T. KENNARD THOMSON, M. AM. SOC. C. E.--Although the author
deserves great credit for the careful and thorough manner in which he
has handled this subject, his paper should be labeled "Dangerous for
Beginners," especially as he is an engineer of great practical
experience; if he were not, comparatively little attention would be paid
to his statements. The paper is dangerous because many will read only
portions of it, or will not read it thoroughly. For instance, at the
beginning, the author cites several experiments in which considerable
force is required to start the lifting of a weight or plunger in sand
and water and much less after the start. This reminds the speaker of the
time when, as a schoolboy, he tried to pick up stones from the bottom of
the river and was told that the "suction" was caused by atmospheric
pressure.
The inference is that tunnels, etc., in sand, etc., are not in any
danger of rising, even though they are lighter than water. Toward the
end of the paper, however, the author states that tunnels should be
weighted, but he rather spoils this by stating that they should be
weighted only enough to overcome the actual water pressure, that is,
between the voids of the sand. It seems to the speaker that the only
really safe way is to make the tunnel at least as heavy as the water
displaced in order to prevent it from coming up, and to take other
measures to prevent it from going down. The City of Toronto, Canada,
formerly pumped its water supply through a 6-ft. iron pipe, buried in
the sand under Toronto Bay and then under Toronto Island, with an intake
in the deep water of the lake. During a storm a mass of seaweed, etc.,
was washed against the intake, completely blocking it, and although the
man at the pumping station knew that something was wrong, he continued
to pump until the water was drawn out of the pipe, with the result that
about half a mile of the conduit started to rise and then broke at
several places, thus allowing it to fill with water. Eventually, the
city went down to bed-rock under the Bay for its water tunnel.
Another reason for calling this paper dangerous for beginners is that it
is improbable that experienced engineers or contractors will omit the
bracing at the bottom, although, since the paper was printed, a glaring
instance has occurred where comparatively little bracing was put in the
bottom of a 40-ft. cut, the result being a bad cave-in from the bottom,
although all the top braces remained in place. Most engineers will agree
that nearly every crib which has failed slipped out from the bottom, and
did not turn over.
The objection to the angle of repose is that it is not possible to
ascertain it for any material deposited by Nature. It could probably be
ascertained for a sand bank deposited by Man, but not for an excavation
to be made in the ground, for it is known that nearly all earth, etc.,
has been deposited under great pressure, and is likely to be cemented
together by clay, loam, roots, trees, boulders, etc., and differs in
character every few feet.
A deep vertical cut can often be made, even in New York quicksand, from
which the water has been drawn, and, if not subjected to jars, water,
etc., this material will stand for considerable time and then come down
like an avalanche, killing any one in its way. In such cases very little
bracing would prevent the slide from starting, provided rain, etc., did
not loosen the material.
The author, of course, treats dry and wet materials differently, but
there are very few places where dry material is not likely to become wet
before the excavation is completed.
In caisson work, if the caisson can be kept absolutely plumb, it can be
sunk without having to overcome much friction, while, on the other hand,
if it is not kept plumb, the material is more or less disturbed and
begins to bind, causing considerable friction. The author claims that
the pressure does not increase with the depth, but all caisson men will
probably remember that the friction to be overcome per square foot of
surface increases with the depth.
In calculating retaining walls, many engineers add the weight of the
soil to the water, and calculate for from 90 to 100 lb. per cu. ft. The
speaker is satisfied that in the so-called New York quicksand it is
sufficient to use the weight of the water only. If the sand increased
the side pressure above the water pressure, engineers would expect to
use more compressed air to hold it back, while, as a matter of fact, the
air pressure used seldom varies much from that called for by the
hydrostatic head.
Although allowance for water pressure is sufficient for designing
retaining walls in New York quicksand, it is far from sufficient in
certain silty materials. For instance, in Maryland, a coffer-dam,
excavated to a depth of 30 ft. in silt and water, had the bottom shoved
in 2 ft., in spite of the fact that the waling pieces were 5 ft. apart
vertically at the top and 3 ft. at the bottom, and were braced with 12
by 12-in. timbers, every 7 ft. horizontally. The walings split, and the
cross-braces cut into the waling pieces from 1 to 2 in.; in other words,
the pressure seemed to be almost irresistible. This is quite a contrast
to certain excavations in Brooklyn, which, without any bracing whatever,
were safely carried down 15 ft.
Any engineer who tries to guess at the angle of repose, and, from the
resulting calculations, economizes on his bottom struts, will find that
sooner or later an accident on one job will cause enough loss of life
and money to pay for conservative timbers for the rest of his life. So
much for side pressures. As to the pressure in the roof of a tunnel,
probably every engineer will agree that almost any material except
unfrozen water will tend to arch more or less, but how much it is
impossible to say. It is doubtful whether any experienced engineer would
ever try to carry all the weight over the roof, except in the case of
back-fill, and even then he would have to make his own assumption (which
sounds more polite than "guess").
The author has stated, however, that when the tunnel roof and sides are
in place, no further trouble need be feared. On the contrary, in 1885,
the Canadian Pacific Railroad built a tunnel through clayey material and
lined it with ordinary 12 by 12-in. timber framing, about 2 or 3 ft.
apart. After the tunnel was completed, it collapsed. It was re-excavated
and lined with 12 by 12-in. timbers side by side, and it collapsed
again; then the tunnel was abandoned, and, for some 20 years, the track,
carried around on a 23° curve, was used until a new tunnel was built
farther in. This trouble could have been caused either by the sliding or
swelling of the material, and the speaker is inclined to believe that it
was caused by swelling, for it is known, of course, that most material
has been deposited by Nature under great pressure, and, by excavating in
certain materials, the air and moisture would cause those materials to
swell and become an irresistible force.
To carry the load, Mr. Meem prefers to rely on the points of the piles
rather than the side friction. In such cases the pile would act as a
post, and would probably fail when ordinarily loaded, unless firmly
supported at the sides. The speaker has seen piles driven from 80 to 90
ft. in 10 min., which offered almost no resistance, and yet, a few days
later, they would sustain 40 tons each. No one would dream of putting 40
tons on a 90-ft. pile resting on rock, if it were not adequately
supported.
It is the speaker's opinion that bracing should not be omitted for
either piles or coffer-dams.
CHARLES E. GREGORY, ASSOC. M. AM. SOC. C. E.--In describing his
last experiment with the hydraulic chambers and plunger, Mr. Meem states
that, after letting the pressure stand at 25 lb., etc., the piston came
up. This suggests that the piston might have been raised at a much lower
pressure, if it had been allowed to stand long enough.
