Strain-time-strength relationships in a marine clay

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Strain-time-strength relationships in a marine clay

Coates, D. F.; Burn, K. N.; McRostie, G. C.

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Ser T r n N21r 2 no. 206 c. 2 BLDG

NATIONAL RESEARCH COUNC I L CANADA

D I V I S I O N O F B U I L D I N G RESEARCH

S T R A I N .- T I M E

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STRENGTH R E L A T I O N S H L P S I N A MARINE CLAY

BY

D . F . COATES, K.N. BURN AND G.C. M c R O S T I E

$f RESEARCH P A P E R NO, 206 O F THE i ! J A N

:

,<:T i - d r , . i l 1.c I' i - ' -

I

D I V I S ION O F B U I L D I N G RESEARCH

LL-

,'

R E P R I N T FROM TRANSACT I O N S O F T H E E N G I N E E R I N G I N S T I T U T E O F CANADA VOL.

6,

NO. A - 1 1 , OCTOBER 1963, P A P E R NO. EIC-63-GEOTECH 11

P R I C E 5 0 C E N T S OTTAWA

DECEMBER 1963

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strain

& L

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ratio of A U ~ to deviator stress at failure

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INTRODUCTION

Although the variation of shear strength of cohesive soils with the duration of stress has been examined by several investigators, allowance for this effect is not wide-spread in foundation engineering design. This situation exists owing to the lack of suffici2nt information on the various causes of change in shear strength with time and of established techniques for determining the basic properties. For example, an apparent change in shear strength can occur with a change in pore pressure. Considerable research has been done on this subject in saturated soils, which may be used in engineering analysis and design. However, the experimental techniques for measuring pore pressures onplanes of failure has not yet been satisfactorily established, which raises some doubts on the conclusions based on pore

pressure measurements at the ends of samples. Where this mechanism applies the passage of time in the field may lead to greater or lesser stability depending on the initial conditions.

Alternatively, a change in shear strength in some soils may result from a viscous reaction to stress. In other words, the soil at some stress levels or some magnitudes of shear stress behaves like a viscous medium which, given sufficient time, may produce excessive strains or even failure. Where such a mechanism operates the passage of time cannot be expected to lead to increased stability.

It is possible that a maximum strain law governs the strength of some soils. In other words, a critical strain might exist, particularly for sensitive clays or loose sands, beyond which the structure would break down and the

inherent strength would be lost. Clearly with such a material Mohr's strength theory or a maximum shear stress law would not likely predict the conditions that would necessarily produce failure. This would be particularly true where strain varied with time under constant stress conditions. For this reason, a maximum strain law might be found to apply to materials with strong rheological or viscous properties.

A number of problems have recently been encountered by the authors where the variation of strain and stability of clays with the passage of time under

constant stress conditions was important. One such job was of a sufficient magnitude to support a detailed laboratory testing program. The main purpose

of this testing program was to determine if the strength of the particular clay deposit would decrease with time while being subjected to a constant shear stress. The strains were also examined to determine if a critical strain might exist for this clay.

This paper presents the test results of that investigation. A decrease in

shear strength with the passage of time was expected in this clay. Consequently, one series of triaxial tests was used to determine if the strength varied with the strain rate of the applied load. A second series of triaxial tests was conducted at constant stress levels to determine if the time to failure was equal or less than that obtained from the controlled strain-rate tests. A third series of triaxial tests was used to determine if there was a value of shear strength below which time would have no effect. Pore pressure measurements were taken in all tests to determine if the reactions were being caused by changes in pore pressures or by a viscous reaction.

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PREVIOUS WORK

The importance of time effects on the strength of clay has received increasing recognition over the past few years. The indications have been that reductions in strength and increase in strain with an increase in time of stress application may be substantial.

House1 (1939) introduced the concept of "yield stressw and showed it to be for some clays about 20% of the unconfined compression strength. Then

Skempton and Bishop (1954) based on the work by Taylor (1943) and Casagrande and Wilson (1951) suggested that there is a stress, for any given clay, below which failure will not occur no matter how long the load is maintained. Vialov and Skibitsky (1957) showed that for their clays there is an '(ultimate continuous shearing resistance" between 0.7 and 0.9 of the strength

obtained in standard tests below which failure will never occur.

