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Model tests in submerged soils
Journal of Terramechanics 1966, Vol. 3, No. 4, pp. 53 to 70. Pergamon Press Ltd.
Printed in Great Britain.
MODEL TESTS IN SUBMERGED SOILS HOWARD DUGOFF* and I. ROBERT EHRLICH* INTRODUCTION AMERICANTelephone and Telegraph Corporation, as part of its overseas communication systems, operates and maintains a number of underwater cables, stretching from the United States and Canada to Great Britain, Ireland and France. At the present time the cables lying exposed on the North American Continental Shelf are subject to considerable damage from the many fishing boats that ply the waters overhead. These vessels, in order to keep their nets on the bottom while trawling, use oneand two-ton trawl boards dragged along the bottom by strong steel cables. Whenever one of these weighted steel cables is dragged across one of the telephone cable loops or bridges, something has to give---it is usually the telephone cable. The cost for repairs and lost usage can be considerable. Two possible solutions to this problem is to bury the cables beneath the ocean floor. The study performed by Davidson Laboratory was concerned with this possibility, in particular with an investigation of the feasibility of employing a selfpropelled tracked vehicle to operate on the floor of the ocean and to entrench and bury the cable. It was financed by and conducted in close cooperation with the Bell Telephone Laboratories. No previous studies of the traflicability characteristics of submerged soils were known. As a first step in gaining an understanding of this very complicated problem, a limited program of submerged soil studies and scale model tractor tests was initiated. One soil mechanical phenomenon particularly associated with the submarine environment was regarded as extremely significant from the vehicle locomotion point of view. This was the phenomenon of soil liquefaction, i.e. the temporary transformation of the submerged soil into a concentrated suspension as a result of a sudden but temporary increase of the pore-water pressure [1 ]. The bearing capacity of liquefied sand is close to zero. In view of this, the prime purpose of the tests was to attempt to create in the soil bin those conditions which would be most conducive to liquefaction of the soil under the treads of the vehicle. It was not anticipated that appropriate scaling factors could be developed for the prediction of actual performance values of the full scale vehicle. Obviously, however, it was a basic assumption that model and prototype behavior would be at least qualitatively related. Another fundamental assumption was that the total hydrostatic pressure existing on a soil is not important to the performance of a vehicle. Therefore, if the soil is totally submerged, it will exhibit the same bearing capacity and tractive qualities *Davidson Laboratory, Stevens Institute of Technology,Castle Point Station, Hoboken, New Jersey, U.S.A. Communicated by C. 3". Nuttall. 53
54
HOWARD DUGOFF and I. ROBERT EHRLICH
whether there are 2 in. or 200 fathoms of water head. This hypothesis is consistent with accepted soil mechanics theory [2]. Test soundings indicated that a path with a predominantly sandy bottom could be found between the New Jersey coast and the edge of the continental shelf. This allowed all tests to be conducted in submerged sand and eliminate the necessity for m a n y different soil conditions. The soundings also indicated probable grain size distributions to be encountered. TEST APPARATUS
Test soils At the outset of the program it was planned to conduct tests using sand samples with each of the three grain size distributions shown in Fig. 1. However, the experiments were terminated after model tests with only one test soil--nominally, the U- 1 curve of Fig. 1".
00
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FIo. 1. Representative grain size distributions to simulate continental shelf.
Soil bins The test program was conducted in a plexiglass bin, 3 ft a and a 30 × 3 ft steel bin, 2ft deep, installed in the Davisdon Laboratory Soil Bin Dynamometer facility. Plexiglass bin. The plexiglass bin which was used in preliminary studies to observe the behavior of a submerged soil, was fitted with a 17 ft length of ~ in. plastic hose, plugged at one end and drilled with ~ in. holes every 2 in. The hose was laid across the bottom in six snake-like loops. Above the hose was laid a course wire grating; over this a layer of fine aluminium window screening. A single layer of loosely woven muslin was then laid over the screening and covered with 11 in. of the U-1 sand. The plastic hose was connected so that the bin could be filled or emptied *Actual grain size distributions of the test sand were slightly finer grained than the nominal--the range is shown in Fig. 2.
M O D E L TESTS I N S U B M E R G E D SOILS
~llJ 11111[ "~ill llllll 1 ~ / R o n g e of
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actual distribution
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Grain size distributions of sand samples used in D.L. tests.
with the water flowing through the sand. Figure 3 is a photograph of the plexiglass bin filled with water to a depth of a few inches above the sand surface. Steel bin. The steel bin which was used for the model tests was fitted out in a manner similar to the smaller plexiglass tank. Three 25 ft lengths of perforated ½ in. plastic hose were laid along the bottom, between a pair of 2 in. dia. transverse header pipes, each fed by an independent water supply. Above these were placed,
FIG. 3.
Plexiglass soil bin filled with U-1 sand and water.
56
HOWARD DUGOFF and I. ROBERT EHRLICH
in order, layers of wire mesh, fine aluminium window screen and loosely woven muslin. Cement was used to bind the muslin to the sides of the tank so that no sand was lost during water draining and filling operations.
Model The test vehicle was a ~-scale model of the U.S. Army M76 Amphibious Vehicle, the "Otter" (Figs. 4, 5). Each of its tracks was 5½ in. in width and had 18½ in. of soil contact length, yielding a nominal ground bearing area of 204 in ~. When unloaded, the model weighed 109 lb, corresponding to slightly over ½ lb/in ~ bearing pressure. The model track was driven through a chain and sprocket power train by a ½HP DC motor. It should be emphasized that the Otter model was by no means regarded as an exact scale model of the ultimate cable burier design; it was viewed merely as a representative tracked vehicle. The primary reason for its utilization in this study was economic - - i t was loaned to Bell Labs by the Land Locomotion Laboratory, U.S. Army TankAutomotive Center.
Soil bin dynamometer The Soil Bin Dynamometer was fitted with two carriages. The main carriage (Figs. 4 and 5), containing the model and measuring devices, was transported along the bin by a cable drive powered by a 5 H P DC motor. The carriage speed, limited to approximately 3 ft/sec, was controlled by a rheostat in conjunction with a standard three-speed automotive transmission. An auxiliary carriage was used to carry the soil processing equipment. Instrumentation The model was fitted with a tachometer to measure track speed. Incorporated in the rig between the model and the dynamometer carriage was a parallelogram-load cell balance which measured the horizontal force reaction, i.e. the drawbar pull. Two linear potentiometers were connected above the model to measure the sinkage at front and rear stations. A tachometer was also mounted for measuring carriage speed. Track and carriage speeds were fed into a special electronic servo multiplier circuit which computed track slip using the equation Track s l i p = i = S t - - S o St where
S t = t r a c k speed S o : c a r r i a g e speed
The six readings were recorded on two Sanborn 150 recorders. In general, all speed measurements appeared accurate to within + 5 per cent. Precision of the drawbar pull measurements, determined from the scatter of repeated data obtained at supposedly identical conditions, was estimated to be better than + 5 lb, precision of sinkage measurements, better than + 0.2 in.
