Chapter II: Part 2

which, by comparison with the former method, would appear to be less safe in its reasoning.

Next is the question of pressure against a wall or braced trench for materials under Class A. The pressure of sand is first calculated independently, as shown in Fig. 6. Reducing this to a basis of 100 lb. for each division of the scale measured horizontally, as shown, gives the line, _B O_, Fig. 12, measuring the outside limit of pressure due to the earth, the horizontal distance at any point between this line and the vertical face equalling the pressure against that face divided by the tangent of the angle of repose, which in this case is assumed to be 45°, equalling unity. If the water pressure line, _C F_, is drawn, it shows the relative pressure of the water. In order to reduce this to the scale of 100 lb. horizontal measurement, the line, _C E_, is drawn, representing the water pressure to scale, that is, so that each horizontal measurement of the scale gives the pressure on the face at that point; and, allowing 50% for voids, halving this area gives the line, _C D_, between which and the vertical face any horizontal line measures the water pressure. Extending these pressure areas where they overlap gives the line, _B D_, which represents the total pressure against the face, measured horizontally.

Next, as to the question of buoyancy in Class A materials. If a submerged structure rests firmly on a bottom of more or less firm sand, its buoyancy, as indicated by the experiments, will only be a percentage of its buoyancy in pure water, corresponding to the voids in the sand. In practice, however, an attempt to show this condition will fail, owing to the fact that in such a structure the water will almost immediately work under the edge and bottom, and cause the structure to rise, and the test can only be made by measuring the difference in uplift in a heavier-than-water structure, as shown in Experiment No. 5. For, if a structure lighter than the displaced water be buried in sand sufficiently deep to insure it against the influx of large volumes of water below, it will not rise. That this is not due entirely to the friction of the solid material on the sides has been demonstrated by the observation of subaqueous structures, which always tend to subside rather than to lift during or following disturbance of the surrounding earth.

The following is quoted from the paper by Charles M. Jacobs, M. Am. Soc. C. E., on the North River Division of the Pennsylvania Railroad Tunnels:[E]

"There was considerable subsidence in the tunnels during
construction and lining, amounting to an average of 0.34 ft.
between the bulkhead lines. This settlement has been constantly
decreasing since construction, and appears to have been due almost
entirely to the disturbances of the surrounding materials during
construction. The silt weighs about 100 lb. per cu. ft. * * * and
contains about 38% of water. It was found that whenever this
material was disturbed outside the tunnels a displacement of the
tunnels followed."

This in substance confirms observations made in the Battery tubes that subsidence of the structure followed disturbance of the outside material, although theoretically the tubes were buoyant in the aqueous material.

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

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

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

As to the question of pure quicksand (if such there be) and other aqueous materials of Class C, such as water, oil, mercury, etc., it has already been shown that they are to be considered as liquids of their normal specific gravity; that is, in calculating the air pressure necessary to displace them, one should consider their specific gravity only, as a factor, and not the total weight per volume including any impurities which they might contain undissolved.

In order to have a clearer conception of aqueous and semi-aqueous materials and their action, they must be viewed under conditions not ordinarily apparent. For instance, ideas of so-called quicksand are largely drawn from seeing structures sinking into it, or from observing it flowing through voids in the sheeting or casing. The action of sand and water under pressure is viewed during or after a slump, when the damage is being done, or has been done, whereas the correct view-point is under static conditions, before the slump takes place.

The following is quoted from the report of Mr. C.M. Jacobs, Chief Engineer of the East River Gas Tunnel, built in 1892-93:

"We found that the material which had heretofore been firm or stiff
had, under erosion, obtained a soup-like consistency, and that a
huge cavity some 3 ft. wide and 26 ft. deep had been washed up
toward the river bed."

This would probably be a fair description of much of the material of this class met with in such work, if compressed air had not been used. The writer believes that in soft material surrounding submerged structures the water actually contained in the voids is not infrequently, after a prolonged period of rest, cut off absolutely from its sources of pressure and that contact with these sources of pressure will not again be resumed until a leak takes place through the structure; and, even when there is a small flow or trickling of water through such material, it confines itself to certain paths or channels, and is largely excluded from the general mass.

The broad principle of the bearing power of soil has been made the subject of too many experiments and too much controversy to be considered in a paper which is intended to be a description of experiments and observed data and notes therefrom. The writer is of the opinion, however, that entirely too little attention has been given to this bearing power of the soil; that while progress has been made in our knowledge of all classes of materials for structures, very little has been done which leads to any real knowledge of the material on which the foundation rests. For instance, it is inconceivable that 1 or 2 tons may sometimes be allowed on a square foot of soft clay, while the load on firm gravel is limited to from 4 to 6 tons. The writer's practical observations have convinced him that it is frequently much safer to put four times 6 tons on a square foot of gravel than it is to put one-fourth of 2 tons on a square foot of soft clay.

