Colinearity of Stresses, Strains, and Strain Increments during Shearing of Soft Clay

The results of torsional shear tests on a normally consolidated marine clay have been analyzed to check the common assumption of colinearity between the principal directions of stress and plastic strain increment. Shear strain increments measured during the tests were divided into elastic and plastic components using three different plastic models: an isotropic hardening model, a kinematic hardening model, and a model in which all shear strain was assumed to be plastic. These three cases are believed to bound the possible divisions, which may be obtained using more sophisticated models. Colinearity was not observed for any of the models over most of the shearing range. All the models indicated that at the onset of yielding, the principal plastic strain increment directions are close to those of stress increment, and they approach those of stress toward failure. A new parameter, psi-ratio, was introduced, defining the principal direction of the plastic strain increment relative to those of the stress and the stress increment. Based on the findings of this investigation, a simple model may be adopted in which the psi-ratio varies from 1 at the onset of yielding (when plastic strain increment and stress increment are assumed to be colinear) to 0 at failure (when plastic strain increment and stress are assumed to be colinear). The psi-ratio is related to the degree of yielding in the soil and, consequently, it may be a useful parameter in studies on the yielding criteria of soils.