Controls A unique feature of the model test setup was the system used to control the model
M O D E L TESTS IN S U B M E R G E D SOILS
FIos. 4 and 5.
Otter model being tested with dry sand in steel soil bin.
57
58
H O W A R D D U G O F F and I. ROBERT E H R L I C H
track speed. This system is illustrated in schematic form in Fig. 6. Note that the feedback signal is a function of the difference between the desired slip (set manually by the experimenter) and the actual slip as computed from carriage and track speeds. With this system it was possible to make test runs of two fundamentally different types: either the carriage speed could be varied while constant slip was maintained, or the slip could be varied while the speed of advance was held constant. During the test program both methods of slip control were employed.
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Schematic diagram of model control system.
Soil processing and measuring equipment A great many different methods were used to process and measure the properties of the submerged soils. Because of the novelty of the submarine environment, the application of each of these methods was in itself an experiment of a sort. For this reason, the processing and measuring equipment will be discussed in detail in the following sections. TEST P R O G R A M
The test program consisted of three major phases: preliminary studies of submerged soil behavior, conducted in the plexiglass soil bin; submerged soil density measurement studies, also in the plexiglass bin; and vehicle model tests in the main (steel) soil bin dynamometer.
Preliminary studies ot submerged soil behavior Attempts to induce liqueJaction. A primary objective of the test program was to create in the laboratory those conditions which would be most conductive to liquefaction of the soil under the treads of the vehicle. It was indicated by the literature that an upward vertical water flow was one such condition. For this reason, extensive tests were made in the plexiglass bin to study the effects of such flows. These
MODEL TESTS IN SUBMERGED SOILS
59
experiments were of a qualitative nature. Relative bearing strength was estimated on the basis of drop tests with a cylindrical steel rod and, even more crudely, by hand prodding of the soil surface. Visual observations through the transparent tank walls were also useful, especially in detecting areas of total liquefaction. No noticeable difference in bearing capacity was observed between the submerged soil in still water and the submerged soil in water flowing upward slowly. At high rates of upward water flow, localized areas of liquefaction could be established. These would manifest themselves as strong soil and water geysers a few square inches in extent. In each of these "quick" areas, the bearing capacity was almost nil. Areas adjacent to the quick stream appeared unaffected and had apparently undiminished bearing capacity. As the supply water flow rate was increased, more vigorous flow occurred in the quick area without any expansion of the quick area. Since, even with maximum water flow, liquefaction was induced over only a very small percentage of the bin, it was concluded that a homogeneous liquefied soil condition could not be achieved in the model tests by water flow.
Attempts to achieve minimum density conditions. It was felt that a very low strength condition, highly susceptible to liquefaction, might exist in a soil formed by particle sedimentation, where a very loose honeycomb or flocculent structure might develop. Attempts were made to simulate this condition in the laboratory by slowly sifting the sand through a fine sieve over a water-filled tank, and allowing the grains to settle naturally to the bottom. Obviously, prohibitively high testing costs would result of this procedure had to be followed in processing the soil between each run of a systematic model test program. Consequently, efforts were also made to produce an equally loose soil condition through some less painstaking process. Both gyrotilling and rototilling procedures were investigated, as well as less conventional techniques such as bubbling compressed air upward through the submerged soil. After each processing, the surface was carefully leveled, and density measurements were made. The slowly sifted sand produced a dry density of 88 lb/fP. This was then set as a minimum density "target" for the various other soil processing techniques under consideration. Figure 7 is a chart showing the dry density of freshly processed and levelled submerged sand as a function of processing technique. The results range from the 88 lb/fP target minimum to a value of 115 lb/ft 3 at maximum compaction. The latter value, however, was obtained* by first loading a small cylinder as tightly as possible with dry sand, then flooding it without any increase in volume. The maximum compaction that was actually attained in the plexiglass soil bin was 95 lb/fP. Thus the total practical achievable density variation (88-95 lb/ft 3) amounted to less than 10 per cent of the minimum attainable value. The processing technique used in the model tests; viz., rototilling and leveling while damp, with later flooding, resulted in a dry density of 90 lb/fP, only 2 lb/ft 3 or 3 per cent greater than the target value. In view of the smallness of the total variation, however, this agreement is perhaps not so impressive as it may seem. Investigation o/ grain segregation due to soil agitation. One problem which was foreseen early in the test program was the possibility that repeated agitation of the *By Haley and Aldrich, Inc., Consulting Soil Engineers.
60
HOWARD DUGOFF and I. ROBERT EHRLICH Processing technique
I
Sedimentation
Rototilling and leveling while damp then flooding
I
Gyrotilling while submerged
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| drop tests Processing of vertical upward compressed air flu W ii
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FIG. 7.
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40 60 80 Dry density, Ib/ft 3
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120
Density of U-1 sand as a function of processing technique.
submerged sand during testing and processing would result in gravity-induced grain size segregation of the test soil. To explore this possibility, an initially homogeneous sample of U-1 sand was loaded in the plexiglass bin, submerged, and subjected to prolonged and very severe agitation by vertical water flow, compressed air flow, hand manipulation and gyrotilling. The sample was then left undisturbed overnight, to allow any particles in suspension to settle out. Visual observations made at this time indicated that some grain segregation had indeed occurred. Samples were extracted from the bin at several locations on the surface, half-way down and at the bottom. These were oven dried and sieve separated. The only quantitatively significant segregation found was of very large particles (¼ in. and greater) which comprised less than 1 per cent of the overall soil composition. It was concluded that this effect should not present any problem for the model test program. Submerged soil density measurement studies On the basis of a recommendation by Haley and Aldrich, Inc., Consulting Soil Engineers, it was stipulated by Bell Telephone Laboratories that the test results be correlated with in-place soil density. Unfortunately, however, no proven method for in-place measurement of density, or porosity, of a submerged soil sample was known
MODEL TESTS IN SUBMERGED SOILS
61
to exist. For this reason, an extensive experimental program to develop such a technique as undertaken. In the course of this phase of the investigation, a number of "blind alleys" were encountered. They are presented here for the benefit of future researchers in this field.
