EFFECT OF RATE OF LOADING ON THE STRENGTH OF CLAYS AND SHALES AT CONSTANT WATER CONTENT

Professor A. CASACRANDE and S. D. WILSON

SYNOPSIS Investigations performed at Harvard Univer- Des recherches effectuees B 1UniversiM de Har-

sity during the past three years show that some vard, au tours de ces trois dernieres arm&es, mon- types of brittle undisturbed clays and clay shales trent que quelques types dar es creep under a sustained load, and that they ulti- !?

et de schistes argileux friables non remues c eminent sous une

mately fail under a sustained load appreciably less charge sontenue, et quils finissent par seffondrer

than the strength indicated by a normal laboratory sous une charge soutenue bien inferieure a la r&&t-

compression test. The shear strengths of six such ante indiquee par un essai normal de compression

soils were found to be reduced to 40-30 per cent. en laboratoire. On a trouve pour six de ces sols

of their normal values in thirty days. This may que leur resistance au cisaillement se trouvait 16 duite de 40 B 80 pour cent de sa valeur normale en

explain some slides which have developed on slopes 30 jours. Ceci peut expliquer quelques glissements that stood for many years without noticeable move- qui se sont prod&s sur des pentes qui &Gent ment. demeur&s pendant de nombreuses arm&es sans

In contrast, it was found that two laboratory *uvement appreciab1e. compacted soils, and one undisturbed soil which Par centre, il a et6 trouve que deux sols com- was not fully saturated, tended to become stronger presses en laboratoire, et un sol non rem& qui

and stiffer under sustained loads, even though n&ait pas completemerit satme, avaient tendance

water content was kept constant. These results B devenir plus r&&ta&s et plus fermes sous des

may prove of value in connexion with the design chargas soutenues, quoique la teneur en eau ait

of embankments. et6 maintenue constante. Ces r&hats peuvent avoir une certalne valeur pour letude des remblais.

INTRODUCTION

Investigations of the effect of rate of loading on the compressive strength of clays and shales at constant water content have been in progress at Harvard University since 1947. The first two years of these investigations were devoted to a study of the stress-deformation and strength characteristics of soils and soft rocks under very rapid loading and unloading (Casagrande and Shannon, 1949 and 1949).

Since the autumn of 1949 the effect of slow rates of load application has been studied under a cooperative research contract between the Waterways Experiment Station and Harvard University (Casagrande and Wilson, 1949 and 1959). This investigation has shown that some types of brittle undisturbed clays and clay shales creep under a sustained load, and that they ultimately fail under a sustained load appreciably less than the strength indicated by a normal laboratory compression test. In contrast, it was found that two laboratory-compacted soils, and one undisturbed soil which was not fully saturated, tended to become stronger and stiffer under sustained loads ; that is, when loaded at very slow rates of load application, they failed under stresses considerably higher than would be indicated by normal laboratory tests.

This Paper describes apparatus and test procedures developed to perform such long-time tests at constant water content, and summarises the results for nine different soils.

DESCRIFTION OF TESTS

Two types of tests were used to study the effect of long-time loading on the compressive strength of soils : (1) creepstrength tests, and (Z), long-time compression tests.

A creepstrength test is one in which a load is built up quickly and maintained constant until the specimen fails. For such tests, time to failure refers to the elapsed time between application of the load and failure. The following nomenclature is used to describe these tests :

251

252 A. CASAGRANDE AND S. D. WILSON

U denotes unconfined creep-strength test. 51 8) quick triaxial creep-strength test in which the specimen is first subjected to

hydrostatic pressure and then loaded. No drainage is permitted during the entire test.

QC *I consolidated-quick triaxial creep-strength test, in which the specimen is first consolidated under a given hydrostatic pressure and then, without permitting further consolidation, is subjected to the sustained load.

A longtime compression test is one in which the specimen is subjected to incremental axial loading, the elapsed time between increments of load varying for different tests. For such tests, time of loading refers to the time which elapses between the application of the first load increment and failure. All long-time tests reported herein are of the unconfined type, and are classified according to time of loading,by using subscripts, as shown in Table 1.

