Time-dependent shear deformation characteristics of sand and their constitutive modelling

Time-dependent (viscous) characteristics of the shear stress and shear strain behaviour of water-saturated or air-dried specimens of clean sands (Hostun and Toyoura sands) observed in a series of drained plane strain compression tests are presented. The overall shear stress-shear strain relationships were very similar in a set of monotonic loading tests performed at constant axial strain rates that were different by a factor of up to 500. Despite the above, significant viscous effects on the stress-strain behaviour were observed, a) when the strain rate was changed stepwise or at a constant rate, b) at creep and stress relaxation stages, and c) immediately after loading was restarted at a constant strain rate following a creep stage. One type of constitutive model was developed in the framework of the general three-component model to simulate these behaviours. According to this model, the strain is decomposed into elastic and irreversible components, while the stress is decomposed into time-independent and dependent components. This model was developed to simulate such experimental results in that the time-dependent stress component changes not only when the irreversible shear strain rate changes but also when loading continues at a certain constant irreversible shear strain rate, while these viscous effects decay with an increase in the irreversible shear strain. The rationales for the structure of the proposed model obtained from the experiment are presented. It is shown that this model can simulate well the experimental observations described above, although they were obtained under certain limited test conditions.