Direct methods In these methods, attempts were made to determine density directly, i.e. as directly as possible for a "specific" quantity, by measuring both mass and volume of a representative soil sample. (a) Height of soil method. Assuming that the soil in a test bin (or section thereof) is perfectly homogeneous, and its surface is perfectly level, its density can easily be determined by measuring its total weight (conveniently done while the test specimen is being loaded into the soil bin) and its overall height, from which the volume can be computed. Tests to evaluate the practical feasibility of this method were made in the plexiglass soil bin. The results of repeated applications of the height of soil method indicated that it would yield a reliable result for the average density of a soil sample. To apply this method, however, it is necessary that an extensive portion of the soil surface be almost perfectly level. This is by no means in itself a disadvantage for model testing where a level surface is generally a requirement in any event. In the cases treated here, however, the surface levelling process resulted in a significant compaction of the soil in the vicinity of the surface. Thus the average density measurement was not representative of local conditions throughout the sample. This is a serious drawback since vehicle mobility is more directly dependent upon local density than average density. Another disadvantage of the height of soil method was the great difficulty in precisely locating the soil surface under its muddy-colored water cover. For these reasons, no attempt was made to apply the height of water method to measuring the volume of soil in the steel bin during the model tests. (b) Coffee-can method. This method drew its name from the way it was conceived, viz. with the idea that if a coffee can were inverted and pressed into the soil, it would entrap a known soil volume which then could be weighed to determine density. The problem, of course, was to devise a "coffee can" that would effectively trap and remove a known volume without significantly disturbing the soil. The approach taken to this problem was a trial and error one; a "can" was designed, tried out, modified, retested, etc. Figure 8 is a schematic drawing of the coffee can method in application. Figure 9 is a drawing of the actual "coffee can" evolved by trial and error as yielding satisfactory submerged soil density measurements. Two important features of this device should be noted. The first is the very large area of the surface bearing plate, necessary to prevent the can from sinking while the sample is being taken because of the low bearing strength of the submerged soil. The second are the notches cut out in the bottom of the slit for inserting the trapping plate. These served the twofold purpose of reducing contact area (and, hence, adhesion) between the trapping plate and the slit surface, and of providing a relief path for the sand (not part of the desired sample) displaced by the trapping plate.
62
HOWARD DUGOFF and I. ROBERT EHRLICH
/
/ SOIL LEVEL
CYLINDER
SURFACE BEARING PLATE
/
TRAPPINGPLATE
FIG. 8. Schematic drawing of "coffee can" for measuring soil density. In use the sharpened circular edge was pressed vertically into the soil until the large bearing plate rested uniformly on the soil surface. Soil was then excavated from the slotted side of the cylinder to a point just below the slot. This process was aided by the contour of the bearing plate which was foreshortened on the slotted side of the cylinder. The trapping plate could then be inserted through the slot without disturbing the entrapped soil sample. Many controlled measurements of dry, moist and submerged sand demonstrated excellent repeatability and correlation with other methods.
Indirect methods In these methods, attempts were made to establish one-to-one correspondence
.
FIG. 9. Coffeecan for measuring submerged soil density.
MODEL TESTS IN SUBMERGED SOILS
63
between density and some other soil property which might be conveniently measured. The obvious shortcoming of any such method was the implicit requirement for some direct density measurement for calibration. It was felt that these might be obtained by using the height of soil method on small, carefully constructed soil samples where, it was hoped, density homogeneity might be achieved. (a) Electrical resistance method. Measurements were made with an accurate ohmmeter of the electrical resistance between two probes a fixed distance apart in submerged soil samples of various densities. Attempts were made to establish densityresistance correlation. These proved to be unsuccessful. Conductivity of the submerged soil samples appeared to be virtually the same as for the free standing water, regardless of the density condition. (b) Plate penetrometer method. Correlation was sought between density and the soil's bearing strength characteristics, as manifested by (force) resistance to sinkage of circular flat plates. The penetrometer was driven at a slow (approx. 1 in./min) uniform rate, and the load was measured with a hydraulic gage mounted in the loading shaft. Plates of both 2 in. and 4 in. dia. were employed. Figure 10 is a graph showing the results of plate penetrometer tests with submerged soils of three different densities. A correlation between density and bearing strength is apparent; in fact, it is clear that bearing capacity is a very sensitive density indicator. The results shown are for the 2 in. dia. circular penetrometer. Similar data were
3ompoctedby
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Gyrotilled ( p =92 Ib/ft 3) /
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64
HOWARD DUGOFF and I. ROBERT EHRL1CH
obtained using the 4 in. plate. However, neither of these penetrometers were used to measure density during the model test program because of their close similarity to the California Bearing Ratio apparatus discussed below. Other methods At the same time the study of submerged soil measuring techniques was being pursued at Davidson Laboratory, various other approaches were being taken by outside investigators concerned with the problem. Engineers at Bell Labs explored the possibility of correlating density with attenuation of nuclear emissions from a radioactive specimen (nuclear probe). In view of the inherent complication and expense of this technique, efforts to perfect it were discontinued when it became apparent that successful results could be achieved using other methods. One of these successful methods, also a density-penetrometer correlation method, was developed by Haley and Aldrich, Inc., Consulting Soil Engineers. The basic difference between this method and the similar technique discussed previously was that the penetrometer apparatus employed was standard California Bearing Ratio test equipment. The penetration piston was 1.95 in. in diameter (base area 3 in2); two l0 lb surcharge weights (10 in. OD) were used. The pcnetrometer was driven by a manually operated loading device at a constant rate of approximately 0- l0 in/min. Load was measured with a 1000 lb capacity proving ring attached to the top of the piston. Figure 11 is a photograph of the CBR apparatus being used to measure the density of submerged soil in the main DL soil bin. The CBR results were plotted in the form of penetration vs. resistance (see Fig. 12). In each case, the zero penetration intercept was established at 3 lb resistance, the seating
FIG. 11. Submergedsoil densitymeasurementby CBR method.
MODEL TESTS IN SUBMERGED SOILS
65
load. When the plotted curve was concave downward* (Fig. 12a), a straight line was drawn connecting the zero penetration intercept with the 0.1 in. penetration point. Another line with the same slope was then drawn through the origin. The intercept of this line at 0-1 in. penetration is considered the "adjusted resistance". When the test plot was initially concave upward1", (Fig. 12b) followed by a reasonably straight line O. CURVE CONCAVE DOWNWARD PENETRATION RESISTANCE 0 O
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*In terms of the familiar Bekker pressure-sinkage representation, p = k z condition n < 1. t n > 1 ; see footnote above.
n
this case corresponds to the
66
HOWARD DUGOFF and I. ROBERT EHRLICH
slope, this slope was translated to the origin, and the 0. I in. penetration force intercept was recorded as adjusted resistance. A calibration curve for the U-1 test sand to relate the CBR 0-1 in. readings with submerged soil density was established by Haley and Aldrich [3] and is shown in Fig. 13.
