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, ANDJ. C. MEEM. 

In the final discussion of the writer's paper, "The Bracing of Trenchesand Tunnels, With Practical Formulas for Earth Pressures, "[B] certainminor experiments were noted in connection with the arching propertiesof sand. In the present paper it is proposed to take up again thequestion of earth pressures, but in more detail, and to note somefurther experiments and deductions therefrom, and also to consider theresistance and stability of earth as applied to piling and foundations, and the pressure on and buoyancy of subaqueous structures in softground. 

In order to make this paper complete in itself, it will be necessary, insome instances, to include in substance some of the matter of the formerpaper, and indulgence is asked from those readers who may note thisfact. 

[Illustration: FIG. 1. SECTIONS OF BOX-FRAME FOR SAND-ARCHEXPERIMENT]

_Experiment No. 1. _--As the sand-box experiments described in the formerpaper were on a small scale, exception might be taken to them, andtherefore the writer has made this experiment on a scale sufficientlylarge 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 filledsolidly with sand. Wooden walls, 3 ft. Apart and 4 ft. High, were thenbuilt crossing the abutments, and solidly cleated and braced frames wereplaced across their ends about 2 ft. Back of each abutment. A falsebottom, made to slide freely up and down between the abutments, andprojecting slightly beyond the walls on each side, was then blocked upsnugly to the bottom edges of the sides, thus obtaining a box 3 by 4 by7 ft. , the last dimension not being important. Bolts, 44 in. Long, withlong threads, were run up through the false bottom and through 6 by 15by 2-in. Pine washers to nuts on the top. The box was filled withordinary coarse sand from the trench, the sand being compacted asthoroughly as possible. The ends were tightened down on the washers, which in turn bore on the compacted sand. The blocking was then knockedout from under the false bottom, and the following was noted:

As soon as the blocking was removed the bottom settled nearly 2 in. , asnoted in Fig. 1, Plate XXIV, due to the initial compacting of the sandunder the arching stresses. A measurement was taken from the bottom ofthe 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 beenbroken, it was decided to test it under greater loading, and four menwere placed on it, four others standing on the haunches, as shown inFig. 2, Plate XXIV. Under this additional loading of about 600 lb. Thebottom settled 2 in. More, or nearly 4 in. In all, due to the furthercompression of the sand arch. About an hour after the superimposed loadhad been removed, the writer jostled the box with his foot sufficientlyto dislodge some of the exposed sand, when the arch at once collapsedand the bottom fell to the ground. 

Referring to Fig. 2, if, instead of being ordinary sand, the blockcomprised within the area, _A U J V X_, had been frozen sand, there canbe no reason to suppose that it would not have sustained itself, forminga perfect arch, with all material removed below the line, _V E J_, infact, the freezing process of tunneling in soft ground is based on thiswell-known principle. 

[Illustration: FIG. 2. ]

[Illustration: FIG. 3. ]

If, then, instead of removing the mass, _J E V_, it is allowed to remainand is supported from the mass above, one must concede to this mass inits normal state the same arching properties it would have had iffrozen, excepting, of course, that a greater thickness of key should beallowed, to offset a greater tendency to compression in moist and dry asagainst 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 assumedthat _A J_, the line bisecting the angle between that of repose and theperpendicular, measures at its intersection with the middle vertical(_A_, Fig. 2) the height which is necessary to give a sufficientthickness of key, it may be concluded that this sand arch will beself-sustaining. That is, it is assumed that the arching effect is takenup virtually within the limits of the area, _A N_{1} V E J N A_, thusrelieving the structure below of the stresses due to the weight orthrust of any of the material above; and that the portion of thematerial below _V E J_ is probably dead weight on any structureunderneath, and when sustained from below forms a natural "centering"for the natural arch above. It is also probably true that the materialin the areas, _X N_{1} A_ and _A N U_, does not add to the archingstrength, more especially in those materials where cohesion may not becounted on as a factor. This is borne out by the fact that, in theexperiment noted, a well-defined crack developed on the surface of thesand at about the point _U_{1}_, and extended apparently a considerabledepth, assumed to be at _N_, where the haunch line is intersected by theslope line from _A_. 

[Illustration: PLATE XXIV, FIG. 1. --INITIAL SETTLEMENT IN 3-FT. SANDARCH, DUE TO COMPRESSION OF MATERIAL ON REMOVING SUPPORTS FROMBOTTOM. ]

[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 thrownloosely in a heap, it assumed an angle of repose of about 45 degrees. Itshould be noted that this material when tested was not compacted asmuch, nor did it possess the same cohesion, as sand in its normalundisturbed condition in a bank, and for this reason it is believed thatthe depth of key given here is absolutely safe for all exceptextraordinary conditions, such as non-homogeneous material and otherswhich may require special consideration. 

Referring again to the area, _A N_{1} V J N A_, Fig. 2, it is probablethat, while self-sustaining, some at least of the lower portion mustderive its initial support from the "centering" below, and the writerhas made the arbitrary assumption that the lower half of it is carriedby the structure while the upper half is entirely independent of it, and, in making this assumption, he believes he is adding a factor ofsafety thereto. The area, then, which is assumed to be carried by anunderground structure the depth of which is sufficient to allow thelines, _V A_ and _J A_, to intersect below the surface, is the lowerhalf of _A N_{1} V E J N A_, or its equivalent, _A V E J A_, plus thearea, _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 andparallel to the lines of repose of their natural slope, and that thethrust of the earth will therefore be measured by the relation betweenthe radius and the tangent of this angle multiplied by the weight ofmaterial affected. The dead weight on a plane, _V J_, due to thematerial 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 verysimple, except that the question of side thrust should be consideredalso as a factor. The thrust against the side of a tunnel in dry sandhaving a flat angle of repose will necessarily be greater than in verymoist sand or clay, which stands at a much steeper angle, and, for thesame reason, the arch thrust is greater in dryer sand and therefore theload on a tunnel structure should not be as great, the material beingcompact and excluding cohesion as a factor. This can be illustrated byreferring to Fig. 3 in which it is seen that the flatter the position ofthe "rakers" keying at _W_{1}_, _W_{2}_, and _W_, the greater will bethe side thrust at _A_, _C_, and _F_. It can also be illustrated byassuming that the arching material is composed of cubes of polishedmarble set one vertically above the other in close columns. There wouldthen be absolutely no side thrust, but, likewise, no arching propertieswould be developed, and an indefinite height would probably be reachedabove the tunnel roof before friction enough would be developed to causeit to relieve the structure of any part of its load. Conversely, if itbe assumed that the superadjacent material is composed of large bowlingballs, interlocking with some degree of regularity, it can be seen thatthose above will form themselves into an arch over the "centering" madeup of those supported directly by the roof of the structure, thusrelieving 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 measuresthe 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 bereduced 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 diminishedthe loading above. The writer makes the arbitrary assumption that thisdecrease in _G F_ should equal 20% of _B C_ = _F D_{1}_. If, then, theline, _B D_{1}_ be drawn, it is conceded that all the material withinthe area, _A B D_{1} G C A_, causes direct pressure against or upon thestructure, _G C A_, the vertical lines being the ordinates of pressuredue to weight, and the horizontal lines (qualified by certain ratios)being the abscissas of pressure due to thrust. An extreme measurement ofthis area of pressure is doubtless approximately more nearly a curvethan the straight lines given, and the curve, _A R T I D_{II}_, istherefore drawn in to give graphically and approximately the safe areaof which any vertical ordinate, multiplied by the weight, gives thepressure on the roof at that point, and any horizontal line, orabscissa, divided by the tangent of the angle of repose and multipliedby 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 materialin the area, _F E C B B_{1}_, forms with the equivalent opposite area anarch reacting against the face, _C B B_{1}_ and that, as heretoforenoted, the lower half (or its equivalent, _B D_{1} G B_) of the weightof this is assumed to be carried by the structure, the upper half beingself-sustaining, as shown by the line, _B_{III} D_{IV}_ (or, forabsolute safety, the curved line), and therefore, if rods could be runfrom sheeting inside the tunnel area to a point outside the line, _FB_{1}_, as indicated by the lines, 5, 6, 7, 8, 11, 12, 13, etc. , thatthe internal bracing of this tunnel could be omitted, or that the tunnelitself would be relieved of all loading, whereas these rods would becarrying some large portion at least of the weight within the areacircumscribed by the curve, _D_{II} I T G_, and further, that a tunnelstructure of the approximate dimensions shown would carry its maximumload with the surface of the ground between _D_{IV}_ and _F_, beyondwhich point the pressure would remain the same for all depths. 

In calculating pressures on circular arches, the arched area shouldfirst be graphically resolved into a rectangular equivalent, as in theright half of Fig. 4, proceeding subsequently as noted. 

The following instances are given as partial evidence that in ordinaryground, not submerged, the pressures do not exceed in any instance thosefound by the above methods, and it is very probable that similarinstances or experiences have been met by every engineer engaged insoft-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 usedfor more than 4, 000 ft. , or for two-thirds of the whole 5, 800 ft. Calledfor in the contract. The external width of opening, measured at thewall-plate, averaged about 19 ft. For the 14½-ft. Circular sewer and 19½ft. For the 15-ft. Sewer. The arch timber segments in the cross-sectionwere 10 by 12-in. North Carolina pine of good grade, with 2 in. Off thebutt for a bearing to take up the thrust. They were set 5 ft. Apart oncenters, and rested on 6 by 12-in. Wall-plates of the same material asnoted above. The ultimate strength of this material, across the grain, when dry and in good condition, as given by the United States ForestryDepartment tests is about 1, 000 lb. In compression. Some tests[C] madein 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 yieldsof ¼ in. Were noted at from 600 to 1, 000 lb. As the tunnel wall-platesdescribed in this paper were subject to occasional saturation, andalways to a moist atmosphere, they could never have been considered asequal to dry material. Had the full loading shown by the foregoingcome on these wall-plates, they would have been subjected to a stress ofabout 25 tons each, or nearly one-half of their ultimate strength. Inonly one or two instances, covering stretches of 100 ft. In one case and200 ft. In another, where there were large areas of quicksand sufficientto cause semi-aqueous pressure, or pockets of the same material causingeccentric loading, did these wall-plates show any signs of heavypressure, and in many instances they were in such good condition thatthey could be taken out and used a second and a third time. Twoespecially interesting instances came under the writer's observation: Inone case, due to a collapse of the internal bracing, the load of anentire section, 25 ft. Long and 19 ft. Wide, was carried for severalhours on ribs spaced 5 ft. Apart. The minimum cross-section of theseribs was 73 sq. In. , and they were under a stress, as noted above, of50, 000 lb. , or nearly up to the actual limit of strength of thewall-plate where the rib bore on it. When these wall-plates wereexamined, after replacing the internal bracing, they did not appear tohave been under any unusual stress. 

[Illustration: PLATE XXV, FIG. 1. --NORMAL SLOPES AND STRATAOF NEWLY EXCAVATED BANKS. ]

[Illustration: PLATE XXV, FIG. 2. --NORMAL SLOPES AND STRATAOF NEWLY EXCAVATED BANKS. ]

In another instance, for a distance of more than 700 ft. , the sub-gradeof the sewer was 4 ft. Below the level of the water in sharp sand. Inexcavating for "bottoms" the water had to be pumped at the rate of morethan 300 gal. Per min. , and it was necessary to close-sheet a trenchbetween the wall-plates in which to place a section of "bottom. " Inspite of the utmost care, some ground was necessarily lost, and this wasshown by the slight subsidence of the wall-plates and a loosening up ofthe wedges in the supports bearing on the arch timbers. During thisoperation of "bottoming, " two men on each side were constantly employedin tightening up wedges and shims above the arch timbers. It isimpossible to explain the fact that these timbers slackened (withoutproportionate roof settlement) by any other theory than that the archingwas so nearly perfect that it relieved the bracing of a large part ofthe load, the ordinary loose material being held in place by the archingor 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 roofwas coarse, sharp sand, through which it had been difficult to tunnelwithout losing ground, and it had admitted water freely after each rainuntil the drainage of a neighboring pond had been completed, the mennever being willing to resume work until the influx of water hadstopped. 