Casagrande and Shannon (1948) and Cadling and Odenstad (1950) found that the strength of some clays increased as the time of loading was decreased. Also, ~ a s a ~ r a n d e and ~ i l s o n (1951) reported decreases in undFained shear strength from 20% to 90% after a load application of one month.

Henkel and Skempton (1955) and Henkel (1956) suggested that the decrease in strength of some clays that might occur in geological time could be due to the dissipation of negative pore pressures which occur on shearing in over- consolidated clays.

Goldstein and Ter-Stepanian (1957) found reductions in the undrained strength of their clays between 30% and 70% in tests lasting from 1 to 3 months.

Goldstein and Misumsky (1958) commented that in some tests soft clay relaxed to zero strength. Bjerrum et a1 (1958) found decreases in strength of

60%

for tests of one month duration. However, Bjerrum (1958) mentioned that the bottom of a shaft with a computed safety factor of 1 did not fail with time. Crawford (1959) also found that the undrained strength of one clay decreased with increasing time to failure. The lowering of the effective stress Mohr envelope as the testing time increased for specimens under confining pressure below the preconsolidation pressure was an important observation.

DESCRIPTION OF CLAY

The soil examined in this work is part of a deposit of marine clay existing extensively in the Lowlands of the Ottawa and St. Lawrence Rivers in Eastern Canada. Distinguishing features of these marine clays are their high sensitivity, high liquidity index, and tendency toward decreasing rather than increasing shear strength with depth. Various local names such as Leda Clay, Champlain Clay or Laurentian Clay have all been applied to this material with

little distinction.

The soil profile has two major layers, aside from the fcrust' where current or past weathering

has

altered the original soil properties. The first layer,

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occurring at depths from 30 to 45 feet at the site near Ottawa, and the second layer occuring at depths from 45 to 75 feet, are the materials with which the paper is mainly concerned. The ground water level currently seems to vary between

6

feet and about 20 feet below the ground surface.

Typical values of physical properties for the material between 30 and 45 feet in depth, described later as the upper layer, are as follows: moisture content 70%to50% decreasing with depth, liquid limit

60%

to 50% decreasing with depth, plasticity index 30%, liquidity index 1.3, activity coefficient 0.4, average undrained shear strength 1 tsf, sensitivity 30 to 150 and the average ratio of preconsolidation to effective overburden pressure 3.9.

Typical values of these properties for the material between 45 and 75 feet in depth, described later as the lower layer, are as follows: moisture content 45% to 55% increasing with depth, liquid limit 30%, plasticity index lo%,

liquidity index 3, activity coefficient 0.2, average undrained shear strength 0.75 tsf, sensitivity 40

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200 and the average ratio of preconsolidation to effective overburden pressure 2.4.

TESTING PROGRAM

The most suitable test for the purpose of this study was considered to be the consolidated

-

undrained triaxial compression test with pore pressure measurements. The samples were obtained with a 2" piston-sampler from several boreholes at one location. It was recognized that the clay was somewhat

layered (but not varved) and that the correlation of tests on the different samples would be difficult. However, rather than tes'ing only one stratum it was considered important to examine the full depth of the deposit. The consolidation pressure for the triaxial tests was selected to minimize the additional consolidation that would be produced on the samples. For this reason a value on the low side of the overburden pressure was used to account for a lateral earth pressure coefficient at rest being less than 1. At the same time, it was considered more important to use one constant value of consolidation pressure for all tests, so that this would not be a variable between tests, rather than using some constant ratio of the overburden pressure. For these reasons, a consolidation pressure of 1 tsf was selected.

After the consolidation pressure had been applied and equilibrium pore pressure achieved, a back pressure of 1 tsf was applied to the sample with a correspond-

ing increase in the confining pressure of 1 tsf. This procedure was followed to eliminate any air in the system and to obtain a quick response in the pore pressure readings when the shear stress was applied.

Vertical strips of filter paper were placed on the sides of the sample. Pore water pressures were measured through a porous base-plate without permitting changes in volume. The samples were all saturated.