The depth and coarseness of the sand were not varied to ascertain
whether any relation exists between them and the pressure required to
lift the piston. If the pressure varied with the depth of sand, it would
indicate that the reduction was due to the resistance of the water when
finely divided by the sand; if it varied with the coarseness of the
sand, as it undoubtedly would, especially if the sand grains were
increased to spheres 1 in. in diameter, it would show that it was
independent of the voids in the sand, but dependent on dividing the
water into thin films.
The speaker believes that the greater part of the reduction of pressure
on the bottom of the piston might be better explained by the viscosity
of the water, than to assume that a considerable part of the plunger is
not in contact with it. The water, being divided by fine sand into very
thin films, has a tensile strength which is capable of resisting the
pressure for at least a limited time.
If the water is capable of exerting its full hydrostatic pressure
through the sand, the total pressure would be the full hydrostatic
pressure on the bottom of the piston where in contact, and, where
separated from it by a grain of sand, the pressure would be decreased
only by the weight of the grain. If a large proportion of the top area
of a grain is in contact, as assumed by the author, this reduction of
pressure would be very small. A correct interpretation can be obtained
only after more complete experiments have been made.
For horizontal pressures exerted by saturated sands on vertical walls,
it has not been demonstrated that anything should be deducted from full
water pressure. No matter how much of the area is in direct contact with
the sand rather than the water, the full water pressure would be
transmitted through each sand grain from its other side and, if
necessary, from and through many other grains which may be in turn in
contact with it. The pressure on such a wall will be water pressure over
its entire surface, and, in addition, the thrust of the sand after
correcting for its loss of weight in the water.
The fact that small cavities may be excavated from the sides of trenches
or tunnels back of the sheeting proves only that there is a local
temporary arching of the material, or that the cohesion of the particles
is sufficient to withstand the stress temporarily, or that there is a
combination of cohesion and arching. The possibility of making such
excavations does not prove that pressure does not exist at such points.
That sand or earth will arch under certain conditions has long been an
accepted fact. The sand arches experimented with developed their
strength only after considerable yielding and, therefore, give no index
of the distribution or intensity of stress before such yielding.
Furthermore, sand and earth in Nature are not constrained by forms and
reinforcing rods.
Mr. Meem's paper is very valuable in that it presents some unusual
phenomena, but many of the conclusions drawn therefrom cannot be
accepted without further demonstration.
FRANCIS W. PERRY, ASSOC. M. AM. SOC. C. E.--Pressure-gauge
observations on a number of pneumatic caissons recently sunk, through
various grades of sand, to rock at depths of from 85 to 105 ft. below
ground-water, invariably showed working-chamber air-pressures equal, as
closely as could be observed, to the hydrostatic pressures computed, for
corresponding depths of cutting-edge, as given in Table 2.
These observations and computations were made by the speaker in
connection with the caisson foundations for the Municipal Building, New
York City.
TABLE 2.--EQUIVALENT FEET OF DEPTH BELOW WATER PER POUND
PRESSURE.
Pressure, |Equivalent |Equivalent |Observed |
in |feet of |elevation |pressure. |
pounds. |depth. |for water | |
| |at--6.85. | |
|___________|_____________| |
| | | |
|M.H.W. |Ground-water.| |
__________|___________|_____________|______________|
| | | |
1 | 2.31 | 9.06 |Practically |
2 | 4.63 | 11.48 |the same as |
3 | 6.94 | 13.79 |computed |
4 | 9.25 | 16.10 |for |
5 | 11.57 | 18.42 |ground-water. |
6 | 13.88 | 20.73 | |
7 | 16.19 | 23.04 | |
8 | 18.50 | 25.35 | |
9 | 20.82 | 27.67 | |
10 | 23.13 | 29.98 | |
11 | 25.44 | 32.29 | |
12 | 27.76 | 34.61 | |
13 | 30.07 | 36.92 | |
14 | 32.38 | 39.23 | |
15 | 34.70 | 41.55 | |
16 | 37.01 | 43.86 | |
17 | 39.32 | 46.17 | |
18 | 41.63 | 48.48 | |
19 | 43.95 | 50.80 | |
20 | 46.26 | 53.11 | |
21 | 48.57 | 55.42 | |
22 | 50.89 | 57.74 | |
23 | 53.20 | 60.05 | |
24 | 55.51 | 62.36 | |
25 | 57.82 | 64.67 | |
26 | 60.14 | 66.99 | |
27 | 62.45 | 69.30 | |
28 | 64.76 | 71.61 | |
29 | 67.08 | 73.93 | |
30 | 69.39 | 76.24 | |
31 | 71.70 | 78.55 | |
32 | 74.01 | 80.86 | |
33 | 76.33 | 83.18 | |
34 | 78.64 | 85.49 | |
35 | 80.95 | 87.80 | |
36 | 83.27 | 90.12 | |
37 | 85.58 | 92.43 | |
38 | 87.89 | 94.74 | |
39 | 90.20 | 97.05 | |
40 | 92.52 | 99.37 | |
41 | 94.83 |101.68 | |
42 | 97.14 |103.99 | |
43 | 99.46 |106.31 | |
44 |101.77 |108.62 | |
45 |104.08 |110.93 | |
46 |106.39 |113.24 | |
__________|___________|_____________|______________|
34
NOTE.--Equivalent depth in feet = ------ × pressure.
14.7
E.P. GOODRICH, M. AM. SOC. C. E. (by letter).--This paper is to
be characterized by superlatives. Parts of it are believed to be
exceptionally good, while other parts are considered equally dangerous.
The author's experimental work is extremely interesting, and the writer
believes the results obtained to be of great value; but the analytical
work, both mathematical and logical, is emphatically questioned.
The writer believes that, in the design of permanent structures,
consideration of arch action should not be included, at least, not until
much more information has been obtained. He also believes that the
design of temporary structures with this inclusion is actually dangerous
in some instances, and takes the liberty of citing the following
statement by the author, with regard to his first experiment:
"About an hour after the superimposed load had been removed, the
writer jostled the box with his foot sufficiently to dislodge some
of the exposed sand, when the arch at once collapsed and the bottom
fell to the ground."
The writer emphatically questions the author's ideas as to "the
thickness of key" which "should be allowed" over tunnels, believing that
conditions within an earth mass, except in very rare instances, are
such that true arch action will seldom take place to any definite
extent, through any considerable depths. Furthermore, the author's
reason for bisecting the angle between the vertical and the angle of
repose of the material, when he undertakes to determine the thickness of
key, is not obvious. This assumption is shown to be absurd when carried
to either limit, for when the angle of repose equals zero, as is the
case with water, this, method would give a definite thickness of key,
while there can be absolutely no arch action possible in such a case;
and, when the angle of repose is 90°, as may be assumed in the case of
rock, this method would give an infinite thickness of key, which is
again seen to be absurd. It would seem as if altogether too many
unknowable conditions had been assumed. In any case, no arch action can
be brought into play until a certain amount of settlement has taken
place so as to bring the particles into closer contact, and in such a
way that the internal stresses are practically those only of
compression, and the shearing stresses are within the limits possible
for the material in question.