In connection with the bearing power of soil, the writer also believes that too little study has been given to the questions of the lateral pressure of earth, and he desires to quote here from some experiments described in a book[F] published in England in 1876, to which his attention has recently been called. This book appears to have been intended for young people, but it is of interest to note the following quotations from a chapter entitled "Sand." This chapter begins by stating that:

"During the course of a lecture on the Suez Canal by Mr. John H.
Pepper, which was delivered nightly by him at the Polytechnic
Institute in London, he illustrated his lecture by some experiments
designed to exhibit certain properties of sand, which had reference
to the construction of the Suez Canal, and it is stated that though
the properties in question were by no means to be classed among
recent discoveries, the experiments were novel in form and served
to interest the public audience."

Further quotation follows:

"When the Suez Canal was projected, many prophesied evil to the
undertaking, from the sand in the desert being drifted by the wind
into the canal, and others were apprehensive that where the canal
was cut through the sand the bottom would be pushed up by the
pressure on the banks * * *.

"The principle of lateral pressure may now be strikingly
illustrated by taking an American wooden pail and, having
previously cut a large circular hole in the bottom, this is now
covered with fine tissue paper, which should be carefully pasted on
to prevent the particles of sand from flowing through the small
openings between the paper and the wood * * * and being placed
upright and rapidly filled with sand, it may be carried about by
the handle without the slightest fear of the weight of the sand
breaking through the thin medium. * * *

"Probably one of the most convincing experiments is that which may
be performed with a cylindrical tube 18 in. long and 2 in. in
diameter, open at both ends. A piece of tissue paper is carefully
pasted on one end, so that when dry no cracks or interstices are
left. The tube is filled with dry sand to a height of say 12 in. In
the upper part is inserted a solid plug of wood 12 in. long and of
the same or very nearly the same diameter as the inside of the
tube, so that it will move freely up and down like the piston of an
air pump. The tube, sand, and piston being arranged as described,
may now be held by an assistant and the demonstrator, taking a
sledge hammer, may proceed to strike steadily on the end of the
piston and, although the paper will bulge out a little, the force
of the blow will not break it.

"If the assistant holding the tube allows it to jerk or rebound
after each blow of the hammer, the paper may break, because air and
sand are driven down by the succeeding blow, and therefore it must
be held steadily so that the piston bears fairly on the sand each
time.

"A still more conclusive and striking experiment may be shown with
a framework of metal constructed to represent a pail, the sides of
which are closed up by pasting sheets of tissue paper inside and
over the lower part. As before demonstrated, when a quantity of
sand is poured into the pail the tissue paper casing at the bottom
does not break, but if a sufficient quantity is used the sides
formed of tissue paper bulge out and usually give way in
consequence of the lateral pressure exerted by the particles of
sand."

The writer has made the second experiment noted, with special apparatus, and finds that with tissue paper over the bottom of a 2-in. pipe, 15 in. long, about 12 in. of sand will stand the blow of a heavy sledge hammer, transmitted through a wooden piston, at least once and sometimes two or three times, while heavy blows given with a lighter hammer have no effect at all. That this is not due in any large measure to inertia can be shown by the fact that more than 200 lb. can safely be put on top of the wooden piston. It cannot be accounted for entirely by the friction, as the removal of the paper allows the sand to drop in a mass. The explanation is that the pressure is transmitted laterally to the sides, and as the friction is directly proportional to the pressure, the load or effect of the blow is carried by the proportional increase in the friction, and any diaphragm which will carry the direct bottom load will not have its stresses largely increased by any greater loading on top.

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

The important consideration of this paper is that all the experiments and observations noted point conclusively to the fact that pressure is transmitted laterally through ground, most probably along or nearly parallel to the angles of repose, or in cases of rock or stiff material, along a line which, until more conclusive experiments are made, may be taken as a mean between the horizontal and vertical, or approximately 45 degrees. There is no reason to believe that this is not the case throughout the entire mass of the earth, that each cubic foot, or yard, or mile is supported or in turn supports its neighboring equivalent along such lines. The theory is not a new one, and its field is too large to encompass within the limits of a single paper, but, for practical purposes, and within the limited areas to which we must necessarily be confined, the writer believes it can be established beyond controversy as true. Certain it is that no one has yet found, in ground free from water pressure or abnormal conditions, any evidence of greater pressure at the bottom of a deep shaft or tunnel than that near the surface. Pressures due to the widening of mines beyond the limits of safety must not be taken as a controversion of this statement, as all arches have limits of safety, more especially if the useless material below the theoretical intrados is only partly supported, or is allowed to be suspended from the natural arch.