Table i

Symbol

-

L_

-

Type of unconfined compression test - Fast transient unconfined compression test Transient unconfined compression test Fast unconiined compression test Normal uncordined compression test Slow unconfined compression test Very slow unconfined compression test

Time of loading

Less than 1 second 1 to 30 seconds 0.5 to 3 minutes 3 to 15 minutes 15 minutes to 8 hours Longer than 8 hours

DESCRIPTION OF APPARATUS AND TEST PROCEDURES

It was necessary to develop simple and inexpensive equipment in order to perform many tests, some of which lasted from 6 months to more than a year. The apparatus shown in Fig. 1 was found blest suited for both U and U, tests. It utilises ball bushings to guide the piston, thus preventing tilting and eccentric loading. The ball bushings have the advantage over other types of bushings in that piston friction is reduced to a tolerable amount even when lateral loads are applied to the piston.

For Q and I& tests, conventional triaxial equipment was generally used, although this was later modified by the substitution of ball bushings for plain bushings.

For all tests reported herein it was of vital importance that the specimens were maintained at unchanged water content, since even a small loss of water would cause a substantial increase in strength. For unconfined compression tests, this was accomplished by encasing the speci- mens in two thin membranes which were separated by a tbin coating of silicone grease. In addition, the membrane covered specimens were totally immersed in water for the duration of the tests. All specimens were weighed both before and after testing and it was found that the change in water content, if any, seldom exceeded O-5 per cent. even if the tests lasted several months.

Most undisturbed specimens were cut cylindrical in shape, l-4 inch in diameter and 3.5 inches long, by sti table trimming devices.

The compacted specimens were prepared in the Harvard compaction apparatus (Wilson, 1950). This apparatus uses a mould l& inch in diameter and 2616 inches long and permits precise control of both moulding water content and unit weight. For this investiga- tion, the specimens were compacted at optimum moisture content by a standard procedure. After compaction, the specimens were extruded from the mould with as little disturbance as possible, and then tested in the same manner as the undisturbed specimens.

All loads were applied directly by means of a dead weight on a hangar which was supported by the piston. In unconfined tests of the U, U, and U,, types, the load was applied in small increments at regular time intervals. The increments of load were chosen such that each.was

EFFECT OF RATE OF LOADING ON STRENGTH OF CLAYS AND SHALES 253 ng.1

komron A-4812

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SCALE INCHES

Unconihed compremionapparatu~~withbailbumbiq guidaforpiston

about 10 to 15 per cent. of the expected failure load, and in most tests the size of increment was kept constant.

In creep-strength tests the load was applied in from four to eight increments at short intervals of time, 1 minute being the most common interval. The size of the increment and the time interval were the same for all tests in a particular series. The load was then maintained constant until failure occurred.

DESCRIPTION OF SOILS TESTED

The following soils were investigated : Cambridge clay.-A grey-green, medium plastic, inorganic clay from North Cambridge,

Massachusetts ; it is medium stiff and brittle in the undisturbed state, and soft and sticky when remoulded. The sample tested had a liquid limit of about 42, a plastic limit of 21, and a natural water content of about 37 per cent.

254 A. CASAGRANDE AND S. D. WILSON

Ohio River sandy c&y.-This is a fairly homogeneous light brown sandy,clay of low plasticity from near Cincinnati, Ohio. It is medium stiff and somewhat brittle in the undisturbed state, and soft when remoulded. The sample tested had a liquid limit of about 32, a plastic limit of 21, and a natural water content of about 27 per cent. The degree of saturation is about 95 per cent.

O&r be&&&--This is a fairly hard and very brittle bentonite from a bed exposed during the driving of an exploratory tunnel in the Pierre shale for the Oahe dam in South Dakota. It is mottled, grey-green in colour and .has a soapy feel. The samples tended to part rather easily along inclined planes which appeared to be existing joints. These planes of weakness were avoided whenever possible in cutting the specimens.

Mexico City day.-An extremely compressible, bentonitic clay of volcanic origin, light grey-brown in colour, and having a natural water content of approximately 490 per cent. It is medium soft, yet brittle, in the nndisturbed state, and extremely soft and sticky when thoroughly remoulded. It has a liquid limit of the same order as the natural water content.

C#carrrcka c&ay-&&.-A mottled grey-green and dark grey, soapy, slickensided clay-shale from the Panama Canal Zone ; this contains slickensided surfaces of varying dimensions, some apparently open and uncemented, other closed and apparently recemented. The material in different cores, and occasionally in the same core, varied in colour, hardness and jointing.

Bearpaw day-shak.-This is a dark grey, slightly organic clay-shale, from the South Saskatchewan River Project, Canada, very stiff and brittle with numerous slickensides in the undisturbed state, of sa consistency when remoulded at natural water content. It is highly plastic and has considerable toughness at the plastic limit.

