Rate Effects And Cyclic Loading of Sensitive Clays

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RATE EFFECTS AND CYCLIC LOADING OF SENSITIVE CLAYS By Guy Lefebvre1 and Denis LeBoeuf2 ABSTRACT: The paper presents the results of a series of monotonic and cyclic triaxial tests carried out to study the influence of the rate of strain and load cycles on the undrained shear strength of three undisturbed sensitive clays from eastern Canada. For structured clays (naturally overconsolidated), pore pres^ sures generated at a given deviator are essentially independent of the strain rate, while the peak strength envelope is lowered as the strain rate is decreased. For destructured clay (normally consolidated), a lower strain rate results in an increase in pore pressure generation during shearing, due to the tendency of the clay skeleton to creep, while the peak strength envelope remains the same. Nevertheless, from a quantitative standpoint, the increase in shear strength caused by an increase in strain rate is similar for both structured and normally consolidated clay, and it is linear for at least five log cycles of strain rate.

INTRODUCTION

The time-dependent behavior of clays has long been recognized in soil mechanics literature. Early works by Bjerrum, et al. (1958), Richardson and Whitman (1963), Crawford (1965), and Jarrett (1967) have clearly demonstrated that the rate of loading has a significant influence on the stress-compressibility and stress-strain behavior of remolded and undisturbed clays. More recently, Crooks and Graham (1976), Graham (1979), Vaid, et al. (1979), and Graham, et al. (1983), have shown that the preconsolidation pressure and the undrained shear strength increase by about 9-20% for a tenfold change in strain rate. Moreover, Graham, et al. (1983), concluded that the influence of strain rate on the undrained shear strength was not related to the soil plasticity, test type, and stress history during laboratory reconsolidation. Leroueil, et al. (1983), have presented a normalizedffp-e„relationship, obtained from more than 150 oedometer tests that were carried out on 11 different Champlain sea clays, which led them to conclude that the influence of strain rate on o'p was unique for all Champlain sea clays. This influence also appeared to be independent of plasticity, degree of overconsolidation, test type, or sampling technique. The strain rate effect should be particularly important in the interpretation of cyclic tests on clay soils performed at high frequencies, whereas the strain rate in such a test is not considered to have a significant effect on the strength of granular materials (Peacock and Seed 1968). The high strain rate generally used in cyclic test could result in a cyclic shear strength 'Prof., Dept. de genie civ., Univ. de Sherbrooke, Sherbrooke, Quebec J1K 2R1, Canada. 2 Grad. Student, Dept. de genie civ., Univ. de Sherbrooke, Sherbrooke, Quebec J1K 2R1, Canada. Note.—Discussion open until October 1, 1987. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on June 20, 1986. This paper is part of the Journal of Geotechnical Engineering, Vol. 113, No. 5, May, 1987. ©ASCE, ISSN 0733-9410/87/0005-0476/$01.00. Paper No. 21508. 476

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for clay soils that appears to be higher than the shear strength determined in monotonic tests at standard strain rates of 0.5-1.0%/hr. It is not known, however, if the strain rate effect that has been reported in the literature for tests sheared at strain rates generally lower than 15%/ hr can be extrapolated to the higher strain rates applied in cyclic tests. This paper presents the results of a series of undrained triaxial compression tests carried out to study the influence of strain rates on three different sensitive clays from eastern Canada. Strain rates varying from 0.05-132.0%/hr were used in monotonic strain-controlled triaxial compression tests. In the stress-controlled tests, strain rates of up to about 6,000%/hr were used, which are equivalent to frequencies of 1-2 Hz, typically applied in cyclic tests. Mechanical behavior of eastern Canadian clay differs significantly if tested at in-situ stresses where the natural clay structure is intact or if tested at stresses higher than the preconsolidation pressure a'p, where the natural structure of the clay is damaged by consolidation. Thus the influence of the strain rate was investigated for consolidation stresses lower than ' j ^ _ » . ~~^~~

^~~^

-—^

-

"

>— EXPERIMENTAL BOUNDS •

-

STRAIN RATE l%/h)

FIG. 16.—Change of Undrained Strength Ratio, Normalized to Undrained Strength Ratio at e, = 1,0%/h, with Strain Rate for All Investigated Clays 485

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TABLE 2.—Change in Percent of Undrained Strength per Log Cycle of Strain Rate Clay (1) Grande Baleine (dyke 12) Grande Baleine (dyke 39) Olga B-6 (7.0 m) B-6 (10.1 m) St. Jean Vianney (upper layer) St. Jean Vianney (lower layer) Average values