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Three series of consolidated-undrained triaxial tests were used. The first series was conducted at controlled rates of strain (Qt) of 202, 2% and 0.2% strain per hour. These tests gave times to failure for most of the samples between six minutes and six hours. This type of test is of value in that stress at failure and time to failure can be obtained for each sample (which is not always the case in creep tests i.e. Qp), and the clay reaction or strength after failure can be measured.

The second series of triaxial tests was performed at constant stress (Qp) with time to failure being measured. These were essentially creep tests and have the virtue of being somewhat analogous to field conditions of static

loading. The drainage conditions, of course, are not necessarily similar. The third series of triaxial tests was run using uniform increments of stress applied for uniform increments of time (Qy). The purpose of this series was to determine the "yield stress" after Housel's concept (see House1 1939). Whereas each of the first two series of tests should be made for comparative purposes on identical samples to eliminate any important effects caused by variation in material, this third series provided tests which were complete within themselves.

Initially in the Qy-tests increments of deviator stress were set at abcut

5% of the ultimate deviator stress with time increments of 5 minutes. When it was seen that the "yield stressl1 would be higher than expected these increments were changed to 10% and 10 minutes. Analysis of the viscous action indicated that these increments should be adequate to obtain strain increments greater than that corresponding to the relaxation time considering the time-strain to be occurring through a Kelvin-type mechanism (see figure 3). After "yieldlf had occurred the sample was failed at a constant strain rate of 20% per hour.

As a separate control for determining comparative ultimate strengths a

sample from each tube was also tested in the normal manner with a consolidated quick test, Qc (now called by some

R ) ,

at a controlled strain rate of 20% per hour.

V RESULTS

In figure 1 classification and strength data are presented for the clay profile. It can be seen that the consolidated-undrained triaxial tests (Qc) produced strengths for the clay strata approximately equal to the results obtained from field vane tests. The results from quick triaxial, Q, and unconfined compression, Qu (now called by some U) tests were found to give

lower strengths.

As the consolidation of the Qc samples produced little measurable change in moisture content (by measuring volume of water expelled and change of weight

of the samp1e)the differences between these and the Q-test results cannot be attributed to changes in moisture content. It is possible, however, that

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during the sonsolidation period for the Qc-tests weaker material adjacent to internal fissures or joints, that had been softened by the release of the confining pressure in the ground, were reconsolidated. In any event, the ultimate strength of the clay for a normal test time would seem to be best represented by the Qc or vane test results.

In figure 2 some of the results of the tests run at constant strain rates (Qt) are shown. These curves show that the decrease in deviator strength as the time to failure increased from 0.1 hours to 1.0 hours is between

6x

and 13%. As the time to failure increased from 0.1 hour to 10 hours the strength decrease varied from 122 to 27%. An attempt was made to decrease the strain rate still further to determine whether these curves continued to slope downwkrds or flattened out to some horizontal asymptote, but equipment difficulties were encountered.

The lack of variation in pore pressure, or Af values (see Table

I),

with time to failure suggests that the strength decrease was not due to pore pressure effects.

The curves in figure 2 also contain results of the constant stress test (Qp). The results of these tests did not differ significantly or consistently with the results of the constant strain rate tests (Qt). The difference between these two tests should assist in appraising whether the clay obeys a maximum strain failure law, in other words, a viscous material might be strained more under a constant ultimate stress than under a stress that is gradually increased to that ultimate stress. However, insufficient comparative results were

obtained for this purpose.

In figure 3 a typical strain-time curve for the constant stress (Qp) tests is shown. There is some question whether the time to failure should be assumed to correspond to the vertical portion of the end of the curve or at the point of inflection in the curve. It is reasonable to assume that at the point of inflection something occurs in the material which leads to the increasing rate of strain with time. Up until this point the strain rate is decreasing. It is possible that the mechanism involved is one of a critical strain beyond which the natural structure on which the strength depends breaks down. It can be seen that the reduction in strength with time is not due to an increase in pore pressure

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clearly the pore pressure, at least at the bottom of the sample, was decreasing.

If the initial part of the curve in figure 3 up to the point of inflection is rep esented by a Kelvin body the coefficient of viscosity is 150,000 lb-

₅

min/in or 62 x lolo poise.

