Viscoplastic constitutive approach for rate-sensitive structured clays

This paper extends an existing elastic-viscoplastic (EVP) constitutive model using a state-dependent viscosity parameter to describe the engineering response of undisturbed structured clay. The term structure refers to the effects of fabric and weak cementation bonds between clay particles. The extended constitutive model is coupled with the Biot consolidation theory and is formulated to describe the intrinsic or unstructured response of clay using overstress viscoplasticity, an elliptical cap yield surface, Drucker-Prager failure envelope, and a hardening law from critical state theory. The clay structure is mathematically accounted for by assuming that the initial fluidity of structured clay at yield and failure is very low and that the fluidity increases with increasing plastic strain. This process is usually referred to as "destructuration." The formulation is evaluated using Saint-Jean-Vianney (SJV) clay by comparing calculated and measured behaviour during consolidated isotropically undrained triaxial compression, triaxial creep, and constant rate-of-strain K-o '-consolidation tests. The comparisons indicate that the EVP constitutive model can describe most of the rate-sensitive behaviour of SJV clay during both drained and undrained laboratory tests involving either constant-volume shear or predominantly volumetric compression of the soil skeleton.