The author has repeatedly made assumptions which are not borne out by
the application of his mathematical formulas to actual extreme
conditions. This method of application to limiting conditions is
concededly sometimes faulty; but the writer believes that no earth
pressure theory, or one concerning arch action, can be considered as
satisfactory which does not apply equally well to hydraulic pressure
problems when the proper assumptions are made as to the factors for
friction, cohesion, etc. For example, when the angle of repose is
considered as zero, in the author's first formula for _W_{1}_, the value
becomes ½ _W_{1}_, whereas it should depend solely on the depth, which
does not enter the formula, and not at all on the width of opening, _l_,
which is thus included.
The author has given no experiments to prove his statement that "the
arch thrust is greater in dryer sand," and the accuracy of the statement
is questioned. Again, no reason is apparent for assuming the direction
of the "rakers" in Fig. 3 as that of the angle of repose. The writer
cannot see why that particular angle is repeatedly used, when almost any
other would give results of a similar kind. The author has made no
experiments which show any connection between the angle of repose, as he
interprets it, and the lines of arch action which he assumes to exist.
With regard to the illustration of the condition which is thought to
exist when the "material is composed of large bowling balls," supposedly
all of the same size, the writer believes the conclusion to be
erroneous, and that this can be readily seen by inspection of a diagram
in which such balls are represented as forming a pile similar to the
well-known "pile of shells" of the algebras, in the diagram of which a
pile of three shells, resting on the base, has been omitted. It is then
seen that unless the pressures at an angle of 60° with the horizontal
are sufficient to produce frictional resistance of a very large amount,
the balls will roll and instantly break the arch action suggested by the
author. Consequently, an almost infinitesimal settlement of the
"centering" may cause the complete destruction of an arch of earth.
The author's logic is believed to be entirely faulty in many cases
because he repeatedly makes assumptions which are not in accordance with
demonstrated fact, and finally sums up the results by the statement: "It
is conceded" (line 2, p. 357, for example), when the writer, for one,
has not even conceded the accuracy of the assumptions. For instance, the
author's well-known theory that pressures against retaining walls are a
maximum at the top and decrease to zero at the bottom, is in absolute
contradiction to the results of experiments conducted on a large scale
by the writer on the new reinforced concrete retaining wall near the St.
George Ferry, on Staten Island, New York City, which will soon be
published, and in which the usual law of increase of lateral pressure
with depth is believed to be demonstrated beyond question. It must be
conceded that a considerable arch action (so-called) actually exists in
many cases; but it should be equally conceded by the advocates of the
existence of such action that changes in humidity, due to moving water,
vibration, and appreciable viscosity, etc., will invariably destroy this
action in time. In consequence, the author's reasoning in regard to the
pressures against the faces of retaining walls is believed to be open to
grave question as to accuracy of assumption, method, and conclusion.
The author is correct in so far as he assumes that "the character of the
stresses due to the thrust of the material will" not "change if bracing
should be substituted for the material in the area" designated by him,
etc., provided he makes the further assumption that absolutely no
motion, however infinitesimal, has taken place meantime; but, unless
such motion has actually taken place, no arch action can have developed.
An arch thrust can result only with true arch action, that is, with
stable abutments, and the mass stressed wholly in compression, with
corresponding shortening of the arch line. The arch thrust must be
proportional to the elastic deformation (shortening) of the arch line.
If any such arch as is shown in Fig. 5 is assumed to carry the whole of
the weight of material above it, that assumed arch must relieve all the
assumed arches below. Therefore each of the assumed arches can carry
nothing more than its own mass. Otherwise the resulting thrust would
increase with the depth, which is opposed to the author's theory.
Turning again to the condition that each arch can carry only its own
weight: if these arches are assumed of thicknesses proportional to the
distance upward from the bottom of the wall, they will be similar
figures, and it is easily demonstrated that the thrust will then be
uniform in amount throughout the whole height of the wall, except,
perhaps, at the very top. This condition is contrary to the author's
ideas and also to the facts as demonstrated by the writer's experiment
on the 40-ft. retaining wall at St. George. Consequently, the author's
statement: "nor can anyone * * * doubt that the top timbers are stressed
more heavily than those at the bottom," is emphatically doubted and
earnestly denied by the writer. Furthermore, "the assumption" made by
the author as to "the tendency of the material to slide" so as to cause
it "to wedge * * * between the face of the sheeting * * * and some plane
between the sheeting and the plane of repose," is considered as
absolutely unwarranted, and consequently the whole conclusion is
believed to be unjustified. Nor is the author's assumption (line 5, p.
361), that "the thrust * * * is measured by its weight divided by the
tangent of the * * * angle of repose" at all obvious.
The author presents some very interesting photographs showing the
natural surface slopes of various materials; but it is interesting to
note that he describes these slopes as having been produced by the
"continual slipping down of particles." The vast difference between
angles of repose produced in this manner by the rolling friction of
particles and the internal angles of friction, which must be used in all
earth-pressure investigations, has been repeatedly called to the
attention of engineers by the writer.[H]
The writer's experiments are entirely in accord with those of the author
in which the latter claims to demonstrate that "earth and water
pressures act independently of each other," and the writer is much
delighted that his own experiments have been thus confirmed.
In Experiment No. 3, the query is naturally suggested: "What would have
been the result if the nuts and washers had first been tightened and
water then added?" Although the writer has not tried the experiment, he
is rather inclined to the idea that the arch would have collapsed. With
regard to Experiment No. 5, there is to be noted an interesting
possibility of its application to the theoretical discussion of masonry
dams, in which films of water are assumed to exist beneath the structure
or in crevices or cracks of capillary dimensions. The writer has always
considered the assumptions made by many designing engineers as
unnecessarily conservative. In regard to the author's conclusions from
Experiment No. 6, it should be noted that no friction can exist between
particles of sand and surrounding water unless there is a tendency of
the latter to move; and that water in motion does not exert pressures
equal to those produced when in a static condition, the reduction being
proportional to the velocity of flow.
The author's conclusion (p. 371), that "pressure will cause the
quicksand to set up hydraulic action," does not seem to have been
demonstrated by his experiments, but to be only his theory. In this
instance, the results of the writer's experiments are contrary to the
author's theory and conclusion.
