A constitutive model for the stress-strain-time behaviour of ‘wet’ clays

A constitutive model is developed to describe the stress-strain-time behaviour of ‘wet’ clays subjected to three-dimensional states of stress and strain. The model is based on Bjerrum’s concept of total strain decomposition into an immediate (time-independent) part and a delayed (time-dependent) part generalized to three-dimensional situations. The classical theory of plasticity is employed to characterize the timeindependent stress-strain behaviour of cohesive soils using the ellipsoidal yield surface of the modified Cam Clay model presented by Roscoe and Burland. The time-independent strain is divided into an elastic part and a plastic part. The plastic part is evaluated using the normality condition and the consistency requirement on the yield surface. The time-dependent (creep) component of the total strain is evaluated by employing the normality rule on the same yield surface as in the time-independent model and the consistency requirement which requires that the creep strain rate reduces to phenomenological creep rate expressions for isotropic or undrained triaxial stress conditions. The mathematical characterization of the constitutive model is given by the constitutive equation expressed in a form suitable for direct numerical implementation (i.e. finite element formulation). The required soil parameters are easily obtainable from conventional laboratory tests. The constitutive equation is shown to predict accurately the stress-strain-time behaviour of an undisturbed ‘wet’ clay in triaxial and plane strain stress conditions.