The writer believes, also, that the question of confined foundations, in contradistinction to that of the spreading of foundations, may be worthy of full discussion, as it applies to safe and economical construction, and he offers, without special comment, the following observations:

He has found that, in soft ground, results are often obtained with small open caissons sunk to a depth of a few feet and cleaned out and filled with concrete, which offer much better resistance than spreading the foundation over four or five times the equivalent area.

He has found that small steel piles and coffer-dams, from 1-ft. cylinders to coffer-dams 4 or 5 ft. square, sunk to a depth of only 1 or 2 ft. below adjacent excavations in ordinary sand, have safely resisted loads four or five times as great as those usually allowed.

He believes that short cylinders, cleaned out and filled with concrete, or coffer-dams of short steel piling with the surface cleaned out to a reasonable depth and filled with concrete horizontally reinforced, will, in many instances, give as good results as, and, in most cases, very much better than, placing the foundation on an equivalent number of small long piles or a proportionately greater spread of foundation area, the idea being that the transmission of pressure to the sides of the coffer-dam will not only confine the side thrust, but will also transfer the loading in mass to a greater depth where the resistance to lateral pressure in the ground will be more stable; that is, the greater depth of foundation is gained without the increased excessive loading, or necessity for deep excavation.

As to the question of the bearing value and friction on piles, the writer believes that while the literature on engineering is full of experimental data relating to friction on caissons, there is little to show the real value of friction on piles. The assumption generally made of an assumed bearing value, and the deduction therefrom of a value for the skin friction is fallacious. Distinction, also, is not made, but should be clearly drawn between skin friction, pure and simple, on smooth surfaces, and the friction due to pressure. Too often the bearing value on irregular surfaces as well as the bearing due to taper in piles, and lastly the resistance offered by binding, enter into the determination of so-called skin friction formulas. The essential condition of sinking a caisson is keeping it plumb; and binding, which is another way of writing increased bearing value, will oftentimes be fatal to success.

The writer believes that a series of observations on caissons sunk plumb under homogeneous conditions of ground and superficial smoothness will show a proportional increase of skin friction per square foot average for each increase in the size of caissons, as well as for increase of depth in the sinking up to certain points, where it may finally become constant, as will be shown later. The determination of the actual friction or coefficient of friction between the surfaces of the pile and the material it encounters, is not difficult to determine. In sand it is approximately 40% of the pressure for reasonably smooth iron or steel, and 45% of the pressure for ordinary wood surfaces. If, for instance, a long shaft be withdrawn vertically from moulding sand, the hole may remain indefinitely as long as water does not get into it or it does not dry out. This is due to the tendency of the sand to arch itself horizontally over small areas. The same operation cannot be performed on dry sand, as the arching properties, while protecting the pile from excessive pressure due to excessive length, will not prevent the loose sand immediately surrounding the pile from exerting a constant pressure against the pile, and it is of this pressure that 45% may be taken as the real value of skin friction on piles in dry sand.

In soft clays or peats which are displaced by driving, the tendency of this material to flow back into the original space causes pressure, of which the friction will be a measured percentage. In this case, however, the friction itself between the material and the clays or peat is usually very much less than 40%, and it is for this reason that piles of almost indefinite length may be driven in materials of this character without offering sufficient resistance to be depended on, as long as no good bearing ground is found at the point.

If this material is under water, and is so soft as to be considered semi-aqueous, the pressure per square foot will increase in diminishing proportion to the depth, and the pressure per area will soon approach and become a constant, due to the resistance offered by the lateral arching of the solid material; whereas, in large circular caissons, or caisson shafts, where the horizontal arching effect is virtually destroyed, or at least rendered non-effective until a great depth is reached, the pressure must necessarily vary under these conditions proportionately to the depth and size of the caisson in semi-aqueous material. On the other hand, in large caisson shafts, especially those which are square, the pressure at the top due to the solid material will also increase proportionately to the depth, as already explained in connection with the pressures of earth against sheeting and retaining walls.

The writer believes that the pressure on these surfaces may be determined with reasonable accuracy by the formulas already given in this paper, and with these pressures, multiplied by the coefficient of friction determined by the simplest experiment on the ground, results may be obtained which will closely approximate the actual friction on caissons at given depths. The friction on caissons, which is usually given at from 200 to 600 lb. per sq. ft., is frequently assumed to be the same on piles 12 in. or less in diameter, whereas the pressures on these surfaces, as shown, are in no way comparable.

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

The writer has in his possession a copy of an official print which was recently furnished to bidders in connection with the foundation for a large public building in New York City. The experiments were made on good sand at a depth of approximately 43 ft. below water and 47 ft. below an adjacent excavation. In this instance a 16-in. pipe was sunk to the depth stated, cleaned out, and a 14-in. piston connected to a 10-in. pipe was inserted and the ground at the bottom of the 16-in. pipe subjected to a loading approximating 28 tons per sq. ft. After an initial settlement of nearly 3 in., there was no further settlement over an extended period, although the load of 28 tons per sq. ft. was continued.