The foregoing applies only to material ordinarily found under ground notsubaqueous, or which cannot be classed as aqueous or semi-aqueousmaterial. These conditions will be noted later. 

[Illustration: FIG. 5. ]

[Illustration: FIG. 6. ]

The writer will take up next the question of pressures against the facesof sheeted trenches or retaining walls, in material of the samecharacter as noted above. Referring to Fig. 2, it is not reasonable tosuppose that having passed the line, _R F J_, the character of thestresses due to the thrust of the material will change, if bracingshould be substituted for the material in the area, _W V J R_, or if, asin Fig. 3, canvas is rolled down along the lines, _E G_ and _A O_, andif, as this section is excavated between the canvas faces, temporarystruts are erected, there is no reason to believe that with properlyadjusted weights at _W_ or _W_{2}_, an exact equilibrium of forces andconditions 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 intheory or practice, nor can anyone, looking at Fig. 5, doubt that thetop timbers are stressed more heavily than those at the bottom. Theassumption is that the tendency of the material to slide toward the toecauses it to wedge itself between the face of the sheeting on the onehand and some plane between the sheeting and the plane of repose on theother, and that the resistance to this tendency will cause an archingthrust 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 curvesapproximating thereto. As the thrust is greatest in that materialdirectly at the face, _A O_, Fig. 6, and is nothing at the plane ofrepose, _C O_, it may be assumed arbitrarily that the line, _B O_, bisecting this angle divides this area into two, in one of which theweight resolves itself wholly into thrust, the other being an area of nothrust, or wholly of weight bearing on the plane of repose. Calling thisline, _B O_, the haunch line, the thrust in the area, _A O B_, ismeasured by its weight divided by the tangent of the angle, _P Q R_ = [phi], which is the angle of repose; that is, the thrust atany given point, _R_ = _R Q_ ÷ tan. [phi]. 

The writer suggests that, in those materials which have steeper anglesof repose than 45°, the area of pressure may be calculated as above, thethrust being computed, however, as for an angle of 45 degrees. 

In calculating the bending moment against a wall or bracing, there isthe weight of the mass multiplied by the distance of its center ofgravity vertically above the toe, or, approximately:

 2Area, _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_ 2then _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 bereduced 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 andsand, which, standing at a general angle of 45°, were in process of"working, " that is, there was continual slipping down of particles ofthe sand, and it may be well to note that in time, under exposure toweather conditions, these banks would finally assume a slope of about 33degrees. They are typical, however, as showing the normal slope offreshly excavated sandy material, and a slope which may be used inordinary calculations. The steps seen in Plate XXV show the differentcharacteristics 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-definedslope of sand adjacent to and in connection with the cohesive propertiesof gravel. 

The next points to be considered are the more difficult problemsconcerning subaqueous or saturated earths. The writer has made someexperiments which appear to be conclusive, showing that, except in purequicksand or wholly aqueous material, as described later, the earth andwater pressures act independently of each other. 

For a better understanding of the scope and purpose of this paper, thewriter divides supersaturated or subaqueous materials into threeclasses:

_Class A. _--Firm materials, such as coarse and fine gravels, gravel andsands mixed, coarse sands, and fine sands in which there is not a largeproportion of fine material, such as loam, clay, or pure quicksand. 

_Class B. _--Semi-aqueous materials, such as fine sands in which there isa large proportion of clay, etc. , pure clays, silts, peats, etc. 

_Class C. _--Aqueous materials, such as pure quicksands, in which thesolid matter is so finely divided that it is amorphous and virtuallyheld in suspension, oils, quicksilver, etc. 

Here it may be stated that the term, "quicksand, " is so illusive that atrue definition of it is badly needed. Many engineers call quicksand anysand which flows under the influence of water in motion. The writerbelieves the term should be applied only to material so "soupy" that itsproperties are practically the same as water under static conditions, itbeing understood that any material may be unstable under the influenceof water at sufficiently high velocities, and that it is with a staticcondition, or one approximately so, that this paper deals. 

A clear understanding of the firm materials noted in Class A will leadto a better solution of problems dealing with those under Class B, as itis to this Class A that the experiments largely relate. 

The experiments noted below were made with varying material, though theprincipal type used was a fine sand, under the conditions in which it isordinarily found in excavations, with less than 40% voids and less than10% of very fine material. 

[Illustration: FIG. 7. ]

_Experiment No. 2. _--The first of these experiments, which in thisseries will be called No. 2, was simple, and was made in order to showthat this material does not flow readily under ordinary conditions, whennot coupled with the discharge of water under high velocity. A bucket 12in. 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 waspartly filled with water. The inside bucket was then given a falsebottom and partly filled with wet sand, resting on the sand in thelarger 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 bucketin the water, the distance of the weight, _W_, from the pivot beingnoted. The false bottom was then removed and the inside bucket, restingon the sand in the larger one, was partly filled with sand and both werefilled with water, the conditions at the point of weighing being exactlythe same, except that the false bottom was removed, leaving the sand incontact through the 6 by 6-in. Opening. It is readily seen that, if thesand had possessed the aqueous properties sometimes attributed to sandunder water, that in the inside bucket would have flowed out through thesquare hole in the bottom, allowing it to be lifted by any weight inexcess of the actual weight of the bucket, less its buoyancy, as wouldbe the case if it contained only water instead of sand and water. It wasfound, however, that the weight, resting at a distance of more thannine-tenths of the original distance from the pivot, would not raise theinside bucket. On lifting this inside bucket bodily, however, the waterat once forced the sand out through the bottom, leaving a hole almostexactly 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 inin small handfuls and thoroughly mixed with water, but not packed, andallowed to stand for some time before the experiments were tried, toinsure the compactness of ordinary conditions. It is seen from Fig. 1, Plate XXVII, that the sand was stable enough to allow the bucket to beput on its side for the moment of being photographed, although it hadbeen pulled out of the water a little less than 3 min. 

[Illustration: PLATE XXVI, FIG. 1. --TYPES OF ARCH TIMBERS USED INBAY 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 ofsand are not destroyed under subaqueous conditions, a small sand-box, having a capacity of about 1 cu. Ft. , and similar to that described inExperiment No. 1, was made. The bottom was cut out, with the exceptionof a ¾-in. Projection on two sides, and a false bottom was placed belowand outside of the original bottom, with bolts running through it, keying to washers on top of the sand, with which the box was partlyfilled. One side of the box contained a glass front, in order thatconditions of saturation could be observed. The box of sand was thenfilled with water and, after saturation had been completed and the nutsand 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 destroyedby the saturation of the sand. 

The next three experiments were intended to show the relative pressureover any given area in contact with the water in the one case or sandand water in the other. 

[Illustration: FIG. 8. ]

_Experiment No. 4. _--The apparatus for this experiment consisted of a3-in. Pipe about 4-in. Long and connected with a ¾-in. Goose-neck pipe17 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 upperface of the machined edge of the bowl and weighted down sufficiently tobalance it against a head of water corresponding to the 17-in. Head inthe goose-neck. The bowl was then to be filled with sand and thedifference, if any, noted between the weight required to hold theflap-valve down under the same head of water flowing through the sand. The results of this experiment were not conclusive, owing to thedifficulty of making contact over the whole area of the sand and the rimof 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. Wouldbe required as opposed to the 4½ lb. Approximate pressure required tohold down the clear water. Again, at times the water would not flowthrough the neck at all, even after several hours, and after increasingthe head by attaching a longer rubber tube thereto. In view of theseconditions, this experiment would not be noted here, except that itunexpectedly developed one interesting fact. In order to insure againsta stoppage of water, as above referred to, gravel was first put into thebottom of the bowl and the flap-valve was then rubbed down and heldtightly while the pipe was filled. On being released, the pressure ofwater 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 eachcase, and the experiment was repeated numberless times, and invariablywith the same result. The sand contained about 40% of voids. Thededuction from this experiment is that the pressure of water is againstrather than through sand and that any excess of voids occurring adjacentto a face against which there is pressure of water will be filled withsand, excepting in so far, of course, as the normal existing voids allowthe pressure of the water to be transmitted through them. 

[Illustration: PLATE XXVII, FIG. 1. --EXPERIMENT SHOWING PROPERTIESOF SAND. ]

[Illustration: PLATE XXVII, FIG. 2. --SAND PUSHED UP FROM BOWL BYWATER PRESSURE THROUGH GOOSE-NECK. ]

If, then, the covering of sand over a structure is sufficiently heavy toallow arching action to be set up, the structure against which thepressure is applied must be relieved of much of the pressure of wateragainst the area of sand not constituted as voids acting outside of thearching area. This is confirmed by the two following experiments:

_Experiment No. 5. _--The same apparatus was used here as in ExperimentNo. 2, Fig. 7, except that the inside bucket had a solid bottom. Theinside and outside buckets were filled with water and the point wasnoted at which the weight would balance the inside bucket at a pointsome 3 in. Off the bottom of the outside bucket. This point wasmeasured, and the bottom of the larger bucket was covered over with sandso that in setting solidly in the sand the inside bucket would occupythe same relative position as it did in the water. The same weight wasthen 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 thepivot, while in the next step the weight, standing at the end of thebar, had no effect, and considerable external pressure had to be exertedbefore the bucket could be lifted. Immediately after it was relieved, however, the weight at 12 in. Would hold it clear of the sand. Noattempt was made to work the bucket into the sand; the sand was leveledup and the bucket was seated on it, turned once or twice to insurecontact, and then allowed to stand for some time before making theexperiment. No attempt was made to establish the relationship betweensands of varying voids, the general fact only being established, by asufficient number of experiments, that the weight required to lift thebucket was more than double in sand having 40% of voids than thatrequired to lift the bucket in water only. 

[Illustration: FIG. 9. ]

_Experiment No. 6. _--The apparatus for this experiment consistedessentially of a hydraulic chamber about 8 in. In diameter and 1 ft. High, the top being removable and containing a collar with suitablepacking, through which a 2½-in. Piston moved freely up and down, thewhole being similar to the cylinder and piston of a large hydraulicjack, as shown in Fig. 1, Plate XXVIII. Just below the collar and abovethe chamber there was a ½-in. Inlet leading to a copper pipe and thenceto a high-pressure pump. Attached to this there was a gauge to show thepressure obtained in the chamber, all as shown in Fig. 9. The purpose ofthe apparatus was to test the difference in pressure on any objectsubmerged in clear water and on the same object buried in the sand underwater. It is readily seen that, if pressure be applied to the water inthis chamber, the amount of pressure (as measured by the gauge)necessary to lift the piston will be that due to the weight of thepiston, less its displacement, plus the friction of the piston in thecollar. 

[Illustration: PLATE XXVIII, FIG. 1. --APPARATUS FOR MEASURING LOSSOF 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 thewater pressure acts be reduced, it will necessarily require aproportionate amount of increase in the pressure to lift this piston. If, therefore, it is found that 10 lb. , for illustration, be required tolift the piston when plunged in clear water, and 20 lb. Be required tolift it when buried in sand, it can be assumed at once that the area ofthe piston has been reduced 50% by being buried in the sand, eliminatingthe question of the friction of the sand itself around the piston. Inorder to determine what this friction might be, the writer arranged atable standing on legs above the bottom of the chamber, allowing thepiston to move freely through a hole in its center. Through this tablepipes were entered (as shown in part of Fig. 9). The whole was thenplaced in the chamber with the piston in place, and the area above wasfilled with sand and water. It is thus seen that, the end of the pistonbeing free and in clear water, the difference, if any, between thepressure required to lift the piston when in clear water alone and inthe case thus noted, where it was surrounded by sand, would measure thefriction of the sand on the piston. After several trials of this, however, it was clearly seen that the friction was too slight to benoted accurately by a gauge registering single pounds, that is, with apiston 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. Itis therefore assumed that the friction on so small a piston in sand neednot be considered as a material factor in the experiments made. 