Also included in figure 3 are the graphical representation of some of the simple rheological models that might be used to analyse the stress-strain-time relations. No elaboration of these general models is presented here as this information can be found in many books.

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In figure

4

typical results of the yield tests (Qy) are presented. The curves of time strain increments ( A L ~ ) (this is not cumulative strain) plotted against deviator stress generally showed a pattern similar to the results froin House1 's double ring shear test. On the other hand, some tests gave points through which a straight line could not be drawn and

consequently might be cases of the step-strain reaction postulated by Trollope and Chan (1960). The pore pressure after 'yield' was constant in all tests. Thus the time and stress required to fail the sample after yield occurred must have been due to a viscous reaction rather than due to pore pressure

effects.

Also in figure

4

a typical curve of deviator stress versus cumulative instantaneous strain ( ci), i.e. excluding all increments of strain due to time, is shown. These curves plotted as straight lines for stress levels varying from about

_-

60%

to 100% of the ultimate stress. This has also been found on other clays (Wilson and Dietrich 1960). This suggests that some type of rheological model, such as a Maxwell or General Linear body, see _--

figure 3, might represent the clay's variation in strain with time under stress. In other words, there is a spring element that gives initial strain and then the viscous element provides additional strain with the passage of time. The straight line portions of these curves represent deformation modulii varying from 3,400 psi to 9,300 psi.

In figure 5 the results of the yield tests (Qy) as they vary with depth are presented. By comparing the "yield stressu (2 f y ) with the ultimate strengths obtained in both the yield tests (Qy) and consolidated quick tests (QC) it may be concluded that the clay "yieldsu at between 76% and

88%

of the ultimate strength.

Figure 5 also shows failure strains at the "yield stressw (2Yy) and at the point of inflection in thestrain-time curve of the creep tests (Qp). The variations of failure strain corresponding to the ultimate strength in the yield tests (Qy) and the consolidated quick tests (Qc) were studied and found to vary widely. These ultimate strains have little significance, as the method of selection of these values is not conducive to the production of accurate results (being the abscissae corresponding to the maxima on rather

flat curves).

It is considered that the strain at which "yielding" occurs is of more

significance than that at which maximum deviator stress or maximum effective

principal stress ratio occurs. Figure 5 shows that these failure or yield strains varied between0.65% and 1.8%. Using the same concept as in the yield tests of examining incremental strain effects, a review of the tests conducted at

constant rates of strain (Qt) showed critical strains varying between 0.7% and 1.0% with no obvious relation to the rate of strain. This is fair corroborat- ion of the results of the yield and creep test results. Hence there may be a critical strain beyond which the natural structure starts to break down and the strength is reduced. In other words, the clay may obey a maximum strain law rather than Mohr's frictional strength law.

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With low failure strains and high sensitivities one might expect the values of Hf to be high. Actually the tests showed Af to be consistently low.

(see Tables I, LI and 111). However, these values are close to those found by Bjerrum and Simons, 1960 for similar ratios of preconsolidation to overburden pressure. However, the Af values in Tables I to I11 did not increase with time to failure as was found by others (Bjerrum et a1 1958).

It has been observed that failure strains are greater if they are determined at the maximum ratio of effective principal stresses (Bjerrum and Simons, 1960) than at maximum deviator stress. This has occurred as a result of the pore pressure continuing to increase after themaximum deviation stress has been reached. This would not seem to apply to the Ottawa clay (see figure 4)

as the pore pressures become essentially constant close to the "yield pointtt below the maximum deviator stress.

In figure

6

typical results of cycling the stress increment in Qy tests are shown. The strain curve shows that the instantaneous strain is of the same order of magnitude as the time-strain. The recovery of strain with release of the stress increment varied from about 30% to 80% of the instantaneous strain for the stress increment. The recovery of the time-strain was found to be very small. The instantaneous strain that occurred with the second application of the stress increment seemed to be equal to the recovery of strain from the first application. The increment of time-strain on the second application of the stress increment was of the order of 20% of the original time-strain. It is possible that if the increment of stress had been maintained long enough on the first application that there would have been no time-strain on the second application (Wilson and Dietrich 1960). The instantaneous recovery being less than the instantaneous strain indicates that besides elastic and viscous reactions to stress there is also a plastic reaction (i.e. an irreversible strain not dependent on time). These results show that for reversible effects no simple rheol.ogica1 model would be

representative.