In connection with some recent underpinning work, 14-in. hollow cylindrical piles 6 ft. long were sunk to a depth of 6 ft. with an ordinary hand-hammer, being excavated as driven. These piles were then filled with concrete and subjected to a loading in some cases approximating 60 tons. After a settlement ranging from 9 to 13 in., no further settlement took place, although the loading was maintained for a considerable period.

In connection with some other pile work, the writer has seen a 10-in. pipe, 3/8 in. thick, 4 ft. below the bottom of an open cylinder, at a depth of about 20 ft., sustain in gravel and sand a load approximating 50 tons when cleaned out to within 2 ft. of the bottom.

He has seen other cylindrical piles with a bearing ring of not more than ¾ in. resting on gravel at a depth of from 20 to 30 ft., cleaned out practically to the bottom, sustain a measured load of 60 tons without settlement.

As to skin friction in sand, a case came under his observation wherein a 14-in. hollow cylindrical pile which had stood for 28 days at a depth of about 30 ft. in the sand, was cleaned out to its bottom and subjected to hydraulic pressure, measured by a gauge, and sunk 2 ft. into the sand without any pressure being registered on the gauge. It should be explained, however, that the gauge could be subjected to a pressure of 250 lb., equal to a total pressure of 7,000 lb. on the piston of the jack without registering, which corresponded, assuming it all as skin friction, to a maximum of not more than 78 lb. per sq. ft., but it should be noted that this included bearing value as well, and that the pressure was very far from 7,000 lb., in all probability, at the beginning of the test.

In the case of the California stove-pipe wells driven by the Board of Water Supply on Long Island, the writer is informed that one of these tubes, 12 in. in diameter, was sunk to a depth of 850 ft. In doing this work the pile was excavated below the footing with a sand pump and was then sunk by hydraulic pressure. Assuming the maximum capacity of the jacks at 100 tons, which is not probable, the skin friction could not have amounted to more than 75 lb. per sq. ft. It cannot be assumed in this case that the excavation of the material below the pile relieved the skin itself of some of its friction, as the operation consumed more than 6 weeks, and, even if excess material was removed, it is certain that a large percentage of it would have had time to adjust itself before the operation was completed.

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

The relation between bearing value and skin friction on a pile, where the end bearing is in good material, is well shown by a case where a wooden pile[G] struck solid material, was distorted under the continual blows of the hammer, and was afterward exposed. It is also shown in the case of a 14-in. California stove-pipe pile, No. 14 gauge, the point of which met firm material. The result, as shown by Fig. 1, Plate XXIX, speaks for itself. Fig. 2, Plate XXIX, shows a Chenoweth pile which was an experimental one driven by its designer. This pile, after getting into hard material, was subjected to the blow of a 4,000-lb. hammer falling the full length of the pile-driver, and the only result was to shatter the head of the pile, and not cause further penetration. Mr. Chenoweth has stated to the writer that he has found material so compact that it could not be penetrated with a solid pile--either with or without jetting--which is in line with the writer's experience.

The writer believes that the foregoing notes will show conclusively that the factor to be sought in pile work is bearing value rather than depth or skin friction, and, however valuable skin friction may be in the larger caissons, it cannot be depended on in the case of small piles, except in values ranging from 25 to 100 lb. per sq. ft.

In conclusion, he desires to thank the following gentlemen, who have contributed to the success of the experiments noted herein: Mr. James W. Nelson, of Richard Dudgeon, New York; Mr. George Noble, of John Simmons and Company, New York; and Mr. Pendleton, of Hindley and Pendleton, Brooklyn, N.Y.; all of whom have furnished apparatus for the experiments and have taken an interest in the results. And lastly, he desires especially to thank Mr. F.L. Cranford, of the Cranford Company, for men and material with which to make the experiments and without whose co-operation it would have been impracticable for the writer to have made them.

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

The writer believes that too much has been taken for granted in connection with earth pressures and resistance; and that, far too often, observations of the results of natural laws have been set down as phenomena. He believes that, both in experimenting and observing, the engineer will frequently find what is being looked for or expected and will fail to see the obvious alternative. He may add that his own experiments and observations may be criticized for the same reason, and he asks, therefore, that all possible light be thrown on this subject. A comparative study of much of our expert testimony or of the plans of almost any of the structures designed in connection with their bearing upon earth, or resistance to earth pressure, will show that under the present methods of interpretation of the underlying principles governing the calculations and designs relating to such structures, the results vary far too widely. Too much is left to the judgment of the engineer, and too frequently no fixed standards can be found for some of the most essential conditions.