Mi.wissi&%gnm& .-A highly plastic alluvial clay with a liquid limit of about 94, a plastic limit of 31, and a natural water content ranging from about 48 to 56 per cent. It is firm and somewhat brittle in the undisturbed state and shows little loss of strength when remoulded.

C&yey sand.This is a subgrade material consisting of a field mixture of a fairly uniform coarse sand with a small amount of a highly plastic clay, from Clinton, MissiGppi. The liquid limit is about 18 and the plastic limit 16. This material was thoroughly investigated in the field and the results were reported by the Waterways Experiment Station (1949).

Silty clay.-A lean silty clay from Vicksburg, Mississippi, with a liquid limit of about 37 and a plastic limit of 23. It was also investigated by the Waterways Experiment Station (1949).

ANALYSIS OF DATA

INSTANTANEOUS MODULUS OF DEFORMATION

The deformatron which accompanied each increment of load was obtained by means of exteusometer readings at speci&d time intervals following the application of load. The following sequence was generally used : 2, 5, 10, 15, and 39 seconds ; I, 2,4,8, 15, and 39 minutes, and so forth. These extensometer readings were then plotted against elapsed time. Detailed study and analysis of these time curves showed that the application of each load increment was accompanied by a sudden deformation, indicated by a jump in the extenso- meter readings, followed by creep deformation which decreased rather quickly with time. The magnitude of the sudden deformation was estimated by extrapolating the time curves back on a smooth curve to zero time. For any one specimen this sudden deformation was found to be proportional to the magnitude of the load. The ratio of the total stress to the summation of the instantaneous strains is referred to as the instantaneous moduhrs of deformation (Ms) (Fig. 2).

Apparatus was later developed to determine the mod&s of elasticity of soils by the dynamic method, &ii forced vibrations to produce resonance. Comparative tests showed excellent agreement between the instantaneous modulus of deformation as defined above and the modulus of elasticity computed from the vibration procedure.

EFFECT OF RATE OF LOADING ON STRENGTH OF CLAYS AND SHALES 255

Fig.2

STRESS: YGPLI) saCY 0. 0s ob 0,

EQQRCT OF SLOW RATES OF LOADING ON THE STRESS-STRAIN CURVE

It Was found that the stress-strain curves of materials which exhibited loss of strength in long-time tests showed a decreasing secant modulus of deformation with decreasing rate of loading. Fig 2 compares the stress-strain curves of three specimens of Mexico City clay with different rates of loading.

The stress-strain curve for Specimen C has been separated into elements of creep and sudden strain. The dashed line shows the cumulative sum of the sudden strains, for which Mi has been computed to be 53 kilograms per square centimetre. For this same specimen the secant modulus of deformation at 50 per cent of compressive strength was computed to be 22 kilograms per square centimetre. The decrease in compressive strength and increase in strain at failure with slow rates of loading, as shown by this plot, was found to be typical for ah tests on Mexico City clay, as well as for other clays and the clay-shales tested.

Fig. 3 shows a typical time curve for each of the three tests shown in Fig. 2. The dashed portions of these curves show how they were extrapolated back to zero time. For normal rates of loading (Specimen A) the time curves were sharply bent near the origin, but for long- time tests (Specimen C) the time curves were quite flat.

The stress-strain curves of materials that gained in strength in long-time tests showed an mcrease in secant modulus of deformation. Typical of this behaviour are the results of long- time tests on the clayey sand, for which four stress-strain ewes are plotted in Fig. 4. No information has yet been obtained on the influence of the rate of loading on Mi.

256 A. CASAGRANDE AND S. D. WILSON

w-3

ELAPSEDTIML:SECONDS

Typicaltime/deformationcurveefor Mexico City clay

STRESS: KG PER saxht.

Typical mtramm-mtrahcurvemfor U, Us, and U,,t@~ti on compact& clayey sand

EFFECT OF RATE OF LOADING ON STRENGTH OF CLAYS AND SHALES 2.57 CREEP-STRENGTH TESTS

In creep-strength tests failure was invariably preceded by a reversal of slope of the time- deformation curve, followed by continuous deformation at an increasing rate. Through the rubber membranes around the specimens it was often possible to see the shear crack develop shortly after the reversal of slope in the time curve took place. For these reasons, time to failure in creep-strength tests has been defined as the elapsed time between the application of the final load increment and this reversal in slope. Typical time curves illustrating this phenomenon are shown in Figs 5, and the corresponding stress-strain curves in Fig. 6.

