A nonlinear viscoelastic-viscoplastic model for clay materials

Although a number of constitutive models has been proposed so far for simulating the delayed behavior of clay, at present a satisfactory constitutive approach is not yet available for predicting the magnitude and rate of viscous deformation of clay materials in real problems. From the observation of the peculiar aspects of stress-strain response of clay under oedometric (1D) and triaxial (2D) conditions, a new nonlinear viscoelastic-viscoplastic model with two yield surfaces has been proposed, in which the viscoplasticity of each yield surface is treated according to the original proposal by Perzyna (1966). The experimental response in oedometric tests along different of loading paths, even involving complex unload-reload cycles, is well approximated by the proposed model. In addition the model can reproduce accurately the linear evolution of creep deformations with respect to the logarithm of time, along a constant load path. Finally the proposed model can simulate very well the typical experimental response measured in triaxial tests performed on normally consolidated or weakly overconsolidated clays, and provides reliable simulations of the primary creep through the tertiary creep failure under undrained conditions.