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

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

TABLE 1.--PRESSURES ON TYPICAL STRUCTURES UNDER VARYING ASSUMED CONDITIONS.

_h_ = exterior height, _l_ = exterior width,

{ [delta] = depth of cover, that is,
{ _D_{E}_ = earth, and _D_{W}_ = water depth,

[phi] = angle of repose, and, for tunnels _D_{W}_ > _D_{E}_ a depth

_l_ [phi]
= ----- ( 45° + ------- )
2 2

_W_{E}_ = weight of 1 cu. ft. of earth = 90 lb.; _W_{W}_ = weight of 1 cu. ft. of water = 62½ lb.

Conditions: 1 = normal sand, 2 = dry sand, 3 = supersaturated firm sand with 40% of voids, 4 = supersaturated semi-aqueous material, 60% aqueous, that is, 60% water and aqueous material.

_______________________________________________________
| | | | |
Combined | | | | |
assumed | _h_ | _l_ | [phi] | _D_{E}_ |
conditions. | | | | |
______________|________|________|________|____________|
| | | | |
I_{1} | 20 | 30 | 45° | 40 |
I_{2} | 20 | 30 | 30° | 40 |
II_{1} | 15 | 15 | 45° | 40 |
II_{2} | 15 | 15 | 30° | 40 |
III_{1} | 15 | | 45° | 15 |
III_{2} | 15 | | 30° | 15 |
IV_{1} | 30 | | 45° | 30 |
IV_{2} | 30 | | 30° | 30 |
______________|________|________|________|____________|

____________________________________________________________________
| | | | | |
Combined | | | | | |
assumed | _h_ | _l_ | [phi] | _D_{E}_ | _D_{W}_ |
conditions. | | | | | |
______________|________|________|________|____________|____________|
| | | | | |
I_{3} | 20 | 30 | 50° | 40 | 60 |
I_{4} | 20 | 30 | 40° | 40 | 60 |
II_{3} | 15 | 15 | 50° | 40 | 60 |
II_{4} | 15 | 15 | 40° | 40 | 60 |
III_{3} | 15 | | 50° | 15 | 15 |
III_{4} | 15 | | 40° | 15 | 15 |
IV_{3} | 30 | | 50° | 30 | 30 |
IV_{4} | 30 | | 40° | 30 | 30 |
______________|________|________|________|____________|____________|

APPROXIMATE PRESSURES ON TUNNELS, PER SQUARE FOOT.

_________________________________________________________________________ | | | | || | | | Pressure | I_{1}| I_{3}| I_{3}| I_{3} || I_{2}| I_{4}| I_{4}| I_{4} per square|Earth.|Earth.|Water.|Combined.||Earth.|Earth.|Water.|Combined. foot, at | | | | || | | | __________|______|______|______|_________||______|______|______|_________ | | | | || | | | A | 3,240| 3,690| 1,500| 5,190 || 2,325| 2,880| 2,250| 5,130 B | 2,745| 3,105| 1,500| 4,605 || 1,845| 2,385| 2,250| 4,635 C | 2,160| 2,475| 1,500| 3,975 || 1,350| 1,800| 2,250| 4,050 D | 450| 540| 1,500| 2,040 || 450| 450| 2,250| 2,700 E | 360| 360| 1,625| 1,985 || 450| 450| 2,438| 2,888 F | 270| 270| 1,750| 2,025 || 450| 360| 2,626| 2,986 G | 225| 225| 1,875| 2,100 || 360| 270| 2,814| 3,084 __________|______|______|______|_________||______|______|______|_________ _________________________________________________________________________ | | | | || | | | Pressure |II_{1}|II_{3}|II_{3}|II_{3} ||II_{2}|II_{4}|II_{4}|II_{4} per square|Earth.|Earth.|Water.|Combined.||Earth.|Earth.|Water.|Combined. foot at | | | | || | | | __________|______|______|______|_________||______|______|______|_________ | | | | || | | | A | 1,485| 1,755| 1,500| 3,255 || 1,035| 1,305| 2,250| 3,555 B | 1,305| 1,485| 1,500| 2,985 || 945| 1,170| 2,250| 3,420 C | 1,125| 1,215| 1,500| 2,715 || 810| 990| 2,250| 3,240 D | 405| 405| 1,500| 1,905 || 540| 450| 2,250| 2,700 E | 405| 405| 1,625| 2,030 || 540| 450| 2,438| 2,888 F | 360| 360| 1,750| 2,110 || 540| 450| 2,626| 3,076 G | 315| 315| 1,875| 2,190 || 360| 360| 2,814| 3,174 H | 180| 225| 2,000| 2,225 || 180| 180| 3,000| 3,180 I | 90| 110| 2,175| 2,285 || 135| 135| 3,188| 3,323 __________|______|______|______|_________||______|______|______|_________