The writer will heartily add his protest to that of the author "against
considering semi-aqueous masses, such as soupy sands, soft concrete,
etc., as exerting hydrostatic pressure due to their weight in bulk,
instead of to the specific gravity of the basic liquid." Again,
similarly hearty concurrence is given to the author's statement:
"If the solid material in any liquid is agitated, so that it is
virtually in suspension, it cannot add to the pressure, and if
allowed to subside it acts as a solid, independently of the water
contained with it, although the water may change somewhat the
properties of the material, by increasing or changing its cohesion,
angle of repose, etc."
On the other hand, it is believed that the author's statement, as to
"the tendency of marbles to arch," a few lines above the one last
quoted, should be qualified by the addition of the words, "only when a
certain amount of deflection has taken place so as to bring the arch
into action." Again, on the following page, a somewhat similar
qualification should be added to the sentence referring to the soft clay
arch, that it would "stand if the rods supporting the intrados of the
arch were keyed back to washers covering a sufficiently large area," by
inserting the words, "unless creeping pressures (such as those
encountered by the writer in his experiments) were exceeded."
The writer considers as very doubtful the formula for _D_{x}_, which is
the same as that for _W_{1}_, already discussed. The author's statement
that "additional back-fill will [under certain circumstances] lighten
the load on the structure," is considered subject to modification by
some such clause as the following, "the word 'lighten' here being
understood to mean the reduction to some extent of what would be the
total pressure due to the combined original and added back-fill,
provided no arch action occurred."
The writer is in entire agreement with the author as to the probability
that water is often "cut off absolutely from its source of pressure,"
with the attendant results described by the author (p. 378); and again,
that too little attention has been given to the bearing power of soil,
with the author's accompanying criticism.
The writer cannot see, however, where the author's experiments
demonstrate his statement "that pressure is transmitted laterally
through ground, most probably along or nearly parallel to the angles of
repose," or any of the conclusions drawn by him in the paragraph (p.
381), which contains this questionable statement. Again the writer is at
a loss as to how to interpret the statement that the author has found
that "better resistance" has been offered by "small open caissons sunk
to a depth of a few feet and cleaned out and filled with concrete" than
by "spreading the foundation over four or five times the equivalent
area." The writer agrees with the author in the majority of his
statements as to the "bearing value and friction on piles," but believes
that he is indulging in pure theory in some of his succeeding remarks,
wherein he ascribes to arch action the results which he believes would
be observed if "a long shaft be withdrawn vertically from moulding
sand." These phenomena would be due rather to capillary action and the
resulting cohesion.
Naturally, the writer doubts the author's conclusions as to the pressure
at the top of large square caisson shafts when he states that "the
pressure at the top * * * will * * * increase proportionately to the
depth." Again, the author is apparently not conversant with experiments
made by the Dock Department of New York City, concerning piles driven in
the Hudson River silt, which showed that a single heavily loaded pile
carried downward with it other unloaded piles, driven considerable
distances away, showing that it was not the pile which lacked in
resistance, as much as the surrounding earth.
In conclusion, the writer heartily concurs with the statement that "too
much has been taken for granted in connection with earth pressures and
resistance," and he is sorry to be forced to add that he believes the
author to be open to the criticism which he himself suggests, that "both
in experimenting and observing, the engineer [and in this case the
author] will frequently find what is being looked for or expected and
will fail to see the obvious alternative."
FRANCIS L. PRUYN, M. AM. SOC. C. E. (by letter).--Mr. Meem
should be congratulated, both in regard to the highly interesting
theories which he advances on the subject of sand pressures--the
pressures of subaqueous material--and on his interesting experiments in
connection therewith.
The experiment in which the plunger on the hydraulic ram is immersed in
sand and covered with water does not seem to be conclusive. By this
experiment the author attempts to demonstrate that the pressure of the
water transmitted through the sand is only about 40% as great as when
the sand is not there. The travel of ground-water through the earth is
at times very slow, and occasionally only at the rate of from 2 to 3 ft.
per hour. In the writer's opinion, Mr. Meem's experiment did not cover
sufficient time during which the pressure was maintained at any given
point. It is quite probable that it may take 15 or 20 min. for the full
pressure to be transmitted through the sand to the bottom of the
plunger, and it is hoped, therefore, that he will make further
experiments lasting long enough to demonstrate this point.
In regard to the question of skin friction on caissons and piles, it may
be of interest to mention an experiment which the writer made during the
sinking of the large caissons for the Williamsburg Bridge. These
caissons were about 70 ft. long and 50 ft. wide. The river bottom was
about 50 ft. below mean high water, and the caissons penetrated sand of
good quality to a depth of from 90 to 100 ft. below that level. On two
occasions calculations were made to determine the skin friction while
the caissons were being settled. With the cutting edge from 20 to 30 ft.
below the river bottom, the calculations showed that the skin friction
was between 500 and 600 lb. per sq. ft. The writer agrees with Mr. Meem
that, in the sinking of caissons, the arch action of sand is, in a great
measure, destroyed by the compressed air which escapes under the cutting
edge and percolates up through the material close to the sides of the
caissons.
With reference to the skin friction on piles, the writer agrees with Mr.
Meem that in certain classes of material this is almost a negligible
quantity. The writer has jacked down 9-in. pipes in various parts of New
York City, and by placing a recording gauge on the hydraulic jack, the
skin friction on the pile could be obtained very accurately. In several
instances the gauge readings did not vary materially from the surface
down to a penetration of 50 ft. In these instances the material inside
the pipe was cleaned out to within 1 ft. of the bottom of the pile, so
that the gauge reading indicated only the friction on the outside of the
pipe plus the bearing value developed by its lower edge. For a 9-in.
pipe, the skin friction on the pile plus the bearing area of the bottom
of the pipe seems to be about 20 tons, irrespective of the depth. After
the pipe had reached sufficient depth, it was concreted, and, after the
concrete had set, the jack was again placed on it and gauge readings
were taken. It was found that in ordinary sands the concreted steel pile
would go down from 3 to 6 in., after which it would bring up to the full
capacity of a 60-ton jack, showing, by gauge reading, a reaction of from
70 to 80 tons.
It is the writer's opinion that, in reasonably compact sands situated at
a depth below the surface which will not allow of much lateral movement,
a reaction of 100 tons per sq. ft. of area can be obtained without any
difficulty whatever.
FRANK H. CARTER, ASSOC. M. AM. SOC. C. E. (by letter).--Mr.
Meem has contributed much that is of value, particularly on water
pressures in sand; just what result would be obtained if coarse crushed
stone or similar material were substituted for sand in Experiment No. 6,
is not obvious.