The piston was plunged into clear water, and it was found that thepressure 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 depthof some 6 in. , water being added so that the sand was completelysaturated. This was allowed to stand until it had regained the stabilityof ordinary sand in place, whereupon the cap with the collar bearing wasset in place over the piston, the machine was coupled up, and the pumpwas started. A series of four experiments, extending over a period oftwo or three days, gave the following results:

_Test 1. _--The piston began to move at a pressure of 25 lb. The pressuregradually dropped to 7½ lb. , at which point, apparently, it came out ofthe sand, and continued at 7½ lb. During the remainder of the test. 

_Test 2. _--The piston was plunged back into the sand, without removingthe cap, and allowed to stand for about 2 hours. No attempt was made topack the sand or to see its condition around the piston, it beingpresumed, however, that it had reasonable time to get a fair amount ofset. At slightly above 20 lb. The piston began to move, and as soon as apocket of water accumulated behind the piston the pressure immediatelydropped to 9 lb. And continued at this point until it came out of thesand. 

_Test 3. _--The piston was plunged into the sand and hammered downwithout waiting for the sand to come to a definite set. In this case theinitial pressure shown by the gauge was 17½ lb. , which immediatelydropped to 8 lb. As soon as the piston had moved sufficiently far toallow water to accumulate below it. 

_Test 4. _--The cap was again removed, the piston set up in place, thesand compacted around it in approximately the same condition it wouldhave had if the sand had been in place underground; the cap was then setin place and, after an hour, the pump was started. The pressureregistered was 25 lb. And extended over a period of several secondsbefore there was any movement in the piston. The piston respondedfinally without any increase of pressure, and, after lifting an inch ortwo, the pressure gradually dropped to 10 lb. , where it remained untilthe piston came out of the sand. 

The sum and average of these tests shows a relation of 22 lb. For thepiston in sand to about 8½ lb. As soon as the volume of water hadaccumulated below it, which would correspond very closely to a sandcontaining 40% of voids, which was the characteristic of the sand usedin this experiment. 

The conclusions from this experiment appear to be absolutely final inillustrating 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 thebuoyant effect upon the tunnel bottom under the same conditions. 

While the apparatus would have to be designed and built on a much largerscale in order to measure accurately the pressures due to sands andearths of varying characteristics, it appears to be conclusive inshowing the principle, and near enough to the theoretical value to betaken for practical purposes in designing structures against waterpressures when buried in sand or earth. 

It should be carefully noted that the friction of the water throughsand, which is always a large factor in subaqueous construction, isvirtually eliminated here, as the water pressure has to be transmittedonly some 6 or 8 in. To actuate the base of the piston, whereas in atunnel only half submerged this distance might be as many feet, andwould be a considerable factor. 

It should be noted also that although the area subject to pressure isdiminished, the pressure on the area remaining corresponds to the fullhydrostatic head, as would be shown by the pressure on an air gaugerequired to hold back the water, except, of course, as it may bediminished more or less by friction. 

The writer understands that experiments of a similar nature and withsimilar apparatus have been tried on clays and peats with resultsconsiderably higher; that is, in one case, there was a pressure of 40lb. Before the piston started to move. 

The following is given, in part, as an analysis and explanation of theabove experiments and notes:

It is well known that if lead be placed in a hydraulic press andsubjected to a sufficient pressure it will exhibit properties somewhatsimilar to soft clay or quicksand under pressure. It will flow out of anorifice or more than one orifice at the same pressure. This is due tothe fact that practically voids do not exist and that the pressure is sogreat, compared with the molecular cohesion, that the latter isvirtually nullified. It is also theoretically true that solid stoneunder infinitely high pressure may be liquefied. If in the cylinder of ahydraulic press there be put a certain quantity of cobblestones, leavinga clearance between the top of the stone and the piston, and if thisspace, together with the voids, be filled with water and subjected to agreat pressure, the sides or the walls of the cylinder are acted on bytwo pressures, one almost negligible, where they are in contact with thestone, restraining the tendency of the stone to roll or slide outward, and the other due to the pressure of the water over the area againstwhich there is no contact of stone. That this area of contact should bededucted from the pressure area can be clearly shown by assuming anothercylinder with cross-sticks jammed into it, as shown in Fig. 10. A glanceat this figure will show that there is no aqueous pressure on the wallsof the cylinder with which the ends of the sticks come in contact andthe loss of the pressure against the walls due to this is equal to theleast sectional area of the stick or tube either at the point of contactor intermediate thereto. 

Following this reasoning, in Fig. 11 it is found that an equivalent areamay be deducted covering the least area of continuous contact of thecobblestones, as shown along the dotted lines in the right half of thefigure. Returning, if, when the pressure is applied, an orifice be madein the cylinder, the water will at once flow out under pressure, allowing the piston to come in contact with the cobblestones. If theflow of the water were controlled, so as to stop it at the point wherethe stone and water are both under direct pressure, it would be foundthat the pressures were totally independent of each other. The aqueouspressure, for instance, would be equal at every point, while thepressure 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% ofthe pressure which a full area of solid stone would stand. If thispressure should be enormously increased after excluding the water, itwould finally result in crushing the stone into a solid mass; and if thepressure should be increased indefinitely, some theoretical point wouldbe reached, as above noted, where the stone would eventually beliquefied and would assume liquid properties. 

[Illustration: FIG. 10. ]

[Illustration: FIG. 11. ]

The same general reasoning applies to pure sand, sand being in effectcobblestones in miniature. In pressing the piston down on dry sand itwill be displaced into every existing abnormal void, but will bedisplaced into these voids rather than pressed into them, in the truedefinition of the word, and while it would flow out of an orifice in thesides or bottom, allowing the piston to be forced down as in asand-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 masstheoretically aqueous. If the positions of cylinder and piston bereversed, the piston pointing vertically upward and the sand "bled" intoan orifice in or through it, the void caused by the outflow of this sandwould be filled by sand displaced by the piston pressing upward ratherthan by sand from above. 

It was the knowledge of this principle which enabled the contractors tojack up successfully the roof of a long section of the cast-iron linedtubes under Joralemon Street in Brooklyn, in connection with thereconstruction of the Battery tubes at that point, the method ofoperation, as partly shown in Fig. 2, Plate XXVIII, being to cut througha section of the roof, 4 by 10 ft. In area, through which holes weredrilled and through which again the sand was "bled, " heavy pressurebeing applied from below through the medium of hydraulic jacks. By acareful manipulation of both these operations, sections of the roof ofthe above dimensions were eventually raised the required height of 30in. And permanently braced there in a single shift. 

If water in excess be put into a cylinder containing sand, and pressurebe applied thereto, the water, if allowed to flow out of an orifice, will carry with it a certain quantity of sand, according to thevelocity, and the observation of this might easily give rise to theerroneous impression that the sand, as well as the water, was flowingout under pressure, and, as heretofore stated, has caused many engineersand contractors to apply the term "quicksand" to any sand flowingthrough an orifice with water. 

Sand in its natural bed always contains some fine material, and wherethis is largely less than the percentage of voids, it has no materialeffect on the pressure exerted by the sand with or without water, asabove noted. If, however, this fine material be largely in excess of thevoids, it allows greater initial compression to take place when dry, andallows 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 quicksandin excess of the voids, pressure will cause the quicksand to set uphydraulic action, and the action of the piston will appear to be similarto that of a piston acting on purely aqueous material. 

Just here the writer desires to protest against considering semi-aqueousmasses, such as soupy sands, soft concrete, etc. , as exertinghydrostatic pressure due to their weight in bulk, instead of to thespecific gravity of the basic liquid. For instance, resorting again tothe illustration of cubes and spheres, it may be assumed that a cubicalreceptacle has been partly filled with small cubes of polished marble, piled vertically in columns. When this receptacle is filled with liquidaround the piles of cubes there will be no pressure on the sides exceptthat 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 weightof the cubes. Again, assume that the receptacle is filled with smallspheres, such as marbles, and that water is then poured in. The pressuredue to the weight of the solids on the bottom is relieved by the loss inweight of the marbles due to the water, and also to the tendency of themarbles to arch over the bottom, and while the pressure on the sides isincreased by this amount of thrust, the aqueous pressure is still thatof a liquid at 62½ lb. , and it is inconceivable that some engineers, incalculating the thrust of aqueous masses, speak of it as a liquidweighing, say, 120 or 150 lb. Per cu. Ft. ; as well might they expect toanchor spherical copper floats in front of a bulkhead and expect thehydrostatic pressure against this bulkhead to be diminished because theactual volume and weight of the water directly in front of the bulkheadhas been diminished. Those who have had experience in tying narrow deepforms for concrete with small wires or bolts and quickly filling themwith liquid concrete, must realize that no such pressures are everdeveloped as would correspond to liquids of 150 lb. Per cu. Ft. If thesolid material in any liquid is agitated, so that it is virtually insuspension, it cannot add to the pressure, and if allowed to subside itacts as a solid, independently of the water contained with it, althoughthe water may change somewhat the properties of the material, byincreasing or changing its cohesion, angle of repose, etc. That is, insubstance, those particles which rest solidly on the bottom and are incontact to the top of the solid material, do not derive any buoyancyfrom the water, while those particles not in contact with the bottomdirectly or through other particles, lose just so much weight throughbuoyancy. If, then, the vertical depth of the earthy particles or sandabove the bottom is so small that the arching effect against the sidesis negligible, the full weight of the particles in contact, directly orvicariously, with the bottom acts as pressure on the bottom, while thefull pressure of the water acts through the voids or on them, or istransmitted through material in contact with the bottom. 

Referring now to materials such as clays, peats, and other soft orplastic materials, it is idle to assume that these do not possesspressure-resisting and arching properties. For instance, a soft clayarch of larger dimensions, under the condition described early in thispaper, would undoubtedly stand if the rods supporting the intrados ofthe arch were keyed back to washers covering a sufficiently large area. 

The fact that compressed air can be used at all in tunnel work isevidence that semi-aqueous materials have arching properties, and thefact that "blows" usually occur in light cover is further evidence ofit. 

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 fullhydrostatic pressure is required to hold back the water, the pressure ofthe 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 itunder the same head in ordinary practice. It is undoubtedly true thatpiles can be driven to an indefinite depth in this material, and it isalso true that the action of the pile is to displace rather thancompress, as shown by the fact of driving portions of the tunnels underthe North River for long distances without opening the doors of theshield or removing any of the material. The case of filling in bogs ormarshes, causing them to sink at the point of filling and riseelsewhere, is readily explained by the fact that the water is confinedin the interstices of the material, admitting of displacement but nocompression. 

The application of the above to pressures over tunnels in materials ofClass A is that the sand or solid matter is virtually assumed to be aseries of columns with their bases in such intimate contact with thetunnel roof that water cannot exert pressure on the tunnel or buoyancyon the sand at the point of contact, and that if these columns aresufficiently deep to have their upper portions wholly or partly carriedby the arching or wedging action, the pressure of any water on theirsurfaces is not transferred to the tunnel, and the only aqueous pressureis that which acts on the tunnel between the assumed columns or throughthe 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 ofany 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 isincapable of arching itself, but, for purposes of safety, the writer hasput this at the point, _F_, as noted above, = _D_{X}_, although hebelieves that experiments on a large scale would show it to be nearer0. 67·_D_{X}_, above which the placing of additional back-fill willlighten the load on the structure. 

We have, then, for _D_{E}_ < _D_{X}_, the weight of the total prism ofthe earth plus the water in the voids, plus the added pressure of thewater 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 ofearth would exert full pressure over the full area of the tunnel, it maybe stated that the water cannot maintain pressure over the whole areawithout likewise giving buoyancy to the sand previously assumed to be incolumns, in which case there is the total weight of the water plus theweight 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 lesssafe in its reasoning. 