The Kelvin coefficient of viscosity for the loading cycle in figure

6

is 13 x 10'~ poise. Considering the loading cycle as a General Linear body (i.e. see figure 3) the calculated viscosity is reduced to 3.7 x 10'~ poise for sample 145-5.

In figure 7 the results of an incremental stress test that was run with time increments sufficient to obtain constant strain at any given stress level are shown. This curve shows that at stress levels below the "yield stresstf failure will not occur. It also indicates that below the "yield stress1* a finite strain will be obtained, given sone minimum amount of time, but that this strain will not increase with periods of time greater than this minimum. By analysing the data shown on figure 7 the Kelvin coefficient of visco

was found to be essentially constant for all stress increments, i .e. 10S6tioise. It had been noted above that the modulus of instantaneous deformation remained constant until high deviator stresses were applied. This would encourage one

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t o b e l i e v e t h a t l i n e a r v i s c o u s e q u a t i o n s c o u l d b e u s e d t o r e p r e s e n t t h i s m a t e r i a l . However, t h e K e l v i n modulus of d e f o r m a t i o n d e c r e a s e d w i t h i n c r e a s e i n d e v i a t o r s t r e s s . Hence i n u s i n g a G e n e r a l L i n e a r body ( s e e f i g u r e 3 ) t o r e p r e s e n t t h e c l a y , t h e r e would b e two c o n s t a n t c o e f f i c i e n t s and one v a r i a b l e .

F i g u r e 7 a l s o shows q u i t e c l e a r l y t h e e f f e c t on s t r a i n of t h e v i s c o u s r e a c t i o n . It c a n b e s e e n t h a t f o r m a t e r i a l s w i t h s u c h a v i s c o u s component o f s t r a i n t h e s t r e s s - s t r a i n p l o t must b e c u r v e d . T h i s a r i s e s from t h e v i s c o u s s t r a i n v a r y i n g w i t h t h e l e v e l of s t r e s s a s w e l l a s w i t h t i m e . E x p r e s s e d mathem- a t i c a l l y , a c o n s t a n t s t r e s s - r a t e t e s t on a K e l v i n body would h a v e a s t r a i n - s t r e s s e q u a t i o n a s f o l l o w s : where s t r a i n

=

s t r e s s E

=

modulus o f d e f o r m a t i o n of t h e s p r i n g i n t h e K e l v i n body

=

c o e f f i c i e n t o f v i s c o s i t y i n t h e d a s h p o t i n t h e K e l v i n body = s t r e s s r a t e t

=

t i m e . C l e a r l y , t h i s would n o t p r o d u c e a s t r a i g h t l i n e i f " t l 1 w e r e g r e a t e r t h a n z e r o . I n f i g u r e 8 t h e normal d e v i a t o r s t r e s s - s t r a i n c u r v e f o r t h e s o i l d e s c r i b e d i n f i g u r e 7 i s i n c l u d e d f o r r e f e r e n c e p u r p o s e s . I t c a n b e s e e n t h a t t h e f a i l u r e s t r a i n was a b o u t 1%, which i s t h e same o r d e r o f m a g n i t u d e a s shown i n f i g u r e 5 . Thus i t would seem t h a t t h e p r o p e r t i e s o f t h e s a m p l e had n o t b e e n s i g n i f i c a n t l y a f f e c t e d by t h e s u s t a i n e d l o a d i n g i n c r e m e n t s . However, t h e s t r e s s r e a c t i o n a f t e r f a i l u r e , c o n s i d e r i n g t h e c l a y had a l i q u i d i t y i n d e x g r e a t e r t h a n 1 , i s s u r p i r i s i n g l y h i g h . It h a s a l w a y s b e e n assumed t h a t t h e s t r e n g t h a f t e r f a i l u r e i n t h i s c l a y would b e v e r y low. F u r t h e r m o r e , few t e s t s h a v e e v e r b e e n p e r - formed on t h i s c l a y , a s i d e from c o m p l e t e r e m o u l d i n g and t e s t i n g , t o d e t e r m i n e t h i s r e a c t i o n . The r e s u l t s o f t h i s t e s t , p a r t i c u l a r l y a s n o n e g a t i v e p o r e p r e s s u r e s d e v e l o p e d , i n d i c a t e s a n a r e a f o r f u r t h e r s t u d y . I n f i g u r e 9 t h e e f f e c t i s shown of n a t u r a l w a t e r c o n t e n t of t h e e n t i r e s a m p l e on t h e s h e a r s t r e n g t h of t h e u p p e r c l a y t e s t e d a t t h e c o n s t a n t r a t e of s t r a i n o f 20% p e r h o u r . A s i m i l a r p a t t e r n was o b t a i n e d f o r t h e o t h e r s t r a i n r a t e s u s e d . T h i s s o i l shows a s u b s t a n t i a l s t r e n g t h v a r i a t i o n w i t h m o i s t u r e c o n t e n t .