Structured

Destructured (2) 8 9 12 14

(3) 7 7 13 9 13 11 9 9.9

— — — 10.7

stress-controlled conditions at a frequency of 0.1 Hz. Fig. 17 shows the static failure envelopes and cyclic stress paths for the Grande Baleine clay. The strength envelope defined at maximum deviator from the monotonic stress paths is typical of intact Canadian

0

EO

100

40

120

MO

160

p'.ITl' 'J3I

FIG. 17.—Static Strength Envelopes and Cyclic Stress Paths of Grande-Baleine Clay I

I

I BROADBACK CLAY IB6)

0.8

•

0 ^ = 266 kPoi K0=0.54

O

O^'IGE KPa;K 0 = 0.54 f=0.IH3

0.6

I

I

~.EXTRAPOLATED STRENGTH AVAILABLE

-

FOR THE FIRST CYCLE

y °- 4

-

02

,l

,,,l NUMBER OF CYCLES, U

FIG. 18.—Cyclic Strength of Destructured B6 Clay 486

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clays and clearly defines the structured and destructured domains. The cyclic stress paths are presented only for a few cycles. For the cyclic tests performed in the structured domain (CIUCV-01), cycling generated very small pore pressures before reaching failure after 32 cycles. Clearly, the failure point is located below the static strength envelope in the structured domain. The cyclic tests on the normally consolidated sample (destructured domain) indicate a different behavior. Large pore pressures were generated during cycling, bringing the stress path to the static maximum deviator envelope. The stress path then ends in failure above this envelope, very close to the locus defined by the monotonic stress paths at large deformation. It is interesting to note that failure in both of the cyclic tests defined points in the stress space that are fairly close to the large deformation envelope defined in the monotonic tests. The difference in behavior observed between structured and destructured clays in the cyclic tests is similar to the difference observed in monotonic tests with different rates of strain. The strength envelope for destructured clay appears to be unique for both monotonic and cyclic tests performed at different strain rates, whereas the peak strength envelope for structured clay is lowered both at low rates of strain and in cyclic tests. The cyclic strength of the destructured B-6 clay is shown in Fig. 18 as the ratio of the maximum applied deviator normalized by u'vc and expressed as a function of the number of cycles to failure. The available undrained shear strength decreases with the number of applied cycles. For 30 cycles and a vertical consolidation pressure of 266 kPa, the relationship shown in Fig. 18 indicates an undrained shear strength of 106 kPa. The static undrained shear strength measured in monotonic tests at a standard strain rate of 0.5%/hr for the same consolidation pressure was equal to only 92 kPa (Fig. 15). One could conclude that the cyclic strength at 30 cycles is about 15% higher than the static strength, but this is neglecting the effect of the difference in strain rates between the monotonic and the cyclic tests. For the B-6 clay, a frequency of 0.1 Hz is roughly equivalent to a strain rate of about 300% /hr. In extrapolating the strain rate effect for the destructured B-6 clay (Fig. 15), one gets an undrained shear strength of about 125 kPa for a strain rate equivalent to a frequency of 0.1 Hz, which is roughly equivalent to what can be obtained when extrapolating the cyclic strength (Fig. 18) for a failure in one cycle. When considering the rate effect, the cyclic strength at 30 cycles is equal to about 85% of the static strength. CONCLUSION

The effect of strain upon undrained shear strength of saturated clay has been recognized for a long time and has been related to excess pore pressure generated during shear (Casagrande and Wilson 1951; Bjerrum, et al. 1958; Crawford 1959). Once more, this effect has been verified for clay specimens destructured by consolidation above the field preconsolidation pressure. Due to different pore pressures generated during shear, the stress paths reach the effective strength envelope at different shear stresses. However, for brittle structured marine clays reconsolidated in the laboratory well below the field preconsolidation pressure, 487