APPROXIMATE PRESSURES ON SHEETED TRENCH FACES OR WALLS

___________________________________________________________________________ | | | | || | | | Pressure |III_{1}|III_{3}|III_{3}|III_{3}||III_{2}|III_{4}|III_{4}|III_{4} per square|Earth. |Earth. |Water. | Total ||Earth. |Earth. |Water. | Total foot at | | | | earth || | | | earth | | | | and || | | | and | | | | water.|| | | | water. __________|_______|_______|_______|_______||_______|_______|_______|_______ | | | | || | | | A | 575 | 510 | 100 | 610 || 1,350 | 810 | 140 | 950 B | 400 | 350 | 190 | 540 || 900 | 540 | 260 | 800 C | 200 | 175 | 280 | 455 || 450 | 270 | 380 | 650 __________|_______|_______|_______|_______||_______|_______|_______|_______ ___________________________________________________________________ | | | | || | | | Pressure |IV_{1}|IV_{3}|IV_{3}|IV_{3}||IV_{2}|IV_{4}|IV_{4}|IV_{4} per square|Earth.|Earth.|Water.|Total ||Earth.|Earth.|Water.|Total foot at | | | |earth || | | |earth | | | | and || | | | and | | | |water.|| | | |water. __________|______|______|______|______||______|______|______|______ | | | | || | | | A | 1,370| 1,210| 100 | 1,310|| 3,175| 1,910| 150| 2,060 B | 1,170| 1,030| 200 | 1,230|| 2,700| 1,610| 290| 1,900 C | 970| 855| 290 | 1,145|| 2,250| 1,355| 430| 1,785 D | 775| 680| 370 | 1,050|| 1,800| 1,100| 570| 1,670 E | 590| 515| 460 | 975|| 1,350| 820| 710| 1,530 F | 400| 350| 560 | 910|| 900| 540| 860| 1,400 G | 190| 170| 650 | 820|| 450| 275| 1,000| 1,275 __________|______|______|______|______||______|______|______|______

FOOTNOTES:

[Footnote A: Presented at the meeting of May 18th, 1910.]

[Footnote B: _Transactions_, Am. Soc. C. E., Vol. LX, p. 1.]

[Footnote C: _Engineering News_, July 1st, 1909.]

[Footnote D: From "Gravel for Good Roads."]

[Footnote E: _Transactions_, Am. Soc. C. E., Vol. LXVIII, pp. 58-60.]

[Footnote F: "Discoveries and Inventions of the Nineteenth Century," by Robert Routledge, Assistant Examiner in Chemistry and in Natural Philosophy to the University of London.]

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

DISCUSSION

T. KENNARD THOMSON, M. AM. SOC. C. E.--Although the author deserves great credit for the careful and thorough manner in which he has handled this subject, his paper should be labeled "Dangerous for Beginners," especially as he is an engineer of great practical experience; if he were not, comparatively little attention would be paid to his statements. The paper is dangerous because many will read only portions of it, or will not read it thoroughly. For instance, at the beginning, the author cites several experiments in which considerable force is required to start the lifting of a weight or plunger in sand and water and much less after the start. This reminds the speaker of the time when, as a schoolboy, he tried to pick up stones from the bottom of the river and was told that the "suction" was caused by atmospheric pressure.

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

Another reason for calling this paper dangerous for beginners is that it is improbable that experienced engineers or contractors will omit the bracing at the bottom, although, since the paper was printed, a glaring instance has occurred where comparatively little bracing was put in the bottom of a 40-ft. cut, the result being a bad cave-in from the bottom, although all the top braces remained in place. Most engineers will agree that nearly every crib which has failed slipped out from the bottom, and did not turn over.

The objection to the angle of repose is that it is not possible to ascertain it for any material deposited by Nature. It could probably be ascertained for a sand bank deposited by Man, but not for an excavation to be made in the ground, for it is known that nearly all earth, etc., has been deposited under great pressure, and is likely to be cemented together by clay, loam, roots, trees, boulders, etc., and differs in character every few feet.

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

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

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

In calculating retaining walls, many engineers add the weight of the soil to the water, and calculate for from 90 to 100 lb. per cu. ft. The speaker is satisfied that in the so-called New York quicksand it is sufficient to use the weight of the water only. If the sand increased the side pressure above the water pressure, engineers would expect to use more compressed air to hold it back, while, as a matter of fact, the air pressure used seldom varies much from that called for by the hydrostatic head.

Although allowance for water pressure is sufficient for designing retaining walls in New York quicksand, it is far from sufficient in certain silty materials. For instance, in Maryland, a coffer-dam, excavated to a depth of 30 ft. in silt and water, had the bottom shoved in 2 ft., in spite of the fact that the waling pieces were 5 ft. apart vertically at the top and 3 ft. at the bottom, and were braced with 12 by 12-in. timbers, every 7 ft. horizontally. The walings split, and the cross-braces cut into the waling pieces from 1 to 2 in.; in other words, the pressure seemed to be almost irresistible. This is quite a contrast to certain excavations in Brooklyn, which, without any bracing whatever, were safely carried down 15 ft.

