Time-dependent shear deformation characteristics of geomaterials and their simulation

The viscous aspects of the stress-strain behaviour of saturated and air-dried clean sands in drained plane strain compression (PSC) and saturated clean sand and soft clays in undrained triaxial compression (TC) are presented. Common as well as different viscous features among the different geomaterials are addressed. The general three-component model is used as the framework for constitutive modelling, in which the total strain rate a is decomposed into elastic and irreversible components epsilon(e) and epsilon(ir) while the stress sigma is decomposed into inviscid (non-viscous) and viscous components of and sigma. In the simplest model (called the new isotach model) among those described in the paper, sigma(f) is a nonlinear function of epsilon(ir), while sigma(v) is a non-linear function of epsilon(ir) and always proportional to sigma(f) for primary loading. This model is relevant to kaolin for the full pre-peak range and a reconstituted low-plasticity clay (Fujinomori clay) at small strains, both in undrained TC. The model is modified to simulate the viscous effect that decays with epsilon(ir), as observed with clean sands and a natural soft clay. It is shown that the second type of model (called the viscous evanescent model and the TESRA model) simulates well the above-mentioned behaviour, not only during primary loading, but also at unloaded conditions. The model is further modified to simulate the behaviour of Fujinomori clay whereby the rate at which the viscous effect decays gradually increases with epsilon(ir) (the general TESRA model). The viscous components sigma of the three models can be represented by a set of common equations, and the other models are specifically simplified versions of the general TESRA model.