[Illustration: COMBINED EARTH AND WATER PRESSURES. FIG. 12. ]

Next is the question of pressure against a wall or braced trench formaterials under Class A. The pressure of sand is first calculatedindependently, as shown in Fig. 6. Reducing this to a basis of 100 lb. For each division of the scale measured horizontally, as shown, givesthe line, _B O_, Fig. 12, measuring the outside limit of pressure due tothe earth, the horizontal distance at any point between this line andthe vertical face equalling the pressure against that face divided bythe tangent of the angle of repose, which in this case is assumed to be45°, equalling unity. If the water pressure line, _C F_, is drawn, itshows the relative pressure of the water. In order to reduce this to thescale of 100 lb. Horizontal measurement, the line, _C E_, is drawn, representing the water pressure to scale, that is, so that eachhorizontal measurement of the scale gives the pressure on the face atthat point; and, allowing 50% for voids, halving this area gives theline, _C D_, between which and the vertical face any horizontal linemeasures the water pressure. Extending these pressure areas where theyoverlap gives the line, _B D_, which represents the total pressureagainst the face, measured horizontally. 

Next, as to the question of buoyancy in Class A materials. If asubmerged structure rests firmly on a bottom of more or less firm sand, its buoyancy, as indicated by the experiments, will only be a percentageof its buoyancy in pure water, corresponding to the voids in the sand. In practice, however, an attempt to show this condition will fail, owingto the fact that in such a structure the water will almost immediatelywork under the edge and bottom, and cause the structure to rise, and thetest can only be made by measuring the difference in uplift in aheavier-than-water structure, as shown in Experiment No. 5. For, if astructure lighter than the displaced water be buried in sandsufficiently deep to insure it against the influx of large volumes ofwater below, it will not rise. That this is not due entirely to thefriction of the solid material on the sides has been demonstrated by theobservation of subaqueous structures, which always tend to subsiderather than to lift during or following disturbance of the surroundingearth. 

The following is quoted from the paper by Charles M. Jacobs, M. Am. Soc. C. E. , on the North River Division of the Pennsylvania RailroadTunnels:[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 thatsubsidence of the structure followed disturbance of the outsidematerial, although theoretically the tubes were buoyant in the aqueousmaterial. 

The writer would urge, however, that, in all cases of submergedstructures only partially buried in solid material, excess weighting beused to cover the contingencies of vibration, oscillation, etc. , towhich such structures may be subjected and which may ultimately allowleads of water to work their way underneath. 

On the other hand, he urges that, in cases of floor areas of deeplysubmerged structures, such as tunnels or cellars, the pressure to beresisted should be assumed to be only slightly in excess of thatcorresponding to the pressure due to the water through the voids. 

The question of pressure, etc. , in Class B, or semi-aqueous materialswill be considered next. Of these materials, as already shown, there aretwo types: (_a_) sand in which the so-called quicksand is largely inexcess of any normal voids, and (_b_) plastic and viscous materials. Thewriter believes that these materials should be treated as mixtures ofsolid and watery particles, in the first of which the quicksand, oraqueous portion, being virtually in suspension, may be treated as water, and it must be concluded that the action here will be similar to that ofsand and pure water, giving a larger value to the properties of waterthan actually exists. If, for instance, it should be found that such amixture contained 40% of pure water, the writer would estimate itspressure on or against a structure as (_a_) that of a moist sandstanding at a steep angle of repose, and (_b_) that of clear water, anallowance of 60% of the total volume being assumed, and the sum of thesetwo results giving the total pressure. Until more definite data can beobtained by experiments on a larger scale, this assumed value of 60% ofthe total volume for the aqueous portion may be taken for all conditionsof semi-aqueous materials, except, of course, where the solid andaqueous particles may be clearly defined, the pressures being computedas described in the preceding pages. 

As to the question of pure quicksand (if such there be) and otheraqueous materials of Class C, such as water, oil, mercury, etc. , it hasalready been shown that they are to be considered as liquids of theirnormal specific gravity; that is, in calculating the air pressurenecessary to displace them, one should consider their specific gravityonly, as a factor, and not the total weight per volume including anyimpurities which they might contain undissolved. 

In order to have a clearer conception of aqueous and semi-aqueousmaterials and their action, they must be viewed under conditions notordinarily apparent. For instance, ideas of so-called quicksand arelargely drawn from seeing structures sinking into it, or from observingit flowing through voids in the sheeting or casing. The action of sandand water under pressure is viewed during or after a slump, when thedamage is being done, or has been done, whereas the correct view-pointis under static conditions, before the slump takes place. 

The following is quoted from the report of Mr. C. M. Jacobs, ChiefEngineer 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 ofthis class met with in such work, if compressed air had not been used. The writer believes that in soft material surrounding submergedstructures the water actually contained in the voids is notinfrequently, after a prolonged period of rest, cut off absolutely fromits sources of pressure and that contact with these sources of pressurewill not again be resumed until a leak takes place through thestructure; and, even when there is a small flow or trickling of waterthrough 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 thesubject of too many experiments and too much controversy to beconsidered in a paper which is intended to be a description ofexperiments and observed data and notes therefrom. The writer is of theopinion, however, that entirely too little attention has been given tothis bearing power of the soil; that while progress has been made in ourknowledge of all classes of materials for structures, very little hasbeen done which leads to any real knowledge of the material on which thefoundation rests. For instance, it is inconceivable that 1 or 2 tons maysometimes be allowed on a square foot of soft clay, while the load onfirm gravel is limited to from 4 to 6 tons. The writer's practicalobservations have convinced him that it is frequently much safer to putfour times 6 tons on a square foot of gravel than it is to putone-fourth of 2 tons on a square foot of soft clay. 

In connection with the bearing power of soil, the writer also believesthat too little study has been given to the questions of the lateralpressure of earth, and he desires to quote here from some experimentsdescribed in a book[F] published in England in 1876, to which hisattention has recently been called. This book appears to have beenintended for young people, but it is of interest to note the followingquotations from a chapter entitled "Sand. " This chapter begins bystating 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 orthree times, while heavy blows given with a lighter hammer have noeffect at all. That this is not due in any large measure to inertia canbe shown by the fact that more than 200 lb. Can safely be put on top ofthe 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. Theexplanation is that the pressure is transmitted laterally to the sides, and as the friction is directly proportional to the pressure, the loador effect of the blow is carried by the proportional increase in thefriction, and any diaphragm which will carry the direct bottom load willnot have its stresses largely increased by any greater loading on top. 

The writer believes that experiments will show that in a sand-jack thetendency will be for the sides to burst rather than the bottom, and thatthe outflow from an orifice at or near the bottom is not either greatlyretarded or accelerated by ordinary pressure on top. The occurrence ofabnormal voids, however, causes the sand to be displaced into them. 

The important consideration of this paper is that all the experimentsand observations noted point conclusively to the fact that pressure istransmitted laterally through ground, most probably along or nearlyparallel to the angles of repose, or in cases of rock or stiff material, along a line which, until more conclusive experiments are made, may betaken as a mean between the horizontal and vertical, or approximately 45degrees. There is no reason to believe that this is not the casethroughout the entire mass of the earth, that each cubic foot, or yard, or mile is supported or in turn supports its neighboring equivalentalong such lines. The theory is not a new one, and its field is toolarge to encompass within the limits of a single paper, but, forpractical purposes, and within the limited areas to which we mustnecessarily be confined, the writer believes it can be establishedbeyond controversy as true. Certain it is that no one has yet found, inground free from water pressure or abnormal conditions, any evidence ofgreater pressure at the bottom of a deep shaft or tunnel than that nearthe surface. Pressures due to the widening of mines beyond the limits ofsafety must not be taken as a controversion of this statement, as allarches have limits of safety, more especially if the useless materialbelow the theoretical intrados is only partly supported, or is allowedto be suspended from the natural arch. 

The writer believes, also, that the question of confined foundations, incontradistinction to that of the spreading of foundations, may be worthyof 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 smallopen caissons sunk to a depth of a few feet and cleaned out and filledwith concrete, which offer much better resistance than spreading thefoundation 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 or2 ft. Below adjacent excavations in ordinary sand, have safely resistedloads 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 areasonable depth and filled with concrete horizontally reinforced, will, in many instances, give as good results as, and, in most cases, verymuch better than, placing the foundation on an equivalent number ofsmall long piles or a proportionately greater spread of foundation area, the idea being that the transmission of pressure to the sides of thecoffer-dam will not only confine the side thrust, but will also transferthe loading in mass to a greater depth where the resistance to lateralpressure in the ground will be more stable; that is, the greater depthof foundation is gained without the increased excessive loading, ornecessity for deep excavation. 

As to the question of the bearing value and friction on piles, thewriter believes that while the literature on engineering is full ofexperimental data relating to friction on caissons, there is little toshow the real value of friction on piles. The assumption generally madeof an assumed bearing value, and the deduction therefrom of a value forthe skin friction is fallacious. Distinction, also, is not made, butshould be clearly drawn between skin friction, pure and simple, onsmooth surfaces, and the friction due to pressure. Too often the bearingvalue on irregular surfaces as well as the bearing due to taper inpiles, and lastly the resistance offered by binding, enter into thedetermination of so-called skin friction formulas. The essentialcondition of sinking a caisson is keeping it plumb; and binding, whichis another way of writing increased bearing value, will oftentimes befatal to success. 

The writer believes that a series of observations on caissons sunk plumbunder homogeneous conditions of ground and superficial smoothness willshow a proportional increase of skin friction per square foot averagefor each increase in the size of caissons, as well as for increase ofdepth in the sinking up to certain points, where it may finally becomeconstant, as will be shown later. The determination of the actualfriction or coefficient of friction between the surfaces of the pile andthe material it encounters, is not difficult to determine. In sand it isapproximately 40% of the pressure for reasonably smooth iron or steel, and 45% of the pressure for ordinary wood surfaces. If, for instance, along shaft be withdrawn vertically from moulding sand, the hole mayremain indefinitely as long as water does not get into it or it does notdry out. This is due to the tendency of the sand to arch itselfhorizontally over small areas. The same operation cannot be performed ondry sand, as the arching properties, while protecting the pile fromexcessive pressure due to excessive length, will not prevent the loosesand immediately surrounding the pile from exerting a constant pressureagainst the pile, and it is of this pressure that 45% may be taken asthe real value of skin friction on piles in dry sand. 

In soft clays or peats which are displaced by driving, the tendency ofthis material to flow back into the original space causes pressure, ofwhich the friction will be a measured percentage. In this case, however, the friction itself between the material and the clays or peat isusually very much less than 40%, and it is for this reason that piles ofalmost indefinite length may be driven in materials of this characterwithout offering sufficient resistance to be depended on, as long as nogood bearing ground is found at the point. 

If this material is under water, and is so soft as to be consideredsemi-aqueous, the pressure per square foot will increase in diminishingproportion to the depth, and the pressure per area will soon approachand become a constant, due to the resistance offered by the lateralarching of the solid material; whereas, in large circular caissons, orcaisson shafts, where the horizontal arching effect is virtuallydestroyed, or at least rendered non-effective until a great depth isreached, the pressure must necessarily vary under these conditionsproportionately to the depth and size of the caisson in semi-aqueousmaterial. On the other hand, in large caisson shafts, especially thosewhich are square, the pressure at the top due to the solid material willalso increase proportionately to the depth, as already explained inconnection with the pressures of earth against sheeting and retainingwalls. 

The writer believes that the pressure on these surfaces may bedetermined with reasonable accuracy by the formulas already given inthis paper, and with these pressures, multiplied by the coefficient offriction determined by the simplest experiment on the ground, resultsmay be obtained which will closely approximate the actual friction oncaissons at given depths. The friction on caissons, which is usuallygiven at from 200 to 600 lb. Per sq. Ft. , is frequently assumed to bethe same on piles 12 in. Or less in diameter, whereas the pressures onthese surfaces, as shown, are in no way comparable. 

The following notes and observations are given in connection with theskin friction and the bearing value of piles:

The writer has in his possession a copy of an official print which wasrecently furnished to bidders in connection with the foundation for alarge public building in New York City. The experiments were made ongood 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 tothe 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. Pipesubjected to a loading approximating 28 tons per sq. Ft. After aninitial settlement of nearly 3 in. , there was no further settlement overan extended period, although the load of 28 tons per sq. Ft. Wascontinued. 