Fig. 7 is a typical plot of compressive strength against time to failure for a series of U tests on Mexico Clay. Within the range of time to failure of from 1 minute to 36 days, the best relationship was found to be a straight line on semilog plot. Deviations of individual tests may possibly be explained by non-homogeneity of the sample from which the specimens

Dial readiq/elapmed time : $jc tests on Cucaracha clay-ohab

258 A. CASAGRANDE AND S. D. WILSON

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were cut, as it was found that there were some differences in deformation characteristics between specimens even when test procedures were identical.

It is emphasised that, even though the relationship shown in Fig. 7 is best represented by a straight line, straight-line extrapolation for duration of sustained loads longer than 1 month is not justified, as this line probably curves and approaches a horizontal asymptote. From the results of later creep-strength tests in which failme did not occur, and in which all creep ceased, it is believed that the lower limit of compressive strength for this sample of Mexico City clay is of the order of O-45 to 055 kilogram per square centimetre.

LONG-TIME UNCONFINED COMPRESSION TESTS

It was found that for some compacted materials creep would cease in a few days or weeks, even for sustained loads only slightly lower than the normal compressive strengths. The compressive strengths of these materials were found to have been increased as a result of the sustained loads. For these materials it was necessary to run a series of long-time tests. Fig. 8 is a plot of compressive strength against time of loading for a series of long-time tests on specimens of the silty clay compacted at optimum moisture content.

EFFECT OF RATE OF LOADING ON STRENGTH OF CLAYS AND SHALES 259

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c.

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compml#aive mtaagthpms to tailura : v kmtm on Mexico city clay

SUMMARY OF RESULTS ON NINE SOILS

STRENGTH RATIO

The effect of long-time and of sustained loading on the compressive strengths of the nine soils tested is summarized in Figs 9 and 10. Fig. 9 is a plot of strength ratio against dura- tion of sustained load for six undisturbed soils in creepstrength tests, and Fig. 10 is a plot of strength ratio against time of loading in long-time unconfined compression tests on four soils. Strength ratio is defined as the ratio of the compressive strength obtained in an actual test to the estimated compressive strength corresponding to a normal rate of loading. In creepstrength tests a time to failure of 1 minute has been used, and for long-time tests a time of loading of 10 minutes. These times, although arbitrary, correspond approximately to normal laboratory test procedures.

CREEP-STRENGTH TESTS

For creepstrength tests at constant water content, the shear strength of Mexico City clay was reduced to about 90 per cent of its normal value in about 30 days, while that of the Cucaracha clay-shale was reduced to approximately 40 per cent. The shear strength of four other soils was reduced in between these values, as shown in Fig. 9.

With the possible exception of the Cucaracha clay-shale, for which data were not obtained, these six soils are considered to be fully saturated and therefore it is assumed that there was no change in void ratio during these tests.

With the exception of the Mississippi gumbo (for which the data were somewhat erratic and the results inconclusive for tests lasting more than one day), these soils were brittle and gave well-defined shear fractures.

LONG-TIME UNCONFINED TESTS

For long-time unconfined tests, also at constant water content, one undisturbed sandy clay and two compacted soils showed a substantial increase of strength with times of loading

260 A. CASAGRANDE AND S. D. WILSON

Compressive strength/time of loading : long-time unconfined compression tests on compacted silty clay

Fig. 9

Strength ratio/elapsed time to failure : creep-strength tests at constant water content on six undisturbed soils

EFFECT OF RATE OF LOADING ON STRENGTH OF CLAYS AND SHALES 261

Fig. 10

TlUE OF LOADING: MlNlrTES

Streqth ratio/time of loading for long-time unconfined compression tests at constant water content on four soils

greater than about 1 day, and a small loss of strength for time of loading greater than 10 minutes but less than 1 day. These three soils were not fully saturated and it is probable that the increase in strength was accompanied by a decrease in void ratio at constant water content. This means that internal consolidation has occurred, the gas in the voids being compressed and absorbed by the water. No attempts were made in this investigation to measure this change in void ratio.

Possibly the strength of the two compacted soils was increased by thixotropic action. The effect of thixotropy was not investigated.