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CONCLUSIONS

From this work certain academic and certain practical results were observed. On the general subject of shear strength of soils it is difficult, in view of current developments, to state definite conclusions. The conclusions are listed as follows:

1. The loss in shear strength with time of up to 25% seems to be a real time effect acting either through some viscous mechanism or through some differential pore pressure effect within the sample not detected by end measurements.

It was seen that Af did not increase with time. In addition, the pore pressure at failure measured at the bottom of the sample did not increase with time and in fact decreased wlth time in the lower silty layer. If these pore pressure measurements were valid for the entire sample it would be expected that the reduction in strength with time would be due to a purely viscous mechanism and hence would occur on a normally consolidated clay and not as would otherwise be expected just on preconsolidated clay. Safety factors should be selected with these possibilities in mind. The incremental stress test (Qy) may be a practical way of determining this long-term strength of clays.

2. The presence of a plastic element of strain (irreversible strain not dependent on time) and the decreasing modulus of deformation with

increasing deviator stress makes the representation of this clay by simple rheological model difficult.

On the other hand, with the coefficient of viscosity remaining fairly constant with stress level with a constant instantaneous modulus of deformation and the plastic element of strain being only significant on release of stress, some encouragement is provided for additional studies using a model for load application only. Nevertheless, the decrease in the Kelvin modulus of deformation with increase in deviator stress will still make this difficult, particularly if the model is to be applied to a non-uniform stress field.

3. With the demonstrated viscous element of strain present it should be recognized that normal stress-strain plots of either constant strain- rate or constant stress-rate tests must be curved, i.e. if the stress- strain curve is a straight line at very high rates of strain then at lower rates the strain at any given level of stress would vary with both time and stress level and hence a non-linear stress strain relationship would be produced.

4. The controlled strain rate test (Qt) may or may not give information equivalent to the constant stress test (Qp). Theoretically for a given time to failure, assuming a constant failure strain, Qp should be less than Qt; however, study of the results corrected for moisture content suggested that they are about equal.

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5. Some of the test data indicated that the strain at failure might be

constant with respect to time for a given clay. However, to substantiate a maximum strain law it would still be necessary to determine if it was constant with respect to the minor principal stress, the intermediate principal stress (when not equal to the minor principal stress) and the degree of saturation.

ACKNOWLEDGEMENTS

This paper represents work performed by joint use of the facilities of Soil Mechanics Section, Division of Building Research, National Research Council and the facilities of the firstly and thirdly named authors. The work was carried out for the Corporation of the City of Ottawa and their consultants, De Leuw Cather C Company of Canada Limited.

REFERENCES

BJERRLJM, L., SIMONS, M. & TORBLAA, I., "The Effect of Time on Shear Strength of a Soft Marine Clayff, Proceedings Brussels Conference 58 on Earth Pressure Problems, Volume 1, 1958.

BJERRUM, L., "Construction of a subway through Soft Clay in Osloq1 a talk to Ottawa Soil Mechanics Group, August 31, 1959.

BJERRLJM, L. & SIMONS, M., "Comparison of Shear Strength Characteristics of Normally Conso?idated Claysf1 Norwegian Geotechnical Institute Publication No. 35, 1960.