In connection with some recent underpinning work, 14-in. Hollowcylindrical piles 6 ft. Long were sunk to a depth of 6 ft. With anordinary hand-hammer, being excavated as driven. These piles were thenfilled with concrete and subjected to a loading in some casesapproximating 60 tons. After a settlement ranging from 9 to 13 in. , nofurther settlement took place, although the loading was maintained for aconsiderable 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 adepth of about 20 ft. , sustain in gravel and sand a load approximating50 tons when cleaned out to within 2 ft. Of the bottom. 

He has seen other cylindrical piles with a bearing ring of not more than¾ in. Resting on gravel at a depth of from 20 to 30 ft. , cleaned outpractically to the bottom, sustain a measured load of 60 tons withoutsettlement. 

As to skin friction in sand, a case came under his observation wherein a14-in. Hollow cylindrical pile which had stood for 28 days at a depth ofabout 30 ft. In the sand, was cleaned out to its bottom and subjected tohydraulic pressure, measured by a gauge, and sunk 2 ft. Into the sandwithout any pressure being registered on the gauge. It should beexplained, however, that the gauge could be subjected to a pressure of250 lb. , equal to a total pressure of 7, 000 lb. On the piston of thejack without registering, which corresponded, assuming it all as skinfriction, to a maximum of not more than 78 lb. Per sq. Ft. , but itshould be noted that this included bearing value as well, and that thepressure was very far from 7, 000 lb. , in all probability, at thebeginning of the test. 

In the case of the California stove-pipe wells driven by the Board ofWater Supply on Long Island, the writer is informed that one of thesetubes, 12 in. In diameter, was sunk to a depth of 850 ft. In doing thiswork the pile was excavated below the footing with a sand pump and wasthen sunk by hydraulic pressure. Assuming the maximum capacity of thejacks at 100 tons, which is not probable, the skin friction could nothave amounted to more than 75 lb. Per sq. Ft. It cannot be assumed inthis case that the excavation of the material below the pile relievedthe skin itself of some of its friction, as the operation consumed morethan 6 weeks, and, even if excess material was removed, it is certainthat a large percentage of it would have had time to adjust itselfbefore the operation was completed. 

[Illustration: PLATE XXIX, FIG. 1. --A 14-GAUGE, 14-IN. , HOLLOW(NON-TELESCOPIC), CALIFORNIA STOVE-PIPE PILE WHICH MET IMPENETRABLEMATERIAL. ]

[Illustration: PLATE XXIX, FIG. 2. --CHENOWETH PILE, PENETRATING HARDMATERIAL. ]

In connection with this, the writer may call attention to the fact thatpiles driven in silt along the North River, and in soft material atother places, are sometimes 90 ft. In length, and even then do not offersufficient resistance to be depended on for loading. This is due to thefact that the end of the pile does not bear in good material. 

The relation between bearing value and skin friction on a pile, wherethe end bearing is in good material, is well shown by a case where awooden pile[G] struck solid material, was distorted under the continualblows of the hammer, and was afterward exposed. It is also shown in thecase of a 14-in. California stove-pipe pile, No. 14 gauge, the point ofwhich met firm material. The result, as shown by Fig. 1, Plate XXIX, speaks for itself. Fig. 2, Plate XXIX, shows a Chenoweth pile which wasan experimental one driven by its designer. This pile, after gettinginto hard material, was subjected to the blow of a 4, 000-lb. Hammerfalling the full length of the pile-driver, and the only result was toshatter the head of the pile, and not cause further penetration. Mr. Chenoweth has stated to the writer that he has found material so compactthat it could not be penetrated with a solid pile--either with orwithout jetting--which is in line with the writer's experience. 

The writer believes that the foregoing notes will show conclusively thatthe factor to be sought in pile work is bearing value rather than depthor skin friction, and, however valuable skin friction may be in thelarger 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 havecontributed to the success of the experiments noted herein: Mr. James W. Nelson, of Richard Dudgeon, New York; Mr. George Noble, of John Simmonsand Company, New York; and Mr. Pendleton, of Hindley and Pendleton, Brooklyn, N. Y. ; all of whom have furnished apparatus for the experimentsand have taken an interest in the results. And lastly, he desiresespecially to thank Mr. F. L. Cranford, of the Cranford Company, for menand material with which to make the experiments and without whoseco-operation it would have been impracticable for the writer to havemade them. 

Throughout this paper the writer has endeavored, as far as possible, todeduce from his observations and from the observations of others, as faras he has been able to obtain them, practical data and formulas whichmay be of use in establishing the relationship between the pressure, resistance, and stability of earths; and, while he does not wish todictate the character of the discussion, he does ask that those who havemade observations of a similar character or who have available data, will, as far as possible, contribute the same to this discussion. It isonly by such observations and experiments, and deductions therefrom, that engineers may obtain a better knowledge of the handling of suchmaterials. 

The writer believes that too much has been taken for granted inconnection with earth pressures and resistance; and that, far too often, observations of the results of natural laws have been set down asphenomena. He believes that, both in experimenting and observing, theengineer will frequently find what is being looked for or expected andwill fail to see the obvious alternative. He may add that his ownexperiments and observations may be criticized for the same reason, andhe asks, therefore, that all possible light be thrown on this subject. Acomparative study of much of our expert testimony or of the plans ofalmost any of the structures designed in connection with their bearingupon earth, or resistance to earth pressure, will show that under thepresent methods of interpretation of the underlying principles governingthe calculations and designs relating to such structures, the resultsvary 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 mostessential conditions. 

Until the engineer can say with certainty that his calculations arereasonably based on facts, he is forced to admit that his design must belacking, either in the elements of safety, on the one hand, or ofeconomy, on the other, and, until he can give to his client a fullmeasure of both these factors in fair proportion, he cannot justly claimthat his profession has reached its full development. 

Table 1 gives approximate calculations of pressures on two types oftunnels and on two heights of sheeted faces or walls, due to fourvarying classes of materials. 

TABLE 1. --PRESSURES ON TYPICAL STRUCTURES UNDER VARYING ASSUMEDCONDITIONS. 

[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 1cu. Ft. Of water = 62½ lb. 

Conditions: 1 = normal sand, 2 = dry sand, 3 = supersaturated firm sandwith 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. | Totalfoot 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. |Totalfoot 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, " byRobert Routledge, Assistant Examiner in Chemistry and in NaturalPhilosophy to the University of London. ]

[Footnote G: _Engineering News_, January 15th, 1909. ]

DISCUSSION

T. KENNARD THOMSON, M. AM. SOC. C. E. --Although the authordeserves great credit for the careful and thorough manner in which hehas handled this subject, his paper should be labeled "Dangerous forBeginners, " especially as he is an engineer of great practicalexperience; if he were not, comparatively little attention would be paidto his statements. The paper is dangerous because many will read onlyportions of it, or will not read it thoroughly. For instance, at thebeginning, the author cites several experiments in which considerableforce is required to start the lifting of a weight or plunger in sandand water and much less after the start. This reminds the speaker of thetime when, as a schoolboy, he tried to pick up stones from the bottom ofthe river and was told that the "suction" was caused by atmosphericpressure. 

The inference is that tunnels, etc. , in sand, etc. , are not in anydanger of rising, even though they are lighter than water. Toward theend of the paper, however, the author states that tunnels should beweighted, but he rather spoils this by stating that they should beweighted only enough to overcome the actual water pressure, that is, between the voids of the sand. It seems to the speaker that the onlyreally safe way is to make the tunnel at least as heavy as the waterdisplaced in order to prevent it from coming up, and to take othermeasures to prevent it from going down. The City of Toronto, Canada, formerly pumped its water supply through a 6-ft. Iron pipe, buried inthe sand under Toronto Bay and then under Toronto Island, with an intakein the deep water of the lake. During a storm a mass of seaweed, etc. , was washed against the intake, completely blocking it, and although theman at the pumping station knew that something was wrong, he continuedto pump until the water was drawn out of the pipe, with the result thatabout half a mile of the conduit started to rise and then broke atseveral places, thus allowing it to fill with water. Eventually, thecity went down to bed-rock under the Bay for its water tunnel. 

Another reason for calling this paper dangerous for beginners is that itis improbable that experienced engineers or contractors will omit thebracing at the bottom, although, since the paper was printed, a glaringinstance has occurred where comparatively little bracing was put in thebottom 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 agreethat nearly every crib which has failed slipped out from the bottom, anddid not turn over. 

The objection to the angle of repose is that it is not possible toascertain it for any material deposited by Nature. It could probably beascertained for a sand bank deposited by Man, but not for an excavationto 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 cementedtogether by clay, loam, roots, trees, boulders, etc. , and differs incharacter every few feet. 

A deep vertical cut can often be made, even in New York quicksand, fromwhich the water has been drawn, and, if not subjected to jars, water, etc. , this material will stand for considerable time and then come downlike an avalanche, killing any one in its way. In such cases very littlebracing would prevent the slide from starting, provided rain, etc. , didnot loosen the material. 

The author, of course, treats dry and wet materials differently, butthere are very few places where dry material is not likely to become wetbefore the excavation is completed. 

In caisson work, if the caisson can be kept absolutely plumb, it can besunk without having to overcome much friction, while, on the other hand, if it is not kept plumb, the material is more or less disturbed andbegins to bind, causing considerable friction. The author claims thatthe pressure does not increase with the depth, but all caisson men willprobably remember that the friction to be overcome per square foot ofsurface increases with the depth. 

In calculating retaining walls, many engineers add the weight of thesoil to the water, and calculate for from 90 to 100 lb. Per cu. Ft. Thespeaker is satisfied that in the so-called New York quicksand it issufficient to use the weight of the water only. If the sand increasedthe side pressure above the water pressure, engineers would expect touse more compressed air to hold it back, while, as a matter of fact, theair pressure used seldom varies much from that called for by thehydrostatic head. 

Although allowance for water pressure is sufficient for designingretaining walls in New York quicksand, it is far from sufficient incertain silty materials. For instance, in Maryland, a coffer-dam, excavated to a depth of 30 ft. In silt and water, had the bottom shovedin 2 ft. , in spite of the fact that the waling pieces were 5 ft. Apartvertically at the top and 3 ft. At the bottom, and were braced with 12by 12-in. Timbers, every 7 ft. Horizontally. The walings split, and thecross-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 contrastto 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 theresulting calculations, economizes on his bottom struts, will find thatsooner or later an accident on one job will cause enough loss of lifeand money to pay for conservative timbers for the rest of his life. Somuch for side pressures. As to the pressure in the roof of a tunnel, probably every engineer will agree that almost any material exceptunfrozen water will tend to arch more or less, but how much it isimpossible to say. It is doubtful whether any experienced engineer wouldever try to carry all the weight over the roof, except in the case ofback-fill, and even then he would have to make his own assumption (whichsounds more polite than "guess"). 

The author has stated, however, that when the tunnel roof and sides arein place, no further trouble need be feared. On the contrary, in 1885, the Canadian Pacific Railroad built a tunnel through clayey material andlined 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-excavatedand lined with 12 by 12-in. Timbers side by side, and it collapsedagain; 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 builtfarther in. This trouble could have been caused either by the sliding orswelling of the material, and the speaker is inclined to believe that itwas caused by swelling, for it is known, of course, that most materialhas been deposited by Nature under great pressure, and, by excavating incertain materials, the air and moisture would cause those materials toswell and become an irresistible force. 

To carry the load, Mr. Meem prefers to rely on the points of the pilesrather than the side friction. In such cases the pile would act as apost, and would probably fail when ordinarily loaded, unless firmlysupported at the sides. The speaker has seen piles driven from 80 to 90ft. In 10 min. , which offered almost no resistance, and yet, a few dayslater, they would sustain 40 tons each. No one would dream of putting 40tons on a 90-ft. Pile resting on rock, if it were not adequatelysupported. 

It is the speaker's opinion that bracing should not be omitted foreither piles or coffer-dams. 

CHARLES E. GREGORY, ASSOC. M. AM. SOC. C. E. --In describing hislast experiment with the hydraulic chambers and plunger, Mr. Meem statesthat, after letting the pressure stand at 25 lb. , etc. , the piston cameup. This suggests that the piston might have been raised at a much lowerpressure, if it had been allowed to stand long enough. 

