of results of
+exploration which may appear to be somewhat radical.
+
+Further, if these views result in stimulating enough interest to lead to
+the hope that eventually the "Pressure, Resistance, and Stability" of
+ground under varying conditions will be known within reasonably accurate
+limits and tabulated, the writer will feel that his efforts have not
+been in vain.
+
+
+FOOTNOTES:
+
+[Footnote H: "Lateral Earth Pressures and Related Phenomena,"
+_Transactions_, Am. Soc. C. E., Vol. LIII, p. 272.]
+
+
+
+
+
+End of the Project Gutenberg EBook of Pressure, Resistance, and Stability of
+Earth, by J. C. Meem
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