Double-yield-surface Cam-clay plasticity model. I: Theory

A constitutive model for the stress-strain-time behavior of cohesive soils is developed using Cam-clay plasticity theory extended to include time-dependent effects. The model adopts the concept of separating the total deformation into immediate and delayed components. The immediate plastic deformation is evaluated by employing the associative flow rule on each of two distinct yield surfaces defined by the ellipsoid of the modified Cam-clay theory and the Von Mises cylinder inscribed in the Cam-clay ellipsoid. The delayed component of deformation is evaluated by employing the normality rule on equivalent ellipsoidal and cylindrical yield surfaces associated with the current state of stress of the soil and forcing the resulting creep strain rate tensor to satisfy phenomenological creep laws. In a companion paper by the same writers, the resulting constitutive equation is shown to predict the stress-strain-time behavior of wet clays more accurately than an earlier version based on a single-yield-surface formulation.