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Fla. 13. Calibration curve for determination of submerged U-I sand density by CBR method. Vehicle model tests
The scale model of the Otter vehicle was tested in submerged U-I sand prepared by (1) rototilling with the soil moist, (2) leveling and smoothing the surface with a flatedged plate, and (3) flooding with water from beneath to a height approximately two inches above the sand surface. After each run the water level was lowered below the depth of tilling, usually at least 10 in. below the surface in preparation for the next processing. Before each model run, measurements were made with a standard U.S. Army Corps of Engineers (WES) cone penetrometer (30 ° cone, ¼ in. max. dia.). These were made to assure uniformity of processing, and also because of their known usefulness in correlation with vehicle performance over a wide range of soil conditions. Since the effective tillage depth of the rototiller was estimated to be about 6 in., readings for only the first 5 in. of penetration were used for comparison. Tests were conducted at model speeds of 0"33. 0"66, 0.99 and 1"32 ft/sec. Measurements were made both with slip held constant and with slip steadily increasing. The drawbar vs. slip curves constructed using constant slip results are presented in this paper.
MODEL TESTS IN SUBMERGED
SOILS
67
To provide a frame of reference for the submerged soil results, similar tests were conducted in both air dry sand and in wet, but unsubmerged, sand. The drawbar pull data from all of the model tests in submerged, air dry, and moist sand are plotted vs. slip in Fig. 14. The data obtained at the four different test velocities are differentiated from one another by use of different symbols. In light of the overall scatter of these data, the influence of velocity variation on drawbar pull is not regarded as significant within the speed range tested. Hence one curve has been drawn in to represent the drawbar-slip characteristic for each soil condition. These curves are superposed in Fig. 15 for a direct comparison of the drawbar pull attainable in the different soils. It will be noted that the drawbar pull degradation due to submergence is significant. In particular, the reduction at the 50 per cent slip point (close to the maximum attainable value) is approximately 25 per cent of the dry sand value and 30 per cent of the value for moist sand (presumably somewhat better due to apparent cohesion resulting from a surface 0-4 0-3 0-2 ..,,_" Oq
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Drawbar-pull data from model tests.
68
HOWARD DUGOFF and I. ROBERT EHRLICH 0.5
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Fro. 15. Comparison of drawbar pull-slip curvvs in submergexl, air dry and moist U-1 sand. tension effect of the water). This amount of degradation, however, is certainly not regarded as indicative of any qualitative breakdown in bearing capacity such as would be expected if the action of the tracks was to induce appreciable local liquefaction of the submerged soil. Figure 16 is a plot of average vehicle sinkage vs. slip for each of three speeds (0.33 ft/sec, 0.66 ft/sec, 0.99 ft/sec) tested in submerged soils. It may be seen that the sinkage increases with slip and decreases with speed. A comparison of sinkage-slip in submerged soil and moist soil, at a speed of 0-66 ft/sec, is shown in Fig. 17. The results conform to expectation, showing the sinkage to be substantially greater in the submerged condition. This raises the suggestion that the degradation of performance due to submergence is perhaps at least as much the result of increased compaction resistance as decreased tractive effort. It will be noted that vehicle sinkage data are not presented for all of the conditions shown in the drawbar pull plots. These measurements were limited for economic reasons, since sinkage is a quantity of importance to the vehicle designer only insofar as it affects the performance qualities more directly described by the drawbar measurements. Cone penetrometer readings in the submerged sand were substantially identical for all tests, giving a 1, 3 and 5 in. penetration average of 12.7. The corresponding value in both the moist sand and the air dry sand was 26.
MODEL TESTS IN SUBMERGED SOILS
69
,,~ Z - -
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0.33 f t/see 0.66 f t / s e c 0.99 f t/sec
t~
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0
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40 SLIP,
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60
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80
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FIG. 16. Sinkage data from submerged soil model tests.
Z -
4
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2
MOIST I
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20
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40
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60
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SLIP, PERCENT
FIG. 17. Comparison of sinkage-slip curves in submerged and moist U-I sand (model speed=0.66 ft/sec).
I00
70
HOWARD DUGOFF and I. ROBERT EHRLICH CONCLUSIONS AND RECOMMENDATIONS
Conclusions
(a) The results o f these tests in no way suggest that the operation of a tracked vehicle in totally submerged sand is not feasible. Because o f the limited nature o f the tests, however, no more affirmative statement along these lines can be made. (b) U n d e r the conditions tested, liquefaction o f the sand under the tracks is not a significant factor affecting vehicle performance. (c) The "coffee-can" m e t h o d of measuring soil density is a promising technique for in situ analysis. Recommendations
(a) Additional research should be performed before any definite general conclusions regarding the feasibility o f vehicle operation in submerged soil are made. (b) A t t e m p t s should be made to correlate these model results with corresponding full scale data to establish quantitative scaling relationships. Acknowledgments--Even more than most research studies, the present program was a cooperative effort between sponsoring agency and contractor. Messrs. C. E. Roden and F. A. Reidy of Bell Telephone Laboratories worked hand in hand with Davidson Laboratory personnel throughout. Major advice and guidance were also forthcoming from BTL consultants Dr. H. P. Aldrich, Jr. and Mr. E. G. Johnson of Haley and Aldrich, Inc. and Mr. C. J. Nuttall, Jr., of Wilson, NuttaU, Raimond, Engineers, Inc. The authors also wish to thank Messrs. R. B. Schwartz, I. O. Kamm and R. Madden of Davidson Laboratory for their help during the experiments and Mr. R. Krukowski of DL who designed and built the special instrumentation.
BIBLIOGRAPHY [1] K. TERZAGmand R. B. PECK. Soil Mechanics in Engineering Practice. John Wiley, New York (1948). [2] K. TERZAOHI. Theoretical Soil Mechanics. John Wiley, New York (1943). [3] Letter, E. G. JOHNSON,Haley and Aldrich to C. E. RODEr4,Bell Telephone Laboratories, April 1, 1964.
Printed in Great Britain.