The depth and coarseness of the sand were not varied to ascertainwhether any relation exists between them and the pressure required tolift the piston. If the pressure varied with the depth of sand, it wouldindicate that the reduction was due to the resistance of the water whenfinely divided by the sand; if it varied with the coarseness of thesand, as it undoubtedly would, especially if the sand grains wereincreased to spheres 1 in. In diameter, it would show that it wasindependent of the voids in the sand, but dependent on dividing thewater into thin films. 

The speaker believes that the greater part of the reduction of pressureon the bottom of the piston might be better explained by the viscosityof the water, than to assume that a considerable part of the plunger isnot in contact with it. The water, being divided by fine sand into verythin films, has a tensile strength which is capable of resisting thepressure for at least a limited time. 

If the water is capable of exerting its full hydrostatic pressurethrough the sand, the total pressure would be the full hydrostaticpressure on the bottom of the piston where in contact, and, whereseparated from it by a grain of sand, the pressure would be decreasedonly by the weight of the grain. If a large proportion of the top areaof a grain is in contact, as assumed by the author, this reduction ofpressure would be very small. A correct interpretation can be obtainedonly 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 fullwater pressure. No matter how much of the area is in direct contact withthe sand rather than the water, the full water pressure would betransmitted through each sand grain from its other side and, ifnecessary, from and through many other grains which may be in turn incontact with it. The pressure on such a wall will be water pressure overits entire surface, and, in addition, the thrust of the sand aftercorrecting for its loss of weight in the water. 

The fact that small cavities may be excavated from the sides of trenchesor tunnels back of the sheeting proves only that there is a localtemporary arching of the material, or that the cohesion of the particlesis sufficient to withstand the stress temporarily, or that there is acombination of cohesion and arching. The possibility of making suchexcavations does not prove that pressure does not exist at such points. That sand or earth will arch under certain conditions has long been anaccepted fact. The sand arches experimented with developed theirstrength only after considerable yielding and, therefore, give no indexof the distribution or intensity of stress before such yielding. Furthermore, sand and earth in Nature are not constrained by forms andreinforcing rods. 

Mr. Meem's paper is very valuable in that it presents some unusualphenomena, but many of the conclusions drawn therefrom cannot beaccepted without further demonstration. 

FRANCIS W. PERRY, ASSOC. M. AM. SOC. C. E. --Pressure-gaugeobservations on a number of pneumatic caissons recently sunk, throughvarious grades of sand, to rock at depths of from 85 to 105 ft. Belowground-water, invariably showed working-chamber air-pressures equal, asclosely as could be observed, to the hydrostatic pressures computed, forcorresponding depths of cutting-edge, as given in Table 2. 

These observations and computations were made by the speaker inconnection with the caisson foundations for the Municipal Building, NewYork City. 

TABLE 2. --EQUIVALENT FEET OF DEPTH BELOW WATER PER POUNDPRESSURE. 

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 | |__________|___________|_____________|______________|

 34NOTE. --Equivalent depth in feet = ------ × pressure. 14. 7

E. P. GOODRICH, M. AM. SOC. C. E. (by letter). --This paper is tobe characterized by superlatives. Parts of it are believed to beexceptionally good, while other parts are considered equally dangerous. The author's experimental work is extremely interesting, and the writerbelieves the results obtained to be of great value; but the analyticalwork, 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 untilmuch more information has been obtained. He also believes that thedesign of temporary structures with this inclusion is actually dangerousin some instances, and takes the liberty of citing the followingstatement 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 "thethickness of key" which "should be allowed" over tunnels, believing thatconditions within an earth mass, except in very rare instances, aresuch that true arch action will seldom take place to any definiteextent, through any considerable depths. Furthermore, the author'sreason for bisecting the angle between the vertical and the angle ofrepose of the material, when he undertakes to determine the thickness ofkey, is not obvious. This assumption is shown to be absurd when carriedto either limit, for when the angle of repose equals zero, as is thecase 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 ofrock, this method would give an infinite thickness of key, which isagain seen to be absurd. It would seem as if altogether too manyunknowable conditions had been assumed. In any case, no arch action canbe brought into play until a certain amount of settlement has takenplace so as to bring the particles into closer contact, and in such away that the internal stresses are practically those only ofcompression, and the shearing stresses are within the limits possiblefor the material in question. 

The author has repeatedly made assumptions which are not borne out bythe application of his mathematical formulas to actual extremeconditions. This method of application to limiting conditions isconcededly sometimes faulty; but the writer believes that no earthpressure theory, or one concerning arch action, can be considered assatisfactory which does not apply equally well to hydraulic pressureproblems when the proper assumptions are made as to the factors forfriction, cohesion, etc. For example, when the angle of repose isconsidered as zero, in the author's first formula for _W_{1}_, the valuebecomes ½ _W_{1}_, whereas it should depend solely on the depth, whichdoes 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 "thearch thrust is greater in dryer sand, " and the accuracy of the statementis questioned. Again, no reason is apparent for assuming the directionof the "rakers" in Fig. 3 as that of the angle of repose. The writercannot see why that particular angle is repeatedly used, when almost anyother would give results of a similar kind. The author has made noexperiments which show any connection between the angle of repose, as heinterprets it, and the lines of arch action which he assumes to exist. 

With regard to the illustration of the condition which is thought toexist when the "material is composed of large bowling balls, " supposedlyall of the same size, the writer believes the conclusion to beerroneous, and that this can be readily seen by inspection of a diagramin which such balls are represented as forming a pile similar to thewell-known "pile of shells" of the algebras, in the diagram of which apile of three shells, resting on the base, has been omitted. It is thenseen that unless the pressures at an angle of 60° with the horizontalare sufficient to produce frictional resistance of a very large amount, the balls will roll and instantly break the arch action suggested by theauthor. 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 casesbecause he repeatedly makes assumptions which are not in accordance withdemonstrated fact, and finally sums up the results by the statement: "Itis conceded" (line 2, p. 357, for example), when the writer, for one, has not even conceded the accuracy of the assumptions. For instance, theauthor's well-known theory that pressures against retaining walls are amaximum at the top and decrease to zero at the bottom, is in absolutecontradiction to the results of experiments conducted on a large scaleby the writer on the new reinforced concrete retaining wall near the St. George Ferry, on Staten Island, New York City, which will soon bepublished, and in which the usual law of increase of lateral pressurewith depth is believed to be demonstrated beyond question. It must beconceded that a considerable arch action (so-called) actually exists inmany cases; but it should be equally conceded by the advocates of theexistence of such action that changes in humidity, due to moving water, vibration, and appreciable viscosity, etc. , will invariably destroy thisaction in time. In consequence, the author's reasoning in regard to thepressures against the faces of retaining walls is believed to be open tograve question as to accuracy of assumption, method, and conclusion. 

The author is correct in so far as he assumes that "the character of thestresses due to the thrust of the material will" not "change if bracingshould be substituted for the material in the area" designated by him, etc. , provided he makes the further assumption that absolutely nomotion, however infinitesimal, has taken place meantime; but, unlesssuch motion has actually taken place, no arch action can have developed. An arch thrust can result only with true arch action, that is, withstable abutments, and the mass stressed wholly in compression, withcorresponding shortening of the arch line. The arch thrust must beproportional 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 ofthe weight of material above it, that assumed arch must relieve all theassumed arches below. Therefore each of the assumed arches can carrynothing more than its own mass. Otherwise the resulting thrust wouldincrease with the depth, which is opposed to the author's theory. 

Turning again to the condition that each arch can carry only its ownweight: if these arches are assumed of thicknesses proportional to thedistance upward from the bottom of the wall, they will be similarfigures, and it is easily demonstrated that the thrust will then beuniform in amount throughout the whole height of the wall, except, perhaps, at the very top. This condition is contrary to the author'sideas and also to the facts as demonstrated by the writer's experimenton the 40-ft. Retaining wall at St. George. Consequently, the author'sstatement: "nor can anyone * * * doubt that the top timbers are stressedmore heavily than those at the bottom, " is emphatically doubted andearnestly denied by the writer. Furthermore, "the assumption" made bythe author as to "the tendency of the material to slide" so as to causeit "to wedge * * * between the face of the sheeting * * * and some planebetween the sheeting and the plane of repose, " is considered asabsolutely unwarranted, and consequently the whole conclusion isbelieved to be unjustified. Nor is the author's assumption (line 5, p. 361), that "the thrust * * * is measured by its weight divided by thetangent of the * * * angle of repose" at all obvious. 

The author presents some very interesting photographs showing thenatural surface slopes of various materials; but it is interesting tonote that he describes these slopes as having been produced by the"continual slipping down of particles. " The vast difference betweenangles of repose produced in this manner by the rolling friction ofparticles and the internal angles of friction, which must be used in allearth-pressure investigations, has been repeatedly called to theattention of engineers by the writer. [H]

The writer's experiments are entirely in accord with those of the authorin which the latter claims to demonstrate that "earth and waterpressures act independently of each other, " and the writer is muchdelighted that his own experiments have been thus confirmed. 

In Experiment No. 3, the query is naturally suggested: "What would havebeen the result if the nuts and washers had first been tightened andwater then added?" Although the writer has not tried the experiment, heis rather inclined to the idea that the arch would have collapsed. Withregard to Experiment No. 5, there is to be noted an interestingpossibility of its application to the theoretical discussion of masonrydams, in which films of water are assumed to exist beneath the structureor in crevices or cracks of capillary dimensions. The writer has alwaysconsidered the assumptions made by many designing engineers asunnecessarily conservative. In regard to the author's conclusions fromExperiment No. 6, it should be noted that no friction can exist betweenparticles of sand and surrounding water unless there is a tendency ofthe latter to move; and that water in motion does not exert pressuresequal to those produced when in a static condition, the reduction beingproportional to the velocity of flow. 

The author's conclusion (p. 371), that "pressure will cause thequicksand to set up hydraulic action, " does not seem to have beendemonstrated by his experiments, but to be only his theory. In thisinstance, the results of the writer's experiments are contrary to theauthor's theory and conclusion. 

The writer will heartily add his protest to that of the author "againstconsidering 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 lastquoted, should be qualified by the addition of the words, "only when acertain amount of deflection has taken place so as to bring the archinto action. " Again, on the following page, a somewhat similarqualification should be added to the sentence referring to the soft clayarch, that it would "stand if the rods supporting the intrados of thearch were keyed back to washers covering a sufficiently large area, " byinserting the words, "unless creeping pressures (such as thoseencountered by the writer in his experiments) were exceeded. "

The writer considers as very doubtful the formula for _D_{x}_, which isthe same as that for _W_{1}_, already discussed. The author's statementthat "additional back-fill will [under certain circumstances] lightenthe load on the structure, " is considered subject to modification bysome such clause as the following, "the word 'lighten' here beingunderstood to mean the reduction to some extent of what would be thetotal 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 probabilitythat 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 experimentsdemonstrate his statement "that pressure is transmitted laterallythrough ground, most probably along or nearly parallel to the angles ofrepose, " or any of the conclusions drawn by him in the paragraph (p. 381), which contains this questionable statement. Again the writer is ata loss as to how to interpret the statement that the author has foundthat "better resistance" has been offered by "small open caissons sunkto a depth of a few feet and cleaned out and filled with concrete" thanby "spreading the foundation over four or five times the equivalentarea. " The writer agrees with the author in the majority of hisstatements as to the "bearing value and friction on piles, " but believesthat he is indulging in pure theory in some of his succeeding remarks, wherein he ascribes to arch action the results which he believes wouldbe observed if "a long shaft be withdrawn vertically from mouldingsand. " These phenomena would be due rather to capillary action and theresulting cohesion. 

Naturally, the writer doubts the author's conclusions as to the pressureat the top of large square caisson shafts when he states that "thepressure at the top * * * will * * * increase proportionately to thedepth. " Again, the author is apparently not conversant with experimentsmade by the Dock Department of New York City, concerning piles driven inthe Hudson River silt, which showed that a single heavily loaded pilecarried downward with it other unloaded piles, driven considerabledistances away, showing that it was not the pile which lacked inresistance, as much as the surrounding earth. 