Any engineer who tries to guess at the angle of repose, and, from the resulting calculations, economizes on his bottom struts, will find that sooner or later an accident on one job will cause enough loss of life and money to pay for conservative timbers for the rest of his life. So much for side pressures. As to the pressure in the roof of a tunnel, probably every engineer will agree that almost any material except unfrozen water will tend to arch more or less, but how much it is impossible to say. It is doubtful whether any experienced engineer would ever try to carry all the weight over the roof, except in the case of back-fill, and even then he would have to make his own assumption (which sounds more polite than "guess").

The author has stated, however, that when the tunnel roof and sides are in place, no further trouble need be feared. On the contrary, in 1885, the Canadian Pacific Railroad built a tunnel through clayey material and lined it with ordinary 12 by 12-in. timber framing, about 2 or 3 ft. apart. After the tunnel was completed, it collapsed. It was re-excavated and lined with 12 by 12-in. timbers side by side, and it collapsed again; then the tunnel was abandoned, and, for some 20 years, the track, carried around on a 23° curve, was used until a new tunnel was built farther in. This trouble could have been caused either by the sliding or swelling of the material, and the speaker is inclined to believe that it was caused by swelling, for it is known, of course, that most material has been deposited by Nature under great pressure, and, by excavating in certain materials, the air and moisture would cause those materials to swell and become an irresistible force.

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

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

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

The depth and coarseness of the sand were not varied to ascertain whether any relation exists between them and the pressure required to lift the piston. If the pressure varied with the depth of sand, it would indicate that the reduction was due to the resistance of the water when finely divided by the sand; if it varied with the coarseness of the sand, as it undoubtedly would, especially if the sand grains were increased to spheres 1 in. in diameter, it would show that it was independent of the voids in the sand, but dependent on dividing the water into thin films.

The speaker believes that the greater part of the reduction of pressure on the bottom of the piston might be better explained by the viscosity of the water, than to assume that a considerable part of the plunger is not in contact with it. The water, being divided by fine sand into very thin films, has a tensile strength which is capable of resisting the pressure for at least a limited time.

If the water is capable of exerting its full hydrostatic pressure through the sand, the total pressure would be the full hydrostatic pressure on the bottom of the piston where in contact, and, where separated from it by a grain of sand, the pressure would be decreased only by the weight of the grain. If a large proportion of the top area of a grain is in contact, as assumed by the author, this reduction of pressure would be very small. A correct interpretation can be obtained only after more complete experiments have been made.

For horizontal pressures exerted by saturated sands on vertical walls, it has not been demonstrated that anything should be deducted from full water pressure. No matter how much of the area is in direct contact with the sand rather than the water, the full water pressure would be transmitted through each sand grain from its other side and, if necessary, from and through many other grains which may be in turn in contact with it. The pressure on such a wall will be water pressure over its entire surface, and, in addition, the thrust of the sand after correcting for its loss of weight in the water.

The fact that small cavities may be excavated from the sides of trenches or tunnels back of the sheeting proves only that there is a local temporary arching of the material, or that the cohesion of the particles is sufficient to withstand the stress temporarily, or that there is a combination of cohesion and arching. The possibility of making such excavations does not prove that pressure does not exist at such points. That sand or earth will arch under certain conditions has long been an accepted fact. The sand arches experimented with developed their strength only after considerable yielding and, therefore, give no index of the distribution or intensity of stress before such yielding. Furthermore, sand and earth in Nature are not constrained by forms and reinforcing rods.

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

FRANCIS W. PERRY, ASSOC. M. AM. SOC. C. E.--Pressure-gauge observations on a number of pneumatic caissons recently sunk, through various grades of sand, to rock at depths of from 85 to 105 ft. below ground-water, invariably showed working-chamber air-pressures equal, as closely as could be observed, to the hydrostatic pressures computed, for corresponding depths of cutting-edge, as given in Table 2.

These observations and computations were made by the speaker in connection with the caisson foundations for the Municipal Building, New York City.

TABLE 2.--EQUIVALENT FEET OF DEPTH BELOW WATER PER POUND PRESSURE.