MODEL TESTS IN SUBMERGED SOILS HOWARD DUGOFF* and I. ROBERT EHRLICH* INTRODUCTION AMERICANTelephone and Telegraph Corporation, as part of its overseas communication systems, operates and maintains a number of underwater cables, stretching from the United States and Canada to Great Britain, Ireland and France. At the present time the cables lying exposed on the North American Continental Shelf are subject to considerable damage from the many fishing boats that ply the waters overhead. These vessels, in order to keep their nets on the bottom while trawling, use oneand two-ton trawl boards dragged along the bottom by strong steel cables. Whenever one of these weighted steel cables is dragged across one of the telephone cable loops or bridges, something has to give---it is usually the telephone cable. The cost for repairs and lost usage can be considerable. Two possible solutions to this problem is to bury the cables beneath the ocean floor. The study performed by Davidson Laboratory was concerned with this possibility, in particular with an investigation of the feasibility of employing a selfpropelled tracked vehicle to operate on the floor of the ocean and to entrench and bury the cable. It was financed by and conducted in close cooperation with the Bell Telephone Laboratories. No previous studies of the traflicability characteristics of submerged soils were known. As a first step in gaining an understanding of this very complicated problem, a limited program of submerged soil studies and scale model tractor tests was initiated. One soil mechanical phenomenon particularly associated with the submarine environment was regarded as extremely significant from the vehicle locomotion point of view. This was the phenomenon of soil liquefaction, i.e. the temporary transformation of the submerged soil into a concentrated suspension as a result of a sudden but temporary increase of the pore-water pressure [1 ]. The bearing capacity of liquefied sand is close to zero. In view of this, the prime purpose of the tests was to attempt to create in the soil bin those conditions which would be most conducive to liquefaction of the soil under the treads of the vehicle. It was not anticipated that appropriate scaling factors could be developed for the prediction of actual performance values of the full scale vehicle. Obviously, however, it was a basic assumption that model and prototype behavior would be at least qualitatively related. Another fundamental assumption was that the total hydrostatic pressure existing on a soil is not important to the performance of a vehicle. Therefore, if the soil is totally submerged, it will exhibit the same bearing capacity and tractive qualities *Davidson Laboratory, Stevens Institute of Technology,Castle Point Station, Hoboken, New Jersey, U.S.A. Communicated by C. 3". Nuttall. 53
54
HOWARD DUGOFF and I. ROBERT EHRLICH
whether there are 2 in. or 200 fathoms of water head. This hypothesis is consistent with accepted soil mechanics theory [2]. Test soundings indicated that a path with a predominantly sandy bottom could be found between the New Jersey coast and the edge of the continental shelf. This allowed all tests to be conducted in submerged sand and eliminate the necessity for m a n y different soil conditions. The soundings also indicated probable grain size distributions to be encountered. TEST APPARATUS
Test soils At the outset of the program it was planned to conduct tests using sand samples with each of the three grain size distributions shown in Fig. 1. However, the experiments were terminated after model tests with only one test soil--nominally, the U- 1 curve of Fig. 1".
00
"
40
c!l
]]
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II II It
ll~]l~J [ \I[Iii I ~-u-'\°-~\llll I
[I I; ]I
~J I]~ I'0
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0 I Groin
size,
Ill I[I
001
0001
turn
FIo. 1. Representative grain size distributions to simulate continental shelf.
Soil bins The test program was conducted in a plexiglass bin, 3 ft a and a 30 × 3 ft steel bin, 2ft deep, installed in the Davisdon Laboratory Soil Bin Dynamometer facility. Plexiglass bin. The plexiglass bin which was used in preliminary studies to observe the behavior of a submerged soil, was fitted with a 17 ft length of ~ in. plastic hose, plugged at one end and drilled with ~ in. holes every 2 in. The hose was laid across the bottom in six snake-like loops. Above the hose was laid a course wire grating; over this a layer of fine aluminium window screening. A single layer of loosely woven muslin was then laid over the screening and covered with 11 in. of the U-1 sand. The plastic hose was connected so that the bin could be filled or emptied *Actual grain size distributions of the test sand were slightly finer grained than the nominal--the range is shown in Fig. 2.
M O D E L TESTS I N S U B M E R G E D SOILS
~llJ 11111[ "~ill llllll 1 ~ / R o n g e of
55
IIIII
actual distribution
40
Ill l o-,-ml IlllUl IllllW ttilllU IIIIll IIitll
IIIII
b_
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I'0
11111 tllll Illll IIIIt IJ]Jl
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O-i
Groin size,
lllll IIIII IIIII IIIII IIIII 11111 tt111
o,ool
mm
Grain size distributions of sand samples used in D.L. tests.
with the water flowing through the sand. Figure 3 is a photograph of the plexiglass bin filled with water to a depth of a few inches above the sand surface. Steel bin. The steel bin which was used for the model tests was fitted out in a manner similar to the smaller plexiglass tank. Three 25 ft lengths of perforated ½ in. plastic hose were laid along the bottom, between a pair of 2 in. dia. transverse header pipes, each fed by an independent water supply. Above these were placed,
FIG. 3.
Plexiglass soil bin filled with U-1 sand and water.
56
HOWARD DUGOFF and I. ROBERT EHRLICH
in order, layers of wire mesh, fine aluminium window screen and loosely woven muslin. Cement was used to bind the muslin to the sides of the tank so that no sand was lost during water draining and filling operations.
Model The test vehicle was a ~-scale model of the U.S. Army M76 Amphibious Vehicle, the "Otter" (Figs. 4, 5). Each of its tracks was 5½ in. in width and had 18½ in. of soil contact length, yielding a nominal ground bearing area of 204 in ~. When unloaded, the model weighed 109 lb, corresponding to slightly over ½ lb/in ~ bearing pressure. The model track was driven through a chain and sprocket power train by a ½HP DC motor. It should be emphasized that the Otter model was by no means regarded as an exact scale model of the ultimate cable burier design; it was viewed merely as a representative tracked vehicle. The primary reason for its utilization in this study was economic - - i t was loaned to Bell Labs by the Land Locomotion Laboratory, U.S. Army TankAutomotive Center.
Soil bin dynamometer The Soil Bin Dynamometer was fitted with two carriages. The main carriage (Figs. 4 and 5), containing the model and measuring devices, was transported along the bin by a cable drive powered by a 5 H P DC motor. The carriage speed, limited to approximately 3 ft/sec, was controlled by a rheostat in conjunction with a standard three-speed automotive transmission. An auxiliary carriage was used to carry the soil processing equipment. Instrumentation The model was fitted with a tachometer to measure track speed. Incorporated in the rig between the model and the dynamometer carriage was a parallelogram-load cell balance which measured the horizontal force reaction, i.e. the drawbar pull. Two linear potentiometers were connected above the model to measure the sinkage at front and rear stations. A tachometer was also mounted for measuring carriage speed. Track and carriage speeds were fed into a special electronic servo multiplier circuit which computed track slip using the equation Track s l i p = i = S t - - S o St where
S t = t r a c k speed S o : c a r r i a g e speed
The six readings were recorded on two Sanborn 150 recorders. In general, all speed measurements appeared accurate to within + 5 per cent. Precision of the drawbar pull measurements, determined from the scatter of repeated data obtained at supposedly identical conditions, was estimated to be better than + 5 lb, precision of sinkage measurements, better than + 0.2 in.
Controls A unique feature of the model test setup was the system used to control the model
M O D E L TESTS IN S U B M E R G E D SOILS
FIos. 4 and 5.
Otter model being tested with dry sand in steel soil bin.
