Evaluation of the viscous behaviour of clay using generalised overstress viscoplastic theory

In this paper, generalised elastic-viscoplastic (EVP) theory combined with a power law is used to derive simplified equations relating undrained shear strength and preconsolidation pressure to strain rate. An additional equation is derived relating the strain-rate parameter in EVP theory to the secondary compression index. The derived equations are used to evaluate the rate-sensitive and drained creep response of 20 clays reported in the literature. The evaluation shows strong evidence that the rate-sensitivity and time-dependency of clay in compression can be described simultaneously using the generalised EVP theory and power law. In addition, the constitutive parameter governing rate-sensitivity in EVP theory appears to be unique for constant rate-of-strain tests such as undrained triaxial compression and drained oedometer compression as well as stress controlled tests such as drained oedometer creep. To conclude, an approach to select the viscous parameters for clay is provided.