Pressure, |Equivalent |Equivalent |Observed | in |feet of |elevation |pressure. | pounds. |depth. |for water | | | |at--6.85. | | |___________|_____________| | | | | | |M.H.W. |Ground-water.| | __________|___________|_____________|______________| | | | | 1 | 2.31 | 9.06 |Practically | 2 | 4.63 | 11.48 |the same as | 3 | 6.94 | 13.79 |computed | 4 | 9.25 | 16.10 |for | 5 | 11.57 | 18.42 |ground-water. | 6 | 13.88 | 20.73 | | 7 | 16.19 | 23.04 | | 8 | 18.50 | 25.35 | | 9 | 20.82 | 27.67 | | 10 | 23.13 | 29.98 | | 11 | 25.44 | 32.29 | | 12 | 27.76 | 34.61 | | 13 | 30.07 | 36.92 | | 14 | 32.38 | 39.23 | | 15 | 34.70 | 41.55 | | 16 | 37.01 | 43.86 | | 17 | 39.32 | 46.17 | | 18 | 41.63 | 48.48 | | 19 | 43.95 | 50.80 | | 20 | 46.26 | 53.11 | | 21 | 48.57 | 55.42 | | 22 | 50.89 | 57.74 | | 23 | 53.20 | 60.05 | | 24 | 55.51 | 62.36 | | 25 | 57.82 | 64.67 | | 26 | 60.14 | 66.99 | | 27 | 62.45 | 69.30 | | 28 | 64.76 | 71.61 | | 29 | 67.08 | 73.93 | | 30 | 69.39 | 76.24 | | 31 | 71.70 | 78.55 | | 32 | 74.01 | 80.86 | | 33 | 76.33 | 83.18 | | 34 | 78.64 | 85.49 | | 35 | 80.95 | 87.80 | | 36 | 83.27 | 90.12 | | 37 | 85.58 | 92.43 | | 38 | 87.89 | 94.74 | | 39 | 90.20 | 97.05 | | 40 | 92.52 | 99.37 | | 41 | 94.83 |101.68 | | 42 | 97.14 |103.99 | | 43 | 99.46 |106.31 | | 44 |101.77 |108.62 | | 45 |104.08 |110.93 | | 46 |106.39 |113.24 | | __________|___________|_____________|______________|

34
NOTE.--Equivalent depth in feet = ------ × pressure.
14.7

E.P. GOODRICH, M. AM. SOC. C. E. (by letter).--This paper is to be characterized by superlatives. Parts of it are believed to be exceptionally good, while other parts are considered equally dangerous. The author's experimental work is extremely interesting, and the writer believes the results obtained to be of great value; but the analytical work, both mathematical and logical, is emphatically questioned.

The writer believes that, in the design of permanent structures, consideration of arch action should not be included, at least, not until much more information has been obtained. He also believes that the design of temporary structures with this inclusion is actually dangerous in some instances, and takes the liberty of citing the following statement by the author, with regard to his first experiment:

"About an hour after the superimposed load had been removed, the
writer jostled the box with his foot sufficiently to dislodge some
of the exposed sand, when the arch at once collapsed and the bottom
fell to the ground."

The writer emphatically questions the author's ideas as to "the thickness of key" which "should be allowed" over tunnels, believing that conditions within an earth mass, except in very rare instances, are such that true arch action will seldom take place to any definite extent, through any considerable depths. Furthermore, the author's reason for bisecting the angle between the vertical and the angle of repose of the material, when he undertakes to determine the thickness of key, is not obvious. This assumption is shown to be absurd when carried to either limit, for when the angle of repose equals zero, as is the case with water, this, method would give a definite thickness of key, while there can be absolutely no arch action possible in such a case; and, when the angle of repose is 90°, as may be assumed in the case of rock, this method would give an infinite thickness of key, which is again seen to be absurd. It would seem as if altogether too many unknowable conditions had been assumed. In any case, no arch action can be brought into play until a certain amount of settlement has taken place so as to bring the particles into closer contact, and in such a way that the internal stresses are practically those only of compression, and the shearing stresses are within the limits possible for the material in question.

The author has repeatedly made assumptions which are not borne out by the application of his mathematical formulas to actual extreme conditions. This method of application to limiting conditions is concededly sometimes faulty; but the writer believes that no earth pressure theory, or one concerning arch action, can be considered as satisfactory which does not apply equally well to hydraulic pressure problems when the proper assumptions are made as to the factors for friction, cohesion, etc. For example, when the angle of repose is considered as zero, in the author's first formula for _W_{1}_, the value becomes ½ _W_{1}_, whereas it should depend solely on the depth, which does not enter the formula, and not at all on the width of opening, _l_, which is thus included.

The author has given no experiments to prove his statement that "the arch thrust is greater in dryer sand," and the accuracy of the statement is questioned. Again, no reason is apparent for assuming the direction of the "rakers" in Fig. 3 as that of the angle of repose. The writer cannot see why that particular angle is repeatedly used, when almost any other would give results of a similar kind. The author has made no experiments which show any connection between the angle of repose, as he interprets it, and the lines of arch action which he assumes to exist.