In conclusion, the writer heartily concurs with the statement that "toomuch has been taken for granted in connection with earth pressures andresistance, " and he is sorry to be forced to add that he believes theauthor to be open to the criticism which he himself suggests, that "bothin experimenting and observing, the engineer [and in this case theauthor] will frequently find what is being looked for or expected andwill fail to see the obvious alternative. "

FRANCIS L. PRUYN, M. AM. SOC. C. E. (by letter). --Mr. Meemshould be congratulated, both in regard to the highly interestingtheories which he advances on the subject of sand pressures--thepressures of subaqueous material--and on his interesting experiments inconnection therewith. 

The experiment in which the plunger on the hydraulic ram is immersed insand and covered with water does not seem to be conclusive. By thisexperiment the author attempts to demonstrate that the pressure of thewater transmitted through the sand is only about 40% as great as whenthe sand is not there. The travel of ground-water through the earth isat 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 coversufficient time during which the pressure was maintained at any givenpoint. It is quite probable that it may take 15 or 20 min. For the fullpressure to be transmitted through the sand to the bottom of theplunger, and it is hoped, therefore, that he will make furtherexperiments lasting long enough to demonstrate this point. 

In regard to the question of skin friction on caissons and piles, it maybe of interest to mention an experiment which the writer made during thesinking of the large caissons for the Williamsburg Bridge. Thesecaissons were about 70 ft. Long and 50 ft. Wide. The river bottom wasabout 50 ft. Below mean high water, and the caissons penetrated sand ofgood quality to a depth of from 90 to 100 ft. Below that level. On twooccasions calculations were made to determine the skin friction whilethe caissons were being settled. With the cutting edge from 20 to 30 ft. Below the river bottom, the calculations showed that the skin frictionwas between 500 and 600 lb. Per sq. Ft. The writer agrees with Mr. Meemthat, in the sinking of caissons, the arch action of sand is, in a greatmeasure, destroyed by the compressed air which escapes under the cuttingedge and percolates up through the material close to the sides of thecaissons. 

With reference to the skin friction on piles, the writer agrees with Mr. Meem that in certain classes of material this is almost a negligiblequantity. The writer has jacked down 9-in. Pipes in various parts of NewYork City, and by placing a recording gauge on the hydraulic jack, theskin friction on the pile could be obtained very accurately. In severalinstances the gauge readings did not vary materially from the surfacedown to a penetration of 50 ft. In these instances the material insidethe pipe was cleaned out to within 1 ft. Of the bottom of the pile, sothat the gauge reading indicated only the friction on the outside of thepipe 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 bottomof the pipe seems to be about 20 tons, irrespective of the depth. Afterthe pipe had reached sufficient depth, it was concreted, and, after theconcrete had set, the jack was again placed on it and gauge readingswere taken. It was found that in ordinary sands the concreted steel pilewould go down from 3 to 6 in. , after which it would bring up to the fullcapacity of a 60-ton jack, showing, by gauge reading, a reaction of from70 to 80 tons. 

It is the writer's opinion that, in reasonably compact sands situated ata 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 anydifficulty whatever. 

FRANK H. CARTER, ASSOC. M. AM. SOC. C. E. (by letter). --Mr. Meem has contributed much that is of value, particularly on waterpressures in sand; just what result would be obtained if coarse crushedstone 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 wellas in New York City, to assume that water pressures on the underside ofinverts is exerted on one-half the area only. The writer, however, hasmade it a practice first to lay a few inches of cracked stone on thebottom of wet excavations in order to keep water from concrete which isto be placed in the invert. In addition to the cracked stone under theinverts, shallow trenches dug laterally across the excavation to insuremore perfect drainage, have been observed. Both these factors no doubtassist the free course of water in exerting pressure on the finishedinvert after the underdrains have been closed up on completion of thework. The writer, therefore, awaits with interest the repetition ofExperiment No. 6, with water on the bottom of a piston buried in coarsegravel or cracked stone. 

As for the arching effect of sand, the writer believes that Mr. Meem hasdemonstrated an important principle, on a small scale. It must beregretted, however, that the box was not made larger, for, to thewriter, it appears unsafe to draw such sweeping conclusions from smallexperiments. As small models of sailboats fail to develop completelylaws for the design and control of large racing yachts, so experimentsin small sand boxes may fail to demonstrate the laws governing actualpressures on full-sized structures. 

For some time the writer has been using a process of reasoning similarto that of the author for assumptions of earth pressure on the roofs oftunnel arches, except that the vertical forces assumed to hold up theweight of the earth have been ascribed to cohesion and friction, alongwhat might be termed the sides of the "trench excavation. "

The writer fails to find proof in this paper of the author's statementthat earth pressures on the sides of a structure buried in earth aregreater 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 similarexpressions used by experienced trench foremen, but, in every casecalled to his attention, local circumstances have caused thetop-heaviness, either undermining at the bottom of the trench, too muchbanked earth on top, or the earth excavated from the trench being toonear the edge of the cut. 

For some years the writer has been making extended observations on deeptrenches, and, thus far, has failed to find evidence, except in aqueousmaterial, of earth pressures which might be expected from the knownnatural slope of the material after exposure to the elements; and thislatter feature may explain why sheeted trenches stand so much betterthan expected. If air had free access to the material, cohesion would bedestroyed, and theoretical pressures would be more easily developed. With closely-sheeted trenches, weathering is practically excluded, andthe bracing, which seemingly is far too light, holds up the trench withscarcely a mark of pressure. As an instance, in 1893, the writer wassuccessfully digging sewer trenches from 10 to 14 ft. Deep, throughgravel, in the central part of Connecticut, without bracing; because ofdemands of the work in another part of the city, a length of severalhundred feet of trench was left open for three days, resulting in thecaving-in of the sides. The elements had destroyed the cohesion, and thesides of the trenches no longer stood vertically. 

Recently, in the vicinity of Boston, trenches, 32 ft. Wide, and from 25to 35 ft. Deep, with heavy buildings on one side, have been braced with8 by 10-in. Stringers, and bracers at 10-ft. Centers longitudinally, andfrom 3 to 5 ft. Apart vertically; this timbering apparently was tooslight for pressures which, theoretically, might be expected from thenatural slope of the material. Just what pressures develop on the sidesof the structures in these deep trenches after pulling the top sheeting(the bottom sheeting being left in place) is, of course, a matter ofconjecture. There can be no doubt that there is an arching of thematerial, as suggested by the author. How much this may be assisted bythe practical non-disturbance of the virgin material is, of course, indeterminate. That substructures and retaining walls designed accordingto the Rankine or similar theories have an additional factor of safetyfrom too generous an assumption in regard to earth pressure ispractically admitted everywhere. It is almost an engineering axiom thatretaining walls generally fail because of insufficient foundation only. 

For the foregoing reasons, and particularly from observations on theeffect of earth pressures on wooden timbers used as bracing, the writerbelieves that, ordinarily, the theoretical earth pressures computed byRankine and Coulomb are not realized by one-half, and sometimes not evenby one-third or one-quarter in trenches well under-drained, rapidlyexcavated, and thoroughly braced. 

J. C. MEEM, M. AM. SOC. C. E. (by letter). --The writer has beenmuch interested in this discussion, and believes that it will be ofgeneral value to the profession. It is unfortunate, however, thatseveral of the points raised have been due to a careless reading of, orfailure to understand, the paper. 

Taking up the discussion in detail, the writer will first answer thecriticisms 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 recognizedand considered? The design of timbering for a structure to rest, forinstance, at a depth of from 200 to 300 ft. In normal dry earth, withoutconsidering this action, would be virtually prohibitive. 

Mr. Goodrich proceeds to show one of the dangers of considering suchaction 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 thedifference between that portion of the sand which acts as so-called"centering" and that which goes to make up the sustaining arch. Thedislodgment of any large portion of this "centering" naturally causescollapse, 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 archis in turn finally established. This, however, does not mean that, during the process of establishing this equilibrium of the archstresses, there is no arching action of any of the material above, butonly that some of the so-called arches are temporarily sustained bythose below. That is, in effect, each area of the material abovebecomes, in turn, a dependent, an independent, and finally aninterdependent arch. 

If Mr. Goodrich's experience has led him to examine any large number oftunnel arches or brick sewers, he will have noted in many of themlongitudinal 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 finalcompactness. Therefore, in adjusting itself to normal conditions, thismaterial causes impact loads to come upon the green arch, and these tendto 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" arestruck. This, again, results in a shock, or impact loading, to thedetriment of the arch. 

_Third. _--The most prolific cause, however, is that in tunneling, aswell as in back-filling open cuts, the material backing up the haunchesis more or less loosened and therefore is not at first compact enough toprevent the spreading of the haunches when the load comes on the arch. This causes cracking, but, as soon as the haunches have been pressed outagainst the solid material, the cracking usually ceases, unless thepressure has been sufficiently heavy to cause collapse. 

An interesting example of this was noted in the Joralemon Street branchof the Rapid Transit Tunnel, in Brooklyn, in which a great many of thecast-iron rings were cracked under the crown of the arch, duringconstruction; but, in spite of this, they sustained, for more than twoyears, a loading which, according to Mr. Goodrich, was continuallyincreasing. In other words, the cracked arch sustained a greater loadingthan that which cracked the plates during construction, according to histheory, 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 havegreat difficulty in convincing naturalists, who are aware that manyanimals live in enlarged burrows the stability of which is dependent onthe arching action of the earth; in fact, many of these burrows haveentrances under water. He would also have some difficulty in convincingthose experienced miners who, after a cave-in, always wait until theground has settled and compacted itself before tunneling, usually withapparent 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 toconsider 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 ormarbles, he seems to reason that the movement of the marbles woulddestroy the arch action. It is very difficult for the writer to conceivehow it would be possible for balls or marbles to move when confined asthey would be confined if the earth were composed of them instead of itspresent 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 theexperiment with sand and water, described by the writer, and is assuredthat, if he will see that the washers are absolutely tight beforeputting the water into the box, he can do this without bringing aboutthe collapse of the arch; the only essential condition is that thebottom shall be keyed up tightly, so as not to allow the escape of anysand. He is also referred to the two photographs, Plate XXIV, illustrating the writer's first experiment, showing how increases in theloading resulted in compacting the material of the arch and in theconsequent lowering of the false bottom. As long as the exposed sandabove this false bottom had cohesion enough to prevent the collapse ofthe "centering, " this arch could have been loaded with safety up to thelimits 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 equalto zero, which is true, but the writer's assumptions applied only tosolid material, and the liquid gives an essentially different conditionof pressure, as shown by a careful reading of the paper. In solid rockMr. Goodrich assumes an angle of repose equal to 90°, for which there isno authority; that is, solid rock has no known angle of repose. In orderto carry these assumptions to a definite conclusion, we must assume forthat material with an angle of repose of 90° some solid material whichhas weight but no thrust, such as blocks of ice piled vertically. Inthis case Mr. Goodrich can readily see that there will be no archingaction over the structure, and that the required thickness of key wouldbe infinite. As to the other case, it is somewhat difficult to conceiveof a solid with an angle of repose of zero; aqueous material does notfulfill this condition, as it is either a liquid or a combination ofwater and solid material. The best illustration, perhaps, would be toassume a material composed of iron filings, into which had been driven apowerful magnet, so that the iron filings would be drawn horizontally inone direction. It is easy to conceive, then, that in tunneling throughthis material there would be no necessity for holding up the roof; thedefinite thickness of key given, as being at the point of intersectionof two 45° angles, would be merely a precautionary measure, and wouldnot be required in practice. 

It is thus seen that both these conditions can be fulfilled withpractical illustrations; that is, for an angle of repose of 90°, thatmaterial which has weight and no thrust, and for an angle of repose ofzero, that solid material which has thrust but no weight. 

