Strain-softening behavior of granular soil in strain-path testing

The strain-softening response of a granular soil along a wide spectrum of strain paths is investigated experimentally. Direct strain-path control is achieved by controlling the ratio of volume to axial strain increment using a digital pressure/volume controller. The questions associated with the definition of strain softening are examined. Provided suitable experimental techniques are used to delay the onset of nonhomogeneous deformation, strain softening as a true material property of a soil element can be observed along certain strain paths. Two types of strain softening, prefailure and postfailure types, have been identified. The conditions for the occurrence of prefailure or postfailure strain softening were established. During postfailure strain softening, the effective stress state of the sample traces along the failure curve toward the origin. The sample, however, will suffer from an irrecoverable loss in shear strength if sheared at a confining stress close to the value prior to onset of postfailure softening. Thus, the shear strength of a sample, although not affected by unidirectional, prefailure shearing, is dependent on post-failure strain history.