57
58
H O W A R D D U G O F F and I. ROBERT E H R L I C H
track speed. This system is illustrated in schematic form in Fig. 6. Note that the feedback signal is a function of the difference between the desired slip (set manually by the experimenter) and the actual slip as computed from carriage and track speeds. With this system it was possible to make test runs of two fundamentally different types: either the carriage speed could be varied while constant slip was maintained, or the slip could be varied while the speed of advance was held constant. During the test program both methods of slip control were employed.
J'~carriagedriveshaft .,llJ~ J
[
-
Colibrat\ion setting
ioch
Model drive
I
shaftj - ~
Slipsetting(manual)
Summing~ ~ inputnetwork
vemotor
(~
Fixedmotor field
Amplid,ne FIG. 6.
Schematic diagram of model control system.
Soil processing and measuring equipment A great many different methods were used to process and measure the properties of the submerged soils. Because of the novelty of the submarine environment, the application of each of these methods was in itself an experiment of a sort. For this reason, the processing and measuring equipment will be discussed in detail in the following sections. TEST P R O G R A M
The test program consisted of three major phases: preliminary studies of submerged soil behavior, conducted in the plexiglass soil bin; submerged soil density measurement studies, also in the plexiglass bin; and vehicle model tests in the main (steel) soil bin dynamometer.
Preliminary studies ot submerged soil behavior Attempts to induce liqueJaction. A primary objective of the test program was to create in the laboratory those conditions which would be most conductive to liquefaction of the soil under the treads of the vehicle. It was indicated by the literature that an upward vertical water flow was one such condition. For this reason, extensive tests were made in the plexiglass bin to study the effects of such flows. These
MODEL TESTS IN SUBMERGED SOILS
59
experiments were of a qualitative nature. Relative bearing strength was estimated on the basis of drop tests with a cylindrical steel rod and, even more crudely, by hand prodding of the soil surface. Visual observations through the transparent tank walls were also useful, especially in detecting areas of total liquefaction. No noticeable difference in bearing capacity was observed between the submerged soil in still water and the submerged soil in water flowing upward slowly. At high rates of upward water flow, localized areas of liquefaction could be established. These would manifest themselves as strong soil and water geysers a few square inches in extent. In each of these "quick" areas, the bearing capacity was almost nil. Areas adjacent to the quick stream appeared unaffected and had apparently undiminished bearing capacity. As the supply water flow rate was increased, more vigorous flow occurred in the quick area without any expansion of the quick area. Since, even with maximum water flow, liquefaction was induced over only a very small percentage of the bin, it was concluded that a homogeneous liquefied soil condition could not be achieved in the model tests by water flow.
Attempts to achieve minimum density conditions. It was felt that a very low strength condition, highly susceptible to liquefaction, might exist in a soil formed by particle sedimentation, where a very loose honeycomb or flocculent structure might develop. Attempts were made to simulate this condition in the laboratory by slowly sifting the sand through a fine sieve over a water-filled tank, and allowing the grains to settle naturally to the bottom. Obviously, prohibitively high testing costs would result of this procedure had to be followed in processing the soil between each run of a systematic model test program. Consequently, efforts were also made to produce an equally loose soil condition through some less painstaking process. Both gyrotilling and rototilling procedures were investigated, as well as less conventional techniques such as bubbling compressed air upward through the submerged soil. After each processing, the surface was carefully leveled, and density measurements were made. The slowly sifted sand produced a dry density of 88 lb/fP. This was then set as a minimum density "target" for the various other soil processing techniques under consideration. Figure 7 is a chart showing the dry density of freshly processed and levelled submerged sand as a function of processing technique. The results range from the 88 lb/fP target minimum to a value of 115 lb/ft 3 at maximum compaction. The latter value, however, was obtained* by first loading a small cylinder as tightly as possible with dry sand, then flooding it without any increase in volume. The maximum compaction that was actually attained in the plexiglass soil bin was 95 lb/fP. Thus the total practical achievable density variation (88-95 lb/ft 3) amounted to less than 10 per cent of the minimum attainable value. The processing technique used in the model tests; viz., rototilling and leveling while damp, with later flooding, resulted in a dry density of 90 lb/fP, only 2 lb/ft 3 or 3 per cent greater than the target value. In view of the smallness of the total variation, however, this agreement is perhaps not so impressive as it may seem. Investigation o/ grain segregation due to soil agitation. One problem which was foreseen early in the test program was the possibility that repeated agitation of the *By Haley and Aldrich, Inc., Consulting Soil Engineers.
60
HOWARD DUGOFF and I. ROBERT EHRLICH Processing technique
I
Sedimentation
Rototilling and leveling while damp then flooding
I
Gyrotilling while submerged
]
Proc.,,,nob,
lil
uO,.,or°,,,a.er,.o,,,
Estimated from
| drop tests Processing of vertical upward compressed air flu W ii
Compacting by tamping with 2"X2" steel plate
[ J
I
Packing cylinders as tightly as possible with dry sand, loading I then saturatincj I
I
20
FIG. 7.
I
I
l
[
J
1
40 60 80 Dry density, Ib/ft 3
L
I
I00
I
I
120
Density of U-1 sand as a function of processing technique.
submerged sand during testing and processing would result in gravity-induced grain size segregation of the test soil. To explore this possibility, an initially homogeneous sample of U-1 sand was loaded in the plexiglass bin, submerged, and subjected to prolonged and very severe agitation by vertical water flow, compressed air flow, hand manipulation and gyrotilling. The sample was then left undisturbed overnight, to allow any particles in suspension to settle out. Visual observations made at this time indicated that some grain segregation had indeed occurred. Samples were extracted from the bin at several locations on the surface, half-way down and at the bottom. These were oven dried and sieve separated. The only quantitatively significant segregation found was of very large particles (¼ in. and greater) which comprised less than 1 per cent of the overall soil composition. It was concluded that this effect should not present any problem for the model test program. Submerged soil density measurement studies On the basis of a recommendation by Haley and Aldrich, Inc., Consulting Soil Engineers, it was stipulated by Bell Telephone Laboratories that the test results be correlated with in-place soil density. Unfortunately, however, no proven method for in-place measurement of density, or porosity, of a submerged soil sample was known
MODEL TESTS IN SUBMERGED SOILS
61
to exist. For this reason, an extensive experimental program to develop such a technique as undertaken. In the course of this phase of the investigation, a number of "blind alleys" were encountered. They are presented here for the benefit of future researchers in this field.
