Viscous stress-strain behaviour of clay under unloaded conditions

The viscous property of several types of clay (undisturbed and reconstituted) was evaluated by performing a series of drained triaxial compression tests including global unloading. The axial strain rate was changed stepwise many times during otherwise monotonic primary loading at a constant strain rate to evaluate the viscous property. Drained creep loading was performed many times during not only primary loading but also global unloading and reloading. The neutral state where the creep strain rate became zero was reached after some amount of unloading from primary loading and the creep strain rate became negative by further unloading, which became more obvious with an increase of unloading. These features and others were successfully simulated based on a non-linear three-component model.