Mr. Goodrich says the author has given no experiments to prove hisstatement that the arch thrust is greater in dryer sand. If Mr. Goodrichwill make the experiment partially described as Experiment No. 3, withabsolutely dry sand, and with moist sand, and on a scale large enoughto eliminate cohesion, he will probably find enough to convince him thatin this assumption the writer is correct. At the same time, the writerhas based his theory in this regard on facts which are not entirelyconclusive, and his mind is open as to what future experiments on alarge scale may develop. It is very probable, however, that ananalytical and practical examination of the English experiments noted onpages 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. Forinstance, 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 thisconcession was one of the writer's as to certain pressures against or ontunnels, and, if Mr. Goodrich does not concede this, he is even moreradical 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 completequotation, which reads:

 "Nor can anyone, looking at Fig. 5, doubt, " etc. 

A glance at Fig. 5 will demonstrate that, under conditions there setforth, the writer is probably correct in his assertion as relating tothat 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 wallsare a maximum at the top and decrease to zero at the bottom" appliesonly to pressures exerted by absolutely dry and normally dry material, and it seems to him that this so-called theory is capable of such easydemonstration, by the simple observation of any bracing in a deep trenchin material of this class, that it ought to be accepted as at leastsafer than the old theory which it reverses. As to this "well-knowntheory" in material subject to water pressure, a careful reading of thepaper, or an examination of Fig. 12 and its accompanying text, or anexamination of Table 1, will convince Mr. Goodrich that, under thewriter's analysis, this pressure does not decrease to zero at thebottom, but that in soft materials it may be approximately constant allthe way down, while, in exceptionally soft material, conditions mayarise where it may increase toward the bottom. The determination shouldbe made by taking the solid material and drying it sufficiently so thatwater does not flow or seep from it. When this material is thencompacted 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 as60 degrees. The very fine matter should then be separated from thecoarser 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 partiallydried in which the natural angle of repose might be 60°, and in whichthe 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 measurementsof pressures on pistons through holes in the sheeting. He desires tostate again that he cannot regard such experiments as conclusive, andbelieves that they are of comparative value only, as such experiments donot measure in any large degree the pressure of the solid material butonly all or a portion of the so-called aqueous matter, that is, theliquid and very fine material which flows with it. Thus it is well knownthat, during the construction of the recent Hudson and North RiverTunnels, pressures were tested in the silt, some of which showed thatthe 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 thewriter, that in 1890 he made some tests of the pressure of this silt innormal 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 ofthe lock in the north tube (in normal air), and about 1000 ft. From theNew Jersey shore. It was found that the silt had become so firm that itdid not flow into the opening. Later, a 4-in. Collar and piston werebuilt into the opening, and, during a period covering at least 3 months, constant observations showed that no pressure came upon it; in fact, itwas stated that the piston was frequently worked back and forth toinduce pressure, but no response was obtained during all this period. The conclusion must then be drawn that when construction, with itsattendant disturbance, has stopped, the solid material surroundingstructures tends to compact itself more or less, and solidify, accordingas it is more or less porous, forming in many instances what may bevirtually 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 throughsmall openings can be verified by any one who will examine such materialback of bracing showing evidences of heavy pressure. The investigatorwill find that, if this material is free from water pressure, paperstuffed lightly into small openings will hold back indefinitely materialwhich in large masses has frequently caused bracing to buckle andsheeting planks to bend and break; and the writer reiterates that suchexperiments should be made in trenches sheeted with horizontal sheetingbearing against short vertical rangers and braces giving horizontalsections absolutely detached and independent of each other. In no otherway can such experiments be of real value (and even then only when madeon a large scale) to determine conclusively the pressure of earth ontrenches. 

As to the questions of the relative thrust of materials under variousangles of repose, and of the necessity of dividing by the tangent, etc. ;these, to the writer, seem to be merely the solution of problems insimple graphics. 

The writer believes that if Mr. Goodrich will make, even on a smallscale, some of the experiments noted by the writer, he will be convincedthat many of the assumptions which he cannot at present endorse arebased on fact, and his co-operation will be welcomed with the greatestinterest. Among the experiments which he is asked to make is the one indry sand, noted as Experiment No. 3, whereby it can be shown veryconclusively that additional back-fill will result in increased archingstability, on an arch which would collapse under lighter loading. 

The writer is indebted to Mr. Goodrich for pointing out some errors inomission and in typography (now corrected), and for his heartyconcurrence in some of the assumptions which the writer believed wouldmeet with greatest disapproval. 

In reply to Mr. Pruyn and Mr. Gregory, the writer assumed that thepiston area in Experiment No. 6 should be reduced only by the actualcontact of material with it. If this material in contact should becomposed of theoretical spheres, resulting in a contact with pointsonly, then the theoretical area reduced should be in proportion to thisamount only. The writer does not believe, however, that this conditionexists in practice, but thinks that the area is reduced very much morethan by the actual theoretical contact of the material. He sees noreason, as far as he has gone, to doubt the accuracy of the deductionsfrom this experiment. 

Regarding the question of the length of time required to raise thepiston, he does not believe that the position of his critics is entirelycorrect in this matter; that is, it must either be conceded that thepiston area is cut off from the source of pressure, or that it is incontact with it through more or less minute channels of water. If it iscut off, then the writer's contention is proved without the need of theexperiment, and it is therefore conclusive that a submerged tunnel isnot under aqueous pressure or the buoyant action of water. If, on theother hand, the water is in contact through channels bearing directlyupon the piston and leading to the clear water chamber, any increase inpressure in the water chamber must necessarily result in a virtuallyinstantaneous increase of the pressure against the piston, and thereforethe action on the latter should follow almost immediately. In all casesduring the experiments the piston did not respond until the pressure wasapproximately twice as great as required in clear water, therefore thewriter must conclude either that the experiments proved it conclusivelyor that his assumption is proved without the necessity of theexperiments. That is, the pressure is virtually not in evidence untilthe piston has commenced to move. 

Mr. Pruyn has added valuable information in his presentation of dataobtained from specific tests of the bearing value of, and friction on, hollow steel piles. These data largely corroborate tests andobservations by the writer, and are commended to general attention. 

Mr. Carter's information is also of special interest to the writer, asmuch of it is in the line of confirming his views. Mr. Carter does notyet accept the theory of increased pressure toward the top, but if hewill examine or experiment with heavy bracing in deep trenches in clearsand, or material with well-defined angles of repose, he will probablyfind much to help him toward the acceptance of this view. 

The writer regrets that he has not now the means or appliances forfurther experiments with the piston chamber, but he does not believethat reliable results could be obtained in broken stone with so small apiston, as it is possible that the point of one stone only might be incontact with the piston. This would naturally leave the base exposedalmost wholly to a clear water area. He does not believe, however, thatin practice the laying of broken stone under inverts will materiallychange the ultimate pressure unless its cross-section represents a largearea. 

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 thefriction 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 casesof failures of certain structures or bracing; and (_b_), that he hasnot in all cases a clear understanding of the paper. For instance, thewriter has not advocated the omission of bottom bracing or sheeting. Hehas seen many instances where it has been, or could have been, safelyomitted, but he desires to make it clear that he does not under anycircumstances advocate its omission in good work; but only that, inwell-designed bracing, its strength may be decreased as it approachesthe bottom. 

Reference is again made to the diagram, Fig. 12, which shows that, inmost cases of coffer-dams in combined aqueous and earth pressure, theremay be nearly equal, and in some cases even greater, loading toward thebottom. 

The writer also specifically states that in air the difference betweenaqueous and earth pressure is plainly noted by the fact that bracing isneeded so frequently to hold back the earth while the air is keeping outthe water. 

The lack of specific data is especially noticeable in the account of therise of the 6-ft. Conduit at Toronto. It would be of great interest toknow with certainly the weight of the pipe per foot, and whether it wasproperly bedded and properly back-filled. In all probability theback-filling over certain areas was not properly done, and as the pipewas exposed to an upward pressure of nearly 1600 lb. Per ft. , withprobably only 500 or 600 lb. Of weight to counterbalance it, it canreadily be seen that it did not conform with the writer's generalsuggestion, 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 thatoftentimes the tunnels under the East River approached very close to thesurface, with the material above them so soupy (owing to the escape ofcompressed air) that their upper surfaces were temporarily in water, yetthere was no instance in which they rose, although some of them wereunder excessive buoyant pressure. 

It is also of interest to note, from the papers descriptive of the NorthRiver Tunnel, that, with shield doors closed, the shield tended to rise, while by opening the doors to take in muck the shield could be broughtdown or kept down. The writer concurs with those who believe that therising of the shield with closed doors was due to the slightly greaterdensity of the material below, and was not in any way due to buoyancy. 

Concerning the collapse of the bracing in the tunnel built under aside-hill, the writer believes it was due to the fact that it was undera sliding side-hill, and that, if it had been possible to haveback-filled over and above this tunnel to a very large extent, thisback-fill would have resulted in checking the sliding of materialagainst the tunnel, and the work would thereafter have been done withsafety. This is corroborated by Mr. Thomson's statement that the tunnelwas subsequently carried through safely by going farther into the hill. 

As to the angle of repose, Mr. Thomson seems to feel that itsdetermination is so often impracticable that it is not to be relied on;and yet all calculations pertaining to earth pressure must be based onthis factor. The writer believes that the angle of repose is notdifficult to determine, and that observations of, and experiments on, exposed banks in similar material, and general experience in relationthereto, will enable one to determine it in nearly all cases within suchreasonably accurate limits that only a small margin of safety need beadded. 

Engineers are sent to Europe to study sewage disposal, waterpurification, transit problems, etc. , but are rarely sent to anadjoining county or State to look at an exposed bank, which wouldperhaps solve a vexed problem in bracing and result in great economy inthe design of permanent structures. 

Mr. Thomson's general views seem to indicate that much of the subjectmatter noted in the paper relates to unsolvable problems, for it appearsthat in many cases he believes the Engineer to be dependent on hiseducated guess, backed perhaps by the experienced guess of the foremanor practical man. The writer, on the contrary, believes that everyproblem relating to work of this class is capable of being solved, within reasonably accurate limits, and that the time is not far distantwhen the engineer, with his study of conditions, and samples of materialbefore him, will be able to solve his earth pressure and earthresistance problems as accurately as the bridge engineer, with hisknowledge of structural materials, solves bridge problems. 

The writer, in the course of his experience, has met with or beeninterested 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 withone at a depth of 90 ft. ?

What difference in timbering or in permanent design should be made for ahorizontally-sheeted shaft, 5 ft. Square, going to a depth of 45 ft. Andone 25 by 70 ft. , for instance, going to the same depth, assuming eachto be braced and sheeted horizontally with independent bracing?

What allowance should be made for the strength of interlock, assumingthat a circular bulkhead of sand, 30 ft. In diameter, is to be carriedby steel sheet-piling exposed around the outside for a depth of 40 ft. ?

What average pressure per square foot of area should be required todrive a section of a 3 by 15-ft. Roof shield, as compared with thepressure needed to drive the whole roof shield with an area four timesas great?

To what depth could a 12 by 12-in. Timber be driven, under graduallyadded 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 hadpenetrated soft, marshy material the bearing resistance of which waspractically valueless?

What allowance should be made for the buoyancy of a tunnel 20 ft. Indiameter, the top of which was buried to a depth of 20 ft. In sand abovewhich there was 40 ft. Of water?

It is believed by the writer that most of the authorities are silent asto the solution of problems similar to the above, and it is because ofthis lack of available data that he has directed his studies to them. The belief that the results of these studies, together with suchobservations and experiments as relate thereto, may be of interest, hascaused him to set them forth in this paper. 

He desires to state his belief that if problems similar to the abovewere given for definite solution, not based on ordinary safe practice, and without conference, to a number of engineers prominently interestedin such matters, the results would vary so widely as to convince some ofthe critics of this paper that the greater danger lies rather in thenon-exploration of such fields than in the setting forth of results ofexploration which may appear to be somewhat radical. 

Further, if these views result in stimulating enough interest to lead tothe hope that eventually the "Pressure, Resistance, and Stability" ofground under varying conditions will be known within reasonably accuratelimits and tabulated, the writer will feel that his efforts have notbeen in vain. 

FOOTNOTES:

[Footnote H: "Lateral Earth Pressures and Related Phenomena, "_Transactions_, Am. Soc. C. E. , Vol. LIII, p. 272. ]