Direct methods In these methods, attempts were made to determine density directly, i.e. as directly as possible for a "specific" quantity, by measuring both mass and volume of a representative soil sample. (a) Height of soil method. Assuming that the soil in a test bin (or section thereof) is perfectly homogeneous, and its surface is perfectly level, its density can easily be determined by measuring its total weight (conveniently done while the test specimen is being loaded into the soil bin) and its overall height, from which the volume can be computed. Tests to evaluate the practical feasibility of this method were made in the plexiglass soil bin. The results of repeated applications of the height of soil method indicated that it would yield a reliable result for the average density of a soil sample. To apply this method, however, it is necessary that an extensive portion of the soil surface be almost perfectly level. This is by no means in itself a disadvantage for model testing where a level surface is generally a requirement in any event. In the cases treated here, however, the surface levelling process resulted in a significant compaction of the soil in the vicinity of the surface. Thus the average density measurement was not representative of local conditions throughout the sample. This is a serious drawback since vehicle mobility is more directly dependent upon local density than average density. Another disadvantage of the height of soil method was the great difficulty in precisely locating the soil surface under its muddy-colored water cover. For these reasons, no attempt was made to apply the height of water method to measuring the volume of soil in the steel bin during the model tests. (b) Coffee-can method. This method drew its name from the way it was conceived, viz. with the idea that if a coffee can were inverted and pressed into the soil, it would entrap a known soil volume which then could be weighed to determine density. The problem, of course, was to devise a "coffee can" that would effectively trap and remove a known volume without significantly disturbing the soil. The approach taken to this problem was a trial and error one; a "can" was designed, tried out, modified, retested, etc. Figure 8 is a schematic drawing of the coffee can method in application. Figure 9 is a drawing of the actual "coffee can" evolved by trial and error as yielding satisfactory submerged soil density measurements. Two important features of this device should be noted. The first is the very large area of the surface bearing plate, necessary to prevent the can from sinking while the sample is being taken because of the low bearing strength of the submerged soil. The second are the notches cut out in the bottom of the slit for inserting the trapping plate. These served the twofold purpose of reducing contact area (and, hence, adhesion) between the trapping plate and the slit surface, and of providing a relief path for the sand (not part of the desired sample) displaced by the trapping plate.
62
HOWARD DUGOFF and I. ROBERT EHRLICH
/
/ SOIL LEVEL
CYLINDER
SURFACE BEARING PLATE
/
TRAPPINGPLATE
FIG. 8. Schematic drawing of "coffee can" for measuring soil density. In use the sharpened circular edge was pressed vertically into the soil until the large bearing plate rested uniformly on the soil surface. Soil was then excavated from the slotted side of the cylinder to a point just below the slot. This process was aided by the contour of the bearing plate which was foreshortened on the slotted side of the cylinder. The trapping plate could then be inserted through the slot without disturbing the entrapped soil sample. Many controlled measurements of dry, moist and submerged sand demonstrated excellent repeatability and correlation with other methods.
Indirect methods In these methods, attempts were made to establish one-to-one correspondence
.
FIG. 9. Coffeecan for measuring submerged soil density.
MODEL TESTS IN SUBMERGED SOILS
63
between density and some other soil property which might be conveniently measured. The obvious shortcoming of any such method was the implicit requirement for some direct density measurement for calibration. It was felt that these might be obtained by using the height of soil method on small, carefully constructed soil samples where, it was hoped, density homogeneity might be achieved. (a) Electrical resistance method. Measurements were made with an accurate ohmmeter of the electrical resistance between two probes a fixed distance apart in submerged soil samples of various densities. Attempts were made to establish densityresistance correlation. These proved to be unsuccessful. Conductivity of the submerged soil samples appeared to be virtually the same as for the free standing water, regardless of the density condition. (b) Plate penetrometer method. Correlation was sought between density and the soil's bearing strength characteristics, as manifested by (force) resistance to sinkage of circular flat plates. The penetrometer was driven at a slow (approx. 1 in./min) uniform rate, and the load was measured with a hydraulic gage mounted in the loading shaft. Plates of both 2 in. and 4 in. dia. were employed. Figure 10 is a graph showing the results of plate penetrometer tests with submerged soils of three different densities. A correlation between density and bearing strength is apparent; in fact, it is clear that bearing capacity is a very sensitive density indicator. The results shown are for the 2 in. dia. circular penetrometer. Similar data were
3ompoctedby
2C
o_
tamping ( p = 9 5 Ib/ft 3)
Gyrotilled ( p =92 Ib/ft 3) /
~0
~ 0
Formedbysedimentation (p =88 Ib/ft 3)
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J
20
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Two in. diameter circular plate penetrometer pressure sinkage results for submerged sand with various densities.
64
HOWARD DUGOFF and I. ROBERT EHRL1CH
obtained using the 4 in. plate. However, neither of these penetrometers were used to measure density during the model test program because of their close similarity to the California Bearing Ratio apparatus discussed below. Other methods At the same time the study of submerged soil measuring techniques was being pursued at Davidson Laboratory, various other approaches were being taken by outside investigators concerned with the problem. Engineers at Bell Labs explored the possibility of correlating density with attenuation of nuclear emissions from a radioactive specimen (nuclear probe). In view of the inherent complication and expense of this technique, efforts to perfect it were discontinued when it became apparent that successful results could be achieved using other methods. One of these successful methods, also a density-penetrometer correlation method, was developed by Haley and Aldrich, Inc., Consulting Soil Engineers. The basic difference between this method and the similar technique discussed previously was that the penetrometer apparatus employed was standard California Bearing Ratio test equipment. The penetration piston was 1.95 in. in diameter (base area 3 in2); two l0 lb surcharge weights (10 in. OD) were used. The pcnetrometer was driven by a manually operated loading device at a constant rate of approximately 0- l0 in/min. Load was measured with a 1000 lb capacity proving ring attached to the top of the piston. Figure 11 is a photograph of the CBR apparatus being used to measure the density of submerged soil in the main DL soil bin. The CBR results were plotted in the form of penetration vs. resistance (see Fig. 12). In each case, the zero penetration intercept was established at 3 lb resistance, the seating
FIG. 11. Submergedsoil densitymeasurementby CBR method.
MODEL TESTS IN SUBMERGED SOILS
65
load. When the plotted curve was concave downward* (Fig. 12a), a straight line was drawn connecting the zero penetration intercept with the 0.1 in. penetration point. Another line with the same slope was then drawn through the origin. The intercept of this line at 0-1 in. penetration is considered the "adjusted resistance". When the test plot was initially concave upward1", (Fig. 12b) followed by a reasonably straight line O. CURVE CONCAVE DOWNWARD PENETRATION RESISTANCE 0 O
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Interpretation of CBR results for determination of density.
*In terms of the familiar Bekker pressure-sinkage representation, p = k z condition n < 1. t n > 1 ; see footnote above.
n
this case corresponds to the
66
HOWARD DUGOFF and I. ROBERT EHRLICH
slope, this slope was translated to the origin, and the 0. I in. penetration force intercept was recorded as adjusted resistance. A calibration curve for the U-1 test sand to relate the CBR 0-1 in. readings with submerged soil density was established by Haley and Aldrich [3] and is shown in Fig. 13.