It has been the practice lately, among some engineers in Boston, as well
as in New York City, to assume that water pressures on the underside of
inverts is exerted on one-half the area only. The writer, however, has
made it a practice first to lay a few inches of cracked stone on the
bottom of wet excavations in order to keep water from concrete which is
to be placed in the invert. In addition to the cracked stone under the
inverts, shallow trenches dug laterally across the excavation to insure
more perfect drainage, have been observed. Both these factors no doubt
assist the free course of water in exerting pressure on the finished
invert after the underdrains have been closed up on completion of the
work. The writer, therefore, awaits with interest the repetition of
Experiment No. 6, with water on the bottom of a piston buried in coarse
gravel or cracked stone.
As for the arching effect of sand, the writer believes that Mr. Meem has
demonstrated an important principle, on a small scale. It must be
regretted, however, that the box was not made larger, for, to the
writer, it appears unsafe to draw such sweeping conclusions from small
experiments. As small models of sailboats fail to develop completely
laws for the design and control of large racing yachts, so experiments
in small sand boxes may fail to demonstrate the laws governing actual
pressures on full-sized structures.
For some time the writer has been using a process of reasoning similar
to that of the author for assumptions of earth pressure on the roofs of
tunnel arches, except that the vertical forces assumed to hold up the
weight of the earth have been ascribed to cohesion and friction, along
what might be termed the sides of the "trench excavation."
The writer fails to find proof in this paper of the author's statement
that earth pressures on the sides of a structure buried in earth are
greater at the top than at the bottom of a trench. That some banks are
"top-heavy," is, no doubt, a fact, the writer having often heard similar
expressions used by experienced trench foremen, but, in every case
called to his attention, local circumstances have caused the
top-heaviness, either undermining at the bottom of the trench, too much
banked earth on top, or the earth excavated from the trench being too
near the edge of the cut.
For some years the writer has been making extended observations on deep
trenches, and, thus far, has failed to find evidence, except in aqueous
material, of earth pressures which might be expected from the known
natural slope of the material after exposure to the elements; and this
latter feature may explain why sheeted trenches stand so much better
than expected. If air had free access to the material, cohesion would be
destroyed, and theoretical pressures would be more easily developed.
With closely-sheeted trenches, weathering is practically excluded, and
the bracing, which seemingly is far too light, holds up the trench with
scarcely a mark of pressure. As an instance, in 1893, the writer was
successfully digging sewer trenches from 10 to 14 ft. deep, through
gravel, in the central part of Connecticut, without bracing; because of
demands of the work in another part of the city, a length of several
hundred feet of trench was left open for three days, resulting in the
caving-in of the sides. The elements had destroyed the cohesion, and the
sides of the trenches no longer stood vertically.
Recently, in the vicinity of Boston, trenches, 32 ft. wide, and from 25
to 35 ft. deep, with heavy buildings on one side, have been braced with
8 by 10-in. stringers, and bracers at 10-ft. centers longitudinally, and
from 3 to 5 ft. apart vertically; this timbering apparently was too
slight for pressures which, theoretically, might be expected from the
natural slope of the material. Just what pressures develop on the sides
of the structures in these deep trenches after pulling the top sheeting
(the bottom sheeting being left in place) is, of course, a matter of
conjecture. There can be no doubt that there is an arching of the
material, as suggested by the author. How much this may be assisted by
the practical non-disturbance of the virgin material is, of course,
indeterminate. That substructures and retaining walls designed according
to the Rankine or similar theories have an additional factor of safety
from too generous an assumption in regard to earth pressure is
practically admitted everywhere. It is almost an engineering axiom that
retaining walls generally fail because of insufficient foundation only.
For the foregoing reasons, and particularly from observations on the
effect of earth pressures on wooden timbers used as bracing, the writer
believes that, ordinarily, the theoretical earth pressures computed by
Rankine and Coulomb are not realized by one-half, and sometimes not even
by one-third or one-quarter in trenches well under-drained, rapidly
excavated, and thoroughly braced.
J.C. MEEM, M. AM. SOC. C. E. (by letter).--The writer has been
much interested in this discussion, and believes that it will be of
general value to the profession. It is unfortunate, however, that
several of the points raised have been due to a careless reading of, or
failure to understand, the paper.
Taking up the discussion in detail, the writer will first answer the
criticisms of Mr. Goodrich. He says:
"The writer believes that, in the design of permanent structures,
consideration of arch action should not be included, at least, not
until more information has been obtained. He also believes that the
design of temporary structures with this inclusion is actually
dangerous in some instances."
If the arching action of earth exists, why should it not be recognized
and considered? The design of timbering for a structure to rest, for
instance, at a depth of from 200 to 300 ft. in normal dry earth, without
considering this action, would be virtually prohibitive.
Mr. Goodrich proceeds to show one of the dangers of considering such
action by quoting the writer, as follows:
"About an hour after the superimposed load had been removed, the
writer jostled the box with his foot sufficiently to dislodge some
of the exposed sand, when the arch at once collapsed and the bottom
fell to the ground."
He fails, as do so many other critics of this theory, to distinguish the
difference between that portion of the sand which acts as so-called
"centering" and that which goes to make up the sustaining arch. The
dislodgment of any large portion of this "centering" naturally causes
collapse, unless it is caught, in which case the void in the "centering"
is filled from the material in the sustaining arch, and this, in turn,
is filled from that above, and so on, until the stability of each arch
is in turn finally established. This, however, does not mean that,
during the process of establishing this equilibrium of the arch
stresses, there is no arching action of any of the material above, but
only that some of the so-called arches are temporarily sustained by
those below. That is, in effect, each area of the material above
becomes, in turn, a dependent, an independent, and finally an
interdependent arch.
If Mr. Goodrich's experience has led him to examine any large number of
tunnel arches or brick sewers, he will have noted in many of them
longitudinal cracks at the soffits of the arches and perhaps elsewhere.
These result from three causes:
_First._--In tunneling, there is more or less loss of material, while,
in back-filling, the material does not at first reach its final
compactness. Therefore, in adjusting itself to normal conditions, this
material causes impact loads to come upon the green arch, and these tend
to crack it.
_Second._--No matter how tightly a brick or other arch is keyed in,
there must always be some slight subsidence when the "centers" are
struck. This, again, results in a shock, or impact loading, to the
detriment of the arch.
_Third._--The most prolific cause, however, is that in tunneling, as
well as in back-filling open cuts, the material backing up the haunches
is more or less loosened and therefore is not at first compact enough to
prevent the spreading of the haunches when the load comes on the arch.
