Stress Relaxation Characteristics of Saturated Clays

Abstract

A series of undrained triaxial stress relaxation tests on four saturated remoulded clays consolidated under isotropic stress condition were performed to investigate the influence of imposed strain level and strain rate prior to the start of relaxation test on the principal stress difference and pore pressure versus elapsed time relationships. Experimental results show that the principal stress difference versus logarithm of time relationship is approximated by the two straight lines independent of strain level, strain rate and mode of stress application in the preceding undrained shear process, and that the stress state corresponding to the point of intersection of the two straight lines locates close to the effective stress path calculated by the Cam-clay model proposed by Roscoe et al. (1963). A sign that the principal stress difference finally approaches to a limiting equilibrium value is found from the test results. It is also shown that the stress points corresponding to the equilibrium state lie below the Cam-clay effective stress path and the curve linking these stress points makes a similar shape of the path. The slope of the initial straight line on the principal stress difference versus logarithm of time plot depends on the magnitude of strain imposed in the preceding undrained shear process. Plastic component of the change of pore water pressure, which was defined by Lo (1969), during stress relaxation test depends not only on the strain level but also on the strain rate prior to the test.