CASAGRANDE, A. & SHANNON, W.L., flStress-deformation and Strength Characteristics of Soils under Dynamic LoadlI, Proceedings, 2nd International

Conference on Soil Mechanics and Foundation Engineering, Volume 5, 1948.

CASAGRANDE, A. & WILSON, S., l1Effect of Rate of Loading on the Strength of Clays and Shales at Constant Water Content", Geotechnique, Volume 2, June 1951.

CADLING, L. & ODENSTAD, S., "The Vane Borer. An Apparatus For Determining the Shear Strength of Clay Soils Directly in the Groundt1, Proceed-

ings No. 2, Royal Swedish Geotechnical Institute, 1950.

CRAWFORD, C., "The Influence of Rate of Strain on Effective Stresses in Sensitive Claytt, ASTM Special Technical Publication No. 254, 1959.

GOLDSTEIN, M. & TER-STEPANIAN, G. "The Long-term Strength of Clays and Deep Creep of Slopes", Proceedings, 4th International Conference on Soil Mechanics and Foundation Engineering, Volume 2, 1957.

(15)

GOLDSTEIN,

M.

& MISUMSKY, V., llDiscussionll, Proceedings, Brussels Conference 58 on Earth Pressure Problems, Volume 3, 1958.

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HENKEL, D., "The Effect of Over-Consolidation on the Behaviour of Clays Dur- ing Shear1! Geotechnique, Volume 6 , December 1956.

HOUSEL, W., "The Shearing Resistance of Soil

-

Its Measurement and Practical Significanceu, Proceedings, ASTM, Volume 39, 1939.

SKEMPTON, A. & BISHOP, A., Chapter X, ! l S ~ i l s ~ ~ in "Building

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Their Elasticity and Inelasticityll, Inter-science 1954.

TROLLOPE, D

.

& CHAN, C

.

,

llSoi 1 Structure and the Step-Strain Phenomenall, Proceedings ASCE, Volume 86, No. SM-2-2431 1960.

VIALOV, S.S. & SKIBITSKY, A.M., "Rheological Processes in Frozen Soils & Dense Claystf, Proceedings, 4th International Conference on Soil Mechanics and Foundation Engineering, Volume 1, Page 120, 1957.

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(16)

0 0 0

k.g$%

I

. . .

_{0 0 0 0 0} _{0 0 0} _{r l r i r l o}

W

. . . .

P - L ~ A O

soma

. . . .

rlnrlrl ~

. * . .

~ r l ~ r \ a n ~ n n

a d d *

c u ~ h n r l

Gl.;Gco'

d d d d , d

.

D d o b m a I I I I P- I

&

d a -P d 0 U' k e, -I=' cd 3 e, k 9 rln .AT?. ~ 3 -

c-.,

rl c-6 .A -I='- .rl>?. G W H adorn O L ~ P - d P- a, M cd d k Q - P

. . . .

4 IT PI

8

3

3 . . . .

*

P - n ~ n a r n 1 1 1 1 1 rr\ I

&

I

,+an,+ a L n • 0 d

*

n d a m 1 1 1 1

=i

&

COD ~ n o ~ n ~ n =f • M d rn:h~Ln 1 1 1 1 Ln I

&

(17)

(18)

Notes: 1.

E

y

=

$

s t r a i n

s t r e s s a t y i e l d .

AUf

0.51

0.71

0.72

0.31 0.1.I.O

0.46

0.51 0.4-3

0.4.0 0.4-2

0.77

0. j7

0.50 ( ~ - D ) Y

(TSF)

1.74

1.94

2.09

1.92

1 79

2.19

1.86

1.51

1.67

1.93

1.98

1.54

1.66

1.96

AUY

0.4.5

0.63

0.68

0.27

0.37

0.39

0.30

0.42

0.32

0.39

0.51

0.28 O.LL5

0 . 5 6 0 . 5 1

Ey

(76)

1 . 3

1.1 1-1

0.9

0.6

0.7

0.7 1 . C

3.9

1 . 2

0.8

0.7

0.5

0.5 SMi1PLE

No.