CREEP EFFECTS

When analysing stress-deformation characteristics for the various materials tested, it appeared that materials which were the most susceptible to loss of strength in creep-strength tests also showed a relatively low ratio of secant modulus of deformation to instantaneous modulus. The trend of this relationship is shown in Fig. 11 for four of the soils tested. In Fig. 11 the abscissae represent the slope of the lines shown in Fig. 9, that is, the decrease in strength ratio per cycle of time, and the ordinates represent the ratio of secant modulus of deformation (at 30 per cent. of normal compressive strength) to instantaneous modulus. Individual tests have not been plotted in Fig. 11 and the ranges shown are only approximate. It is interesting that, of the four materials, the Cucuracha clay-shale was the strongest, yet showed the greatest relative amount of creep. In contrast, the Mexico City clay had the lowest compressive strength, yet showed the least amount of relative creep.

CORRELATION OF CREEP-STRENGTH TESTS WITH TRANSIENT TRSTS

It was possible to correlate the results of creep-strength tests on Cucaracha clay-shale with results of transient tests as determined in the Panama Research Project. To do this, it was

R

262 A. CASAGRANDE AND S. D. WILSON

Fig. 11

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Fig. 12

TIME 10 FAILURE : UmTES -. lo lo- I IO IO Id

Effect of rate of loading on the compramsive mtrmgth of Cucaracha CliIy-mhalO

EFFECT OF RATE OF LOADING ON STRENGTH OF CLAYS AND SHALES 263 necessary to correlate the time of loading as defined in the Panama Report with the elapsed time to failure as defined herein. The results are summarized in Fig. 12, which covers the entire range of time from O*OOl second to 1 year. Within this range the strength ratio varies from about 2-O for the fastest time of loading to about 02.5 for the slowest.

CONCLUSIONS

The following tentative conclusions can be drawn from the tests performed during this investigation :

(1) Sustained loads at constant water content reduce the strength of fully saturated, brittle clays and clay-shales. This may explain, at least in part, slides which develop on slopes that have been standing for many years without noticeable movement, such as, for example, the slides in the Cucaracha formation along the Panama Canal (Casagrande, 1949).

(2) The strengths of compacted soils and of undisturbed soils which are not fully saturated increase with time, even when the water content is kept constant. From the standpoint of earth dam design these results have important practical implica- tions. It would seem that most compacted soils of which earth dams are made increase in strength with time, even without additional consolidation.

It is realized that the results obtained will have to be verified by additional investigations using triaxial compression tests, rather than unconfined tests. There is also a possibility that even at unchanged water content there is a redistribution of water content within a sample which may change its strength characteristics in a manner not necessarily representative for the effects which take place within a large mass. The Authors realize these and other limita- tions of the present work which are all part of the great complexity of strength properties of soils, of which probably only little is really understood.

ACKNOWLEDGEMENT

The investigations described herein were performed under a cooperative research contract with the Waterways Experiment Station, Department of the Army, Vicksburg, Mississippi. The tests were performed in the Soil Mechanics Laboratory at Harvard University by various members of the research staff. In particular, J. M. Corso, G. J. Kling and H. B. Seed, Re- search Associates, have contributed materially to the success of the investigations.

REFERENCES CASAGRANDE. A.,and SHANNON,W. L., 1948. Research on stress-deformation and strength characteristics

$~i; and soft rocks under transient loading. Pub. Haward Univ. Grad. Sch. Eug. Soil Mech. Series, . . 132 pp.

CASAGRANDE, A., and SHANNON, W. L., 1949. Strength of soils under dynamic loads. Trans Amer. SOC. Civ. Eng. 114 : 755-772.

CASAGRANDE, A., and WILSON. S. D., 1949. Final report to U.S. Waterways Experiment Station on investieation of effect of lona-time loading on the strength of clays and shales at constant water conten;. Harvard Univevsity_ 77 pp. -

CASAGRANDE, A., CORSO, J. M., and WILSON, S. D., 1959. Report to Waterways Experiment Station on the 1949-1959 program of investigation of effect of long-time loading on the strength of clays and shales at constant water content. Harvard University.

WILSON, S. D.. 1950. Small soil compaction apparatus duplicates field results closely. Engineering News- Record. 145: 18: 34-36.

CASAGRANDE, A., 1949. Discussion of the paper on excavation slopes by Binger and Thompson, symposium on the Panama Canal-the sea level project. Trams. Amer. Sot. Civ. Eng. 114 : 870-874.

U.S. War Department, 1949. Soil compaction investigation. ?ortno l*

Compaction studies on clayey sands, Tech memo. U.S. Watem. Expt. Sk No. 271-3. 4 pp.

U.S. War Department 1949. Soil compaction investigation, report no. 2. Compaction studies on silty clay, U.S. Waterw. Expt. Sta.