This causes cracking, but, as soon as the haunches have been pressed out
against the solid material, the cracking usually ceases, unless the
pressure has been sufficiently heavy to cause collapse.
An interesting example of this was noted in the Joralemon Street branch
of the Rapid Transit Tunnel, in Brooklyn, in which a great many of the
cast-iron rings were cracked under the crown of the arch, during
construction; but, in spite of this, they sustained, for more than two
years, a loading which, according to Mr. Goodrich, was continually
increasing. In other words, the cracked arch sustained a greater loading
than that which cracked the plates during construction, according to his
theory, as noted in the following quotation:
"But it should be equally conceded by the advocates of the
existence of such action that changes in humidity, due to moving
water, vibration, and appreciable viscosity, etc., will invariably
destroy this action in time."
As to the correctness of this theory Mr. Goodrich would probably have
great difficulty in convincing naturalists, who are aware that many
animals live in enlarged burrows the stability of which is dependent on
the arching action of the earth; in fact, many of these burrows have
entrances under water. He would also have some difficulty in convincing
those experienced miners who, after a cave-in, always wait until the
ground has settled and compacted itself before tunneling, usually with
apparent safety, over the scene of the cave-in.
The writer quotes as follows from Mr. Goodrich's discussion:
"In any case, no arch action can be brought into play until a
certain amount of settlement has taken place so as to bring the
particles into closer contact, and in such a way that the internal
stresses are practically those only of compression, and the
shearing stresses are within the limits possible for the material
in question."
Further:
"Consequently, an almost infinitesimal settlement of the
'centering' may cause the complete destruction of an arch of
earth."
And further:
"On the other hand, it is believed that the author's statement, as
to the 'tendency of marbles to arch,' * * * should be qualified by
the addition of the words, 'only when a certain amount of
deflection has taken place so as to bring the arch into action.'"
In a large measure the writer agrees with the first and last quotations,
but sees no reason to endorse the second, as it is impossible to
consider any arch being built which does not settle slightly, at least,
when the "centers" are struck.
Regarding his criticism of the lack of arching action in balls or
marbles, he seems to reason that the movement of the marbles would
destroy the arch action. It is very difficult for the writer to conceive
how it would be possible for balls or marbles to move when confined as
they would be confined if the earth were composed of them instead of its
present ingredients, and under the same conditions otherwise. Mr.
Goodrich can demonstrate the correctness of the writer's theories,
however, if he will repeat the writer's Experiment No. 3, with marbles,
with buckshot, and with dry sand. He is also advised to make the
experiment with sand and water, described by the writer, and is assured
that, if he will see that the washers are absolutely tight before
putting the water into the box, he can do this without bringing about
the collapse of the arch; the only essential condition is that the
bottom shall be keyed up tightly, so as not to allow the escape of any
sand. He is also referred to the two photographs, Plate XXIV,
illustrating the writer's first experiment, showing how increases in the
loading resulted in compacting the material of the arch and in the
consequent lowering of the false bottom. As long as the exposed sand
above this false bottom had cohesion enough to prevent the collapse of
the "centering," this arch could have been loaded with safety up to the
limits of the compressive strength of the sand.
To quote again from Mr. Goodrich:
"Furthermore, the author's reason for bisecting the angle between
the vertical and the angle of repose of the material, when he
undertakes to determine the thickness of key, is not obvious. This
assumption is shown to be absurd when carried to either limit, for
when the angle of repose equals zero, as is the case with water,
this method would give a definite thickness of key, while there can
be absolutely no arch action possible in such a case; and, when the
angle of repose is 90°, as may be assumed in the case of rock, this
method would give an infinite thickness of key, which is again seen
to be absurd."
Mr. Goodrich assumes that water or liquid has an angle of repose equal
to zero, which is true, but the writer's assumptions applied only to
solid material, and the liquid gives an essentially different condition
of pressure, as shown by a careful reading of the paper. In solid rock
Mr. Goodrich assumes an angle of repose equal to 90°, for which there is
no authority; that is, solid rock has no known angle of repose. In order
to carry these assumptions to a definite conclusion, we must assume for
that material with an angle of repose of 90° some solid material which
has weight but no thrust, such as blocks of ice piled vertically. In
this case Mr. Goodrich can readily see that there will be no arching
action over the structure, and that the required thickness of key would
be infinite. As to the other case, it is somewhat difficult to conceive
of a solid with an angle of repose of zero; aqueous material does not
fulfill this condition, as it is either a liquid or a combination of
water and solid material. The best illustration, perhaps, would be to
assume a material composed of iron filings, into which had been driven a
powerful magnet, so that the iron filings would be drawn horizontally in
one direction. It is easy to conceive, then, that in tunneling through
this material there would be no necessity for holding up the roof; the
definite thickness of key given, as being at the point of intersection
of two 45° angles, would be merely a precautionary measure, and would
not be required in practice.
It is thus seen that both these conditions can be fulfilled with
practical illustrations; that is, for an angle of repose of 90°, that
material which has weight and no thrust, and for an angle of repose of
zero, that solid material which has thrust but no weight.
Mr. Goodrich says the author has given no experiments to prove his
statement that the arch thrust is greater in dryer sand. If Mr. Goodrich
will make the experiment partially described as Experiment No. 3, with
absolutely dry sand, and with moist sand, and on a scale large enough
to eliminate cohesion, he will probably find enough to convince him that
in this assumption the writer is correct. At the same time, the writer
has based his theory in this regard on facts which are not entirely
conclusive, and his mind is open as to what future experiments on a
large scale may develop. It is very probable, however, that an
analytical and practical examination of the English experiments noted on
pages 379 and 380, will be sufficient to develop this fact conclusively.
The writer is forced to conclude that some of the criticisms by Mr.
Goodrich result from a not too careful reading of the paper. For
instance, he states:
"'It is conceded' (line 2, p. 357, for example) when the writer,
for one, has not even conceded the accuracy of the assumptions."
A more careful reading would have shown Mr. Goodrich that this
concession was one of the writer's as to certain pressures against or on
tunnels, and, if Mr. Goodrich does not concede this, he is even more
radical than the writer.
And again:
"'Nor can anyone * * * doubt that the top timbers are stressed more
heavily than those at the bottom' is emphatically doubted and
earnestly denied by the writer."
It is unfortunate that Mr. Goodrich failed to make the complete
quotation, which reads:
"Nor can anyone, looking at Fig. 5, doubt," etc.
