Viscous soft-soil modeling

Viscosity is in fact a relaxation of the shear stresses over time, which results for constant stