Part 4

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.