145-4

5

6

7

8

9

10

11

1 2

1 3

1 4

1 5

94-34

94-6-7

Average

Depth

F e e t

21.0

26.0

31.0

36.0

4.1.0 4-6.0

51.0

56.0

61.0

66.0

71.0

76.0

37.5

46.4 A

f

0.25

0.31

0.30

0.14

0.20

0.19 0.1%

0.25

0.19

0.18

0.16

0.19

0.24

0.24 Tf

h r s .

2.1

51.1 1 . 8

1 . 5

1 . 6

-

1.6

2.2

2.0

3.3 2 . 0

2.0

2.3

2.6 c f

(

1.9 1 . 6

1 . 8

1 . 3

0.7 1 . 2

1 . 2

3.0

2.4

2.2 1 . 6

2.4 1 . 0

0.9 '.later

I n i t i a l

(%)

G I . 0

64.5

60.0

61.2

59.7

54.

G

43.9

45.3

4.9

5

51.8

52.8

42.4

57.8

53.5 C o n t e n t

F i n a l

(

4;)

64.4

61.8

59.2 3 . 8

4;'

-1

6 4 . 9

4

51.2

8

41.6

57.6

53.4 Ay

0.25

0.32

0.33 0.14-

0.21

0.18

0.16

0.28

0.19

0.20

0.16

0.18

0.27 0 . 2 6 7 8 5

Ty

h r s .

1.6

2.6 1 . 4

1 . 4

1 . 2

1.6

1 . 4 -

1.7 1 . 4

2.G

1 . 6

1 . 5

1 . 3

1.7 E t

[

T

SF)

275 4-00

4-50

/-1-15

4.50

625

640

195

380

370

480

375

560 ( n - O ) f

(TSF)

2.00

2.28

2.35

2.18

1.96

2 4-5

2.59 1..

72

2.07

2.29

2.36

1.97

2.08

2.38

(19)

(20)

O/o

WATER CONTENT

N A T U R A L HOLE 1 4 5

-

Qc

L I Q U I D L l M l T

S T R E N G T H

A N D C O N S I S T E N C Y PROFILE

(21)

(22)

T Y P I C A L S T R A I N

vs

T I M E CURVE FOR

Q P

TESTS

F I G U R E

-

3

2 5 0 0 e

, _{INITIAL DIAL READINCi}

-

1.008

/MAX.

READING 0.87 TONS/F-- _{PORE WATER SYSTEM}

*

2 4 0 0 ' I 0 . 8 6 4 0

-

_cV X C v) 2300 0.720

?

a

I

U

z

0 -

I- W

Z

2 2 0 0 0.576 3 0

cn

z

_-

v) W L3

u

2100 0 . 4 3 2

a

(1: (1: W J I- 4

i

3

2000

-+z+-

-

0 . 2 8 8

$

W Z

_-

GENERAL L I N E A R

u

a

0 a

Q: k 1900 0.1 4 4

cn

I N I T I c ~ L PORE 1000 WATER T I M E PRESSUFIE I N M l h U T E S

_-

0.000 0 4 0 80 1 20 160 200 2 4 0 280 320 360

(23)

T Y P I C A L INCREMENTAL STRESS - T I M E TEST (QY)

(24)

NOTE:

Ef (QP)

P L O T T E D H E R E R E P R E S E N T S P O I N T OF I N F L E C T I O N I N T H E S T R A I N - T I M E CURVE,NOT T H E U L T I M A T E S T R A I N . 1-0 2.0 3.0 S H E A R S T R E N G T H I N T.S.F. 1-0 2.0 3.0 I % S T R A I N AT F A I L U R E Ef

"

Y I

ELD" S T R E S S - S T R A I N P R O F I L E

FIGURE,

5

(25)

0 ₅ ₁₀ _{I S} ₂₀ ₂₅ ₃₀ ₃₅ _{4 0} ₄₅ ₅₀ ₅₅

TIME IN M I N U T E S

CYCLING

OF STRESS INCREMENTS

(QY)

S A M P L E 1 4 5 -5,

DEPTH

26.0'

(26)

0 5 10 I5 2 0 25 3 0 35 4 0 4 5 5 0

T I M E IN M I N U T E S

T I M E EFFECTS BELOW

"YIELD

STRESS"

(27)

0 0.5 1.0 115 2.0 2.5 3.0 3.5 4.0 4.5 5.0 S T R A I N

%

(28)