A glance at Fig. 5 will demonstrate that, under conditions there set
forth, the writer is probably correct in his assertion as relating to
that particular instance. Further:
"For instance, the author's well-known theory that the pressures
against retaining walls are a maximum at the top and decrease to
zero at the bottom, is in absolute contradiction to the results of
experiments conducted on a large scale by the writer on the new
reinforced concrete retaining wall near the St. George Ferry, on
Staten Island."
The writer's "well-known theory that pressures against retaining walls
are a maximum at the top and decrease to zero at the bottom" applies
only to pressures exerted by absolutely dry and normally dry material,
and it seems to him that this so-called theory is capable of such easy
demonstration, by the simple observation of any bracing in a deep trench
in material of this class, that it ought to be accepted as at least
safer than the old theory which it reverses. As to this "well-known
theory" in material subject to water pressure, a careful reading of the
paper, or an examination of Fig. 12 and its accompanying text, or an
examination of Table 1, will convince Mr. Goodrich that, under the
writer's analysis, this pressure does not decrease to zero at the
bottom, but that in soft materials it may be approximately constant all
the way down, while, in exceptionally soft material, conditions may
arise where it may increase toward the bottom. The determination should
be made by taking the solid material and drying it sufficiently so that
water does not flow or seep from it. When this material is then
compacted to the condition in which it would be in its natural state,
its angle of repose may be measured, and may be found to be as high as
60 degrees. The very fine matter should then be separated from the
coarser material, and the latter weighed, to determine its proportion.
Subtracting this from the total, the remainder could be credited to
"aqueous matter." It is thus seen that with a material when partially
dried in which the natural angle of repose might be 60°, and in which
the percentage of water or aqueous matter when submerged might be 60%,
there would be an increase of pressure toward the bottom.
The writer does not know the exact nature of the experiments made at St.
George's Ferry by Mr. Goodrich, but he supposes they were measurements
of pressures on pistons through holes in the sheeting. He desires to
state again that he cannot regard such experiments as conclusive, and
believes that they are of comparative value only, as such experiments do
not measure in any large degree the pressure of the solid material but
only all or a portion of the so-called aqueous matter, that is, the
liquid and very fine material which flows with it. Thus it is well known
that, during the construction of the recent Hudson and North River
Tunnels, pressures were tested in the silt, some of which showed that
the silt exerted full hydrostatic pressure. At the same time, W.I. Aims,
M. Am. Soc. C. E., stated in a public lecture, and recently also to the
writer, that in 1890 he made some tests of the pressure of this silt in
normal air for the late W.R. Hutton, M. Am. Soc. C. E. A hole, 12 in.
square, was cut through the brickwork and the iron lining, just back of
the lock in the north tube (in normal air), and about 1000 ft. from the
New Jersey shore. It was found that the silt had become so firm that it
did not flow into the opening. Later, a 4-in. collar and piston were
built into the opening, and, during a period covering at least 3 months,
constant observations showed that no pressure came upon it; in fact, it
was stated that the piston was frequently worked back and forth to
induce pressure, but no response was obtained during all this period.
The conclusion must then be drawn that when construction, with its
attendant disturbance, has stopped, the solid material surrounding
structures tends to compact itself more or less, and solidify, according
as it is more or less porous, forming in many instances what may be
virtually a compact arch shutting off a large percentage of the normal,
and some percentage even of the aqueous, pressure.
That the pressure of normally dry material cannot be measured through
small openings can be verified by any one who will examine such material
back of bracing showing evidences of heavy pressure. The investigator
will find that, if this material is free from water pressure, paper
stuffed lightly into small openings will hold back indefinitely material
which in large masses has frequently caused bracing to buckle and
sheeting planks to bend and break; and the writer reiterates that such
experiments should be made in trenches sheeted with horizontal sheeting
bearing against short vertical rangers and braces giving horizontal
sections absolutely detached and independent of each other. In no other
way can such experiments be of real value (and even then only when made
on a large scale) to determine conclusively the pressure of earth on
trenches.
As to the questions of the relative thrust of materials under various
angles of repose, and of the necessity of dividing by the tangent, etc.;
these, to the writer, seem to be merely the solution of problems in
simple graphics.
The writer believes that if Mr. Goodrich will make, even on a small
scale, some of the experiments noted by the writer, he will be convinced
that many of the assumptions which he cannot at present endorse are
based on fact, and his co-operation will be welcomed with the greatest
interest. Among the experiments which he is asked to make is the one in
dry sand, noted as Experiment No. 3, whereby it can be shown very
conclusively that additional back-fill will result in increased arching
stability, on an arch which would collapse under lighter loading.
The writer is indebted to Mr. Goodrich for pointing out some errors in
omission and in typography (now corrected), and for his hearty
concurrence in some of the assumptions which the writer believed would
meet with greatest disapproval.
In reply to Mr. Pruyn and Mr. Gregory, the writer assumed that the
piston area in Experiment No. 6 should be reduced only by the actual
contact of material with it. If this material in contact should be
composed of theoretical spheres, resulting in a contact with points
only, then the theoretical area reduced should be in proportion to this
amount only. The writer does not believe, however, that this condition
exists in practice, but thinks that the area is reduced very much more
than by the actual theoretical contact of the material. He sees no
reason, as far as he has gone, to doubt the accuracy of the deductions
from this experiment.
Regarding the question of the length of time required to raise the
piston, he does not believe that the position of his critics is entirely
correct in this matter; that is, it must either be conceded that the
piston area is cut off from the source of pressure, or that it is in
contact with it through more or less minute channels of water. If it is
cut off, then the writer's contention is proved without the need of the
experiment, and it is therefore conclusive that a submerged tunnel is
not under aqueous pressure or the buoyant action of water. If, on the
other hand, the water is in contact through channels bearing directly
upon the piston and leading to the clear water chamber, any increase in
pressure in the water chamber must necessarily result in a virtually
instantaneous increase of the pressure against the piston, and therefore
the action on the latter should follow almost immediately. In all cases
during the experiments the piston did not respond until the pressure was
approximately twice as great as required in clear water, therefore the
writer must conclude either that the experiments proved it conclusively
or that his assumption is proved without the necessity of the
experiments. That is, the pressure is virtually not in evidence until
the piston has commenced to move.
Mr. Pruyn has added valuable information in his presentation of data
obtained from specific tests of the bearing value of, and friction on,
hollow steel piles. These data largely corroborate tests and
observations by the writer, and are commended to general attention.
Mr. Carter's information is also of special interest to the writer, as
much of it is in the line of confirming his views. Mr. Carter does not
yet accept the theory of increased pressure toward the top, but if he
will examine or experiment with heavy bracing in deep trenches in clear
sand, or material with well-defined angles of repose, he will probably
find much to help him toward the acceptance of this view.