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variation in the rate of loading does not result in a significant change in the pore pressure generated during shear, but a reduction in the rate of strain clearly results in a lowering of the peak strength envelope. Similar behavior was observed in cyclic tests carried out at strain rates equivalent to those used in monotonic tests. Pore pressures generated during cyclic tests on destructured clay are sufficient to bring the stress path to failure on the monotonic test strength envelope. However, in cyclic tests carried out on structured clay, relatively low pore pressure was generated, and failure took place at a maximum deviator stress below the peak strength envelope defined from monotonic tests. The shear resistance of a given destructured clay is a function of the effective stress. Reducing the rate of loading or cycling the load reduces effective stresses and, consequently, reduces the undrained shear strength. For structured Canadian clays, part of the shear resistance is due to the brittle bonds of the clay skeleton. Reducing the strain rate or cycling the load appears to weaken the resistance of the clay skeleton by a fatigue phenomenon. The peak strength envelope is then lowered. For structured marine clays, the reduction in undrained shear strength either at low strain rates or in cyclic testing is not due to the difference in stress path but mainly to the lowering of the strength envelope itself. While the cause of the reduction in undrained shear strength with lower strain rate appears to be different for structured and destructured clays, the magnitude of the strain rate effect is similar. For both structured and destructured clays, the variation of the undrained shear strength with the log of the strain rate defined a linear relationship. Test results indicate that this linear relationship can be extended up to larger strain rates representative of the frequencies at which cyclic tests are usually performed. The variation of undrained shear strength with strain rate observed in the study varied from 7-14% /log cycle, the average being 10% for destructured as well as structured clays. When comparing monotonic and cyclic undrained shear strengths, it is important to consider the difference in strain rate in the tests as well as the degradation of undrained shear strength due to cycling. Significant strain rate effects in saturated clays cause the cyclic strength mobilized at high frequencies to be higher than the monotonic strength measured at standard strain rates. As a first approximation, the cyclic strength of a clay at a given frequency could be evaluated from a monotonic test by first applying a correction for the strain rate effect to obtain the monotonic strength at the given frequency and then applying a degradation function to obtain the cyclic strength that can be mobilized for a given number of cycles. ACKNOWLEDGMENTS

The results presented in this paper were obtained in research programs conducted at the Universite de Sherbrooke and supported by the National Sciences and Engineering Research Council of Canada, the Ministere de l'Education du Quebec, Hydro-Quebec, and the Societe d'Energie de la Baie James. 488

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APPENDIX.—REFERENCES

Bishop, A. W., and Henkel, D. J. (1962). The measurements of soil properties in the triaxial test. E. Arnold Ltd., London, U.K. Bjerrum, L., Simons, N., and Torblaa, I. (1958). "The effect of time on the shear strength of a soft marine clay." Proceedings, Brussels Conference on Earth Pressure Problems, Vol. I, 148-158. Casagrande, A., and Wilson, S. D. (1951). "Effect of rate of loading on the strength of clays and shales at constant water content." Geotechnique, 2(3), 251-263. Crawford, C. B. (1965). "The resistance of soil structure to consolidation." Canad. Geotech. J., 2(2), 90-97. Crooks, J. H. A., and Graham, J. (1976). "Geotechnical properties of the Belfast estuarine deposits." Geotechnique, 26(3), 293-315. Demers, B. (1980). "Resistance cyclique de I'argile de Grande Baleine." Memoire de maitrise, Departement de genie civil, Universite de Sherbrooke, Sherbrooke, Quebec, Canada. Graham, J. (1979). "Embankment stability on anisotropic soft clays." Canad. Geotech. } . , 16(2), 295-308. Graham, J., Crooks, J. H. A., and Bell, A. L. (1983). "Time effects on the stressstrain behaviour of soft natural clays." Geotechnique, 33(3), 327-340. Jarrett, P. M. (1967). "Time-dependant consolidation of a sensitive clay." Materials Research and Standards (ASTM), 7(7), 300-304. Lefebvre, G., Bosse, J. P., and Beliveau, J. G. (1979). "Etude de I'argile du site Olga sous sollicitations cycliques." Rapport Gio-79-06, Departement de g£nie civil, Universite de Sherbrooke, Sherbrooke, Quebec, Canada. Lefebvre, G. (1982). "Results of cyclic triaxial tests on the B-2 and B-6 clays." Rapport Geo-82-11, Departement de genie civil, Universite de Sherbrooke, Sherbrooke, Quebec, Canada. Lefebvre, G., Bosse, J. P., and Beliveau, J. G. (1979). "Etude de L'argile du site Olga sous sollicitations cycliques." Rapport Gio-79-06, Departement de genie civil, Universite de Sherbrooke, Sherbrooke, Quebec, Canada. Lefebvre, G., and Poulin, C. (1979). "A new method of sampling in sensitive clays." Canad. Geotech. ]., 16(1), 226-233. Leroueil, S., Tavenas, F., Samson, L., and Morin, P. (1983). "Preconsolidation pressure of Champlain clays: Part II. Laboratory determination." Canad. Geotech. } . , 20(4), 803-816. Peacock, W. H., and Seed, H. B. (1968). "Sand liquefaction under cyclic loading simple shear conditions." /. Soil Mech. Foundation Div., ASCE, 94(3), 689-708. Richardson, A., and Whitman, R. V. (1963). "Effect of strain-rate upon undrained shear resistance." Geotechnique, 13(3), 310-324. Vaid, Y. P., Robertson, P. K., and Campanella, R. G. (1979). "Strain rate behaviour of St. Jean Vianney clay." Canad. Geotech. /., 16(1), 34-42.

489

J. Geotech. Engrg. 1987.113:476-489.