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Triaxial compression tests on an undisturbed sensitive clay Hirst, Terence John


An experimental investigation into the stress-strain behavior of an undisturbed sensitive clay is presented. The stress-strain characteristics of both drained and undrained triaxial tests are considered. The drained and undrained shear strengths are compared for both the maximum principal stress difference and the maximum effective principal stress ratio failure criteria. An attempt is made to correlate the drained and undrained shear strength through the use of energy equations which account for volume change. The magnitude of pore pressures that develop during drained tests is estimated, and a brief discussion of the effect of rate of strain on the behavior of the clay is also included. The soil tested was a sensitive laminated silty-clay of marine origin. The experimental work consisted of standard strain-controlled triaxial compression tests performed on saturated, normally consolidated, 2.8 ins. by 1.4 ins. diameter samples. The strain rate in both the drained and undrained tests was 0.5 percent per hour, except for one drained test sheared at 2.5 percent per hour. All consolidation and drained shear was conducted under a back pressure of 10 lbs./sq. in. Drainage was permitted from both ends of the sample but no filter paper side drains were used. Pore pressures were measured at the base of the sample using a Bishop and Henkel null-indicator. The samples were sheared until approximately 30 percent axial strain had been developed or until failure had occurred. A discussion of the testing procedures is included. The results of the investigation indicated that the sensitivity of the clay is of primary importance in determining the behavior of soil under load. A relationship between void ratio and strength that is independent of stress path does not exist in undisturbed sensitive clays, nor does there appear to be a common drained and undrained strength envelope at the maximum principal stress difference failure criterion. Application of the Bishop and Rowe energy corrections to the drained strength obtained at the maximum effective principal stress ratio did not yield the same effective strength envelope as that determined from undrained tests at the same failure criterion, but the value of M (the slope of the q(w)-pˈ curve) in the Roscoe, Schofield, and Thurairajah energy equation was approximately constant. The uncorrected effective angle of shearing resistance, ⌽ˈ, was found to be a function of failure criterion and drainage condition. The strain at which failure occurred in drained tests, although decreasing with increase in consolidation stress, was large, indicating that the generally accepted failure criteria of maximum principal stress difference and maximum effective principal stress ratio are not satisfactory for sensitive clay. Although calculations showed that pore pressures were developed at low strains in drained tests, increasing the rate of strain from 0.5 percent per hour to 2.5 percent per hour did not noticeably affect the strength or stress-strain behavior of the 2.8 ins. by 1.4 ins. diameter samples.

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