The writer regrets that he has not now the means or appliances for
further experiments with the piston chamber, but he does not believe
that reliable results could be obtained in broken stone with so small a
piston, as it is possible that the point of one stone only might be in
contact with the piston. This would naturally leave the base exposed
almost wholly to a clear water area. He does not believe, however, that
in practice the laying of broken stone under inverts will materially
change the ultimate pressure unless its cross-section represents a large
area.
Mr. Perry will find the following on page 369:
"It should be noted also that although the area subject to pressure
is diminished, the pressure on the area remaining corresponds to
the full hydrostatic head, as would be shown by the pressure on an
air gauge."
This, of course, depends on the porosity of the material and the
friction the water meets in passing through it.
As to Mr. Thomson's discussion, the writer notes with regret two points:
(_a_) that specific data are not given in many of the interesting cases
of failures of certain structures or bracing; and (_b_), that he has
not in all cases a clear understanding of the paper. For instance, the
writer has not advocated the omission of bottom bracing or sheeting. He
has seen many instances where it has been, or could have been, safely
omitted, but he desires to make it clear that he does not under any
circumstances advocate its omission in good work; but only that, in
well-designed bracing, its strength may be decreased as it approaches
the bottom.
Reference is again made to the diagram, Fig. 12, which shows that, in
most cases of coffer-dams in combined aqueous and earth pressure, there
may be nearly equal, and in some cases even greater, loading toward the
bottom.
The writer also specifically states that in air the difference between
aqueous and earth pressure is plainly noted by the fact that bracing is
needed so frequently to hold back the earth while the air is keeping out
the water.
The lack of specific data is especially noticeable in the account of the
rise of the 6-ft. conduit at Toronto. It would be of great interest to
know with certainly the weight of the pipe per foot, and whether it was
properly bedded and properly back-filled. In all probability the
back-filling over certain areas was not properly done, and as the pipe
was exposed to an upward pressure of nearly 1600 lb. per ft., with
probably only 500 or 600 lb. of weight to counterbalance it, it can
readily be seen that it did not conform with the writer's general
suggestion, that structures not compactly, or only partially, buried,
should have a large factor of safety against the upward pressure.
Opposed to Mr. Thomson's experience in this instance is the fact that
oftentimes the tunnels under the East River approached very close to the
surface, with the material above them so soupy (owing to the escape of
compressed air) that their upper surfaces were temporarily in water, yet
there was no instance in which they rose, although some of them were
under excessive buoyant pressure.
It is also of interest to note, from the papers descriptive of the North
River Tunnel, that, with shield doors closed, the shield tended to rise,
while by opening the doors to take in muck the shield could be brought
down or kept down. The writer concurs with those who believe that the
rising of the shield with closed doors was due to the slightly greater
density of the material below, and was not in any way due to buoyancy.
Concerning the collapse of the bracing in the tunnel built under a
side-hill, the writer believes it was due to the fact that it was under
a sliding side-hill, and that, if it had been possible to have
back-filled over and above this tunnel to a very large extent, this
back-fill would have resulted in checking the sliding of material
against the tunnel, and the work would thereafter have been done with
safety. This is corroborated by Mr. Thomson's statement that the tunnel
was subsequently carried through safely by going farther into the hill.
As to the angle of repose, Mr. Thomson seems to feel that its
determination is so often impracticable that it is not to be relied on;
and yet all calculations pertaining to earth pressure must be based on
this factor. The writer believes that the angle of repose is not
difficult to determine, and that observations of, and experiments on,
exposed banks in similar material, and general experience in relation
thereto, will enable one to determine it in nearly all cases within such
reasonably accurate limits that only a small margin of safety need be
added.
Engineers are sent to Europe to study sewage disposal, water
purification, transit problems, etc., but are rarely sent to an
adjoining county or State to look at an exposed bank, which would
perhaps solve a vexed problem in bracing and result in great economy in
the design of permanent structures.
Mr. Thomson's general views seem to indicate that much of the subject
matter noted in the paper relates to unsolvable problems, for it appears
that in many cases he believes the Engineer to be dependent on his
educated guess, backed perhaps by the experienced guess of the foreman
or practical man. The writer, on the contrary, believes that every
problem relating to work of this class is capable of being solved,
within reasonably accurate limits, and that the time is not far distant
when the engineer, with his study of conditions, and samples of material
before him, will be able to solve his earth pressure and earth
resistance problems as accurately as the bridge engineer, with his
knowledge of structural materials, solves bridge problems.
The writer, in the course of his experience, has met with or been
interested in the solution of many problems similar to the following:
What difference in timbering should be made for a tunnel in ordinary,
normally dry ground at a depth of 20 ft. to the roof, as compared with
one at a depth of 90 ft.?
What difference in timbering or in permanent design should be made for a
horizontally-sheeted shaft, 5 ft. square, going to a depth of 45 ft. and
one 25 by 70 ft., for instance, going to the same depth, assuming each
to be braced and sheeted horizontally with independent bracing?
What allowance should be made for the strength of interlock, assuming
that a circular bulkhead of sand, 30 ft. in diameter, is to be carried
by steel sheet-piling exposed around the outside for a depth of 40 ft.?
What average pressure per square foot of area should be required to
drive a section of a 3 by 15-ft. roof shield, as compared with the
pressure needed to drive the whole roof shield with an area four times
as great?
To what depth could a 12 by 12-in. timber be driven, under gradually
added pressure, up to 60 tons, for instance, in normal sand?
What frictional resistance should be assumed on a hollow, steel,
smooth-bore pile which had been driven through sharp sand and had
penetrated soft, marshy material the bearing resistance of which was
practically valueless?
What allowance should be made for the buoyancy of a tunnel 20 ft. in
diameter, the top of which was buried to a depth of 20 ft. in sand above
which there was 40 ft. of water?
It is believed by the writer that most of the authorities are silent as
to the solution of problems similar to the above, and it is because of
this lack of available data that he has directed his studies to them.
The belief that the results of these studies, together with such
observations and experiments as relate thereto, may be of interest, has
caused him to set them forth in this paper.
He desires to state his belief that if problems similar to the above
were given for definite solution, not based on ordinary safe practice,
and without conference, to a number of engineers prominently interested
in such matters, the results would vary so widely as to convince some of
the critics of this paper that the greater danger lies rather in the
non-exploration of such fields than in the setting forth of results of
exploration which may appear to be somewhat radical.
Further, if these views result in stimulating enough interest to lead to
the hope that eventually the "Pressure, Resistance, and Stability" of
ground under varying conditions will be known within reasonably accurate
limits and tabulated, the writer will feel that his efforts have not
been in vain.