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Fla. 13. Calibration curve for determination of submerged U-I sand density by CBR method. Vehicle model tests
The scale model of the Otter vehicle was tested in submerged U-I sand prepared by (1) rototilling with the soil moist, (2) leveling and smoothing the surface with a flatedged plate, and (3) flooding with water from beneath to a height approximately two inches above the sand surface. After each run the water level was lowered below the depth of tilling, usually at least 10 in. below the surface in preparation for the next processing. Before each model run, measurements were made with a standard U.S. Army Corps of Engineers (WES) cone penetrometer (30 ° cone, ¼ in. max. dia.). These were made to assure uniformity of processing, and also because of their known usefulness in correlation with vehicle performance over a wide range of soil conditions. Since the effective tillage depth of the rototiller was estimated to be about 6 in., readings for only the first 5 in. of penetration were used for comparison. Tests were conducted at model speeds of 0"33. 0"66, 0.99 and 1"32 ft/sec. Measurements were made both with slip held constant and with slip steadily increasing. The drawbar vs. slip curves constructed using constant slip results are presented in this paper.
MODEL TESTS IN SUBMERGED
SOILS
67
To provide a frame of reference for the submerged soil results, similar tests were conducted in both air dry sand and in wet, but unsubmerged, sand. The drawbar pull data from all of the model tests in submerged, air dry, and moist sand are plotted vs. slip in Fig. 14. The data obtained at the four different test velocities are differentiated from one another by use of different symbols. In light of the overall scatter of these data, the influence of velocity variation on drawbar pull is not regarded as significant within the speed range tested. Hence one curve has been drawn in to represent the drawbar-slip characteristic for each soil condition. These curves are superposed in Fig. 15 for a direct comparison of the drawbar pull attainable in the different soils. It will be noted that the drawbar pull degradation due to submergence is significant. In particular, the reduction at the 50 per cent slip point (close to the maximum attainable value) is approximately 25 per cent of the dry sand value and 30 per cent of the value for moist sand (presumably somewhat better due to apparent cohesion resulting from a surface 0-4 0-3 0-2 ..,,_" Oq
t-
/
--
+
4- x + X O X x o"<~"~°
°
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Submerged U - I sand
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0
20
I
40
60
I
80
~ -0"] o
~
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+ V=0,33 ft/sec o 0,66 ft/sec x 0,99 ft/se¢ n l'32 ft/sec
-0.2 --04
~
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i
0"3
,~.~"~o
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0.2
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0.5
g
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I
40
60
I
8O
%
0-4
Moist U - I sond
~. 0.2
~I 3 0.1
o Q
0
I
20
L Slip,
FIG. 14.
1
40
60
I
80
%
Drawbar-pull data from model tests.
68
HOWARD DUGOFF and I. ROBERT EHRLICH 0.5
AIR DRY
0.4 SUBMERGED 0.3 ° I-Z W (..)
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0.2
I
u.
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!
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-0.2
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60
t
80
SLIP, PERCENT
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-0.3
Fro. 15. Comparison of drawbar pull-slip curvvs in submergexl, air dry and moist U-1 sand. tension effect of the water). This amount of degradation, however, is certainly not regarded as indicative of any qualitative breakdown in bearing capacity such as would be expected if the action of the tracks was to induce appreciable local liquefaction of the submerged soil. Figure 16 is a plot of average vehicle sinkage vs. slip for each of three speeds (0.33 ft/sec, 0.66 ft/sec, 0.99 ft/sec) tested in submerged soils. It may be seen that the sinkage increases with slip and decreases with speed. A comparison of sinkage-slip in submerged soil and moist soil, at a speed of 0-66 ft/sec, is shown in Fig. 17. The results conform to expectation, showing the sinkage to be substantially greater in the submerged condition. This raises the suggestion that the degradation of performance due to submergence is perhaps at least as much the result of increased compaction resistance as decreased tractive effort. It will be noted that vehicle sinkage data are not presented for all of the conditions shown in the drawbar pull plots. These measurements were limited for economic reasons, since sinkage is a quantity of importance to the vehicle designer only insofar as it affects the performance qualities more directly described by the drawbar measurements. Cone penetrometer readings in the submerged sand were substantially identical for all tests, giving a 1, 3 and 5 in. penetration average of 12.7. The corresponding value in both the moist sand and the air dry sand was 26.
MODEL TESTS IN SUBMERGED SOILS
69
,,~ Z - -
V:
0.33 f t/see 0.66 f t / s e c 0.99 f t/sec
t~
. ,,,,-,. X "
0
0
~(
X
I
20
I
40 SLIP,
I
60
I
80
I00
PERCENT
FIG. 16. Sinkage data from submerged soil model tests.
Z -
4
~s SUBMERGED
2
MOIST I
I
20
I
40
I
60
I
80
SLIP, PERCENT
FIG. 17. Comparison of sinkage-slip curves in submerged and moist U-I sand (model speed=0.66 ft/sec).
I00
70
HOWARD DUGOFF and I. ROBERT EHRLICH CONCLUSIONS AND RECOMMENDATIONS
Conclusions
(a) The results o f these tests in no way suggest that the operation of a tracked vehicle in totally submerged sand is not feasible. Because o f the limited nature o f the tests, however, no more affirmative statement along these lines can be made. (b) U n d e r the conditions tested, liquefaction o f the sand under the tracks is not a significant factor affecting vehicle performance. (c) The "coffee-can" m e t h o d of measuring soil density is a promising technique for in situ analysis. Recommendations
(a) Additional research should be performed before any definite general conclusions regarding the feasibility o f vehicle operation in submerged soil are made. (b) A t t e m p t s should be made to correlate these model results with corresponding full scale data to establish quantitative scaling relationships. Acknowledgments--Even more than most research studies, the present program was a cooperative effort between sponsoring agency and contractor. Messrs. C. E. Roden and F. A. Reidy of Bell Telephone Laboratories worked hand in hand with Davidson Laboratory personnel throughout. Major advice and guidance were also forthcoming from BTL consultants Dr. H. P. Aldrich, Jr. and Mr. E. G. Johnson of Haley and Aldrich, Inc. and Mr. C. J. Nuttall, Jr., of Wilson, NuttaU, Raimond, Engineers, Inc. The authors also wish to thank Messrs. R. B. Schwartz, I. O. Kamm and R. Madden of Davidson Laboratory for their help during the experiments and Mr. R. Krukowski of DL who designed and built the special instrumentation.
BIBLIOGRAPHY [1] K. TERZAGmand R. B. PECK. Soil Mechanics in Engineering Practice. John Wiley, New York (1948). [2] K. TERZAOHI. Theoretical Soil Mechanics. John Wiley, New York (1943). [3] Letter, E. G. JOHNSON,Haley and Aldrich to C. E. RODEr4,Bell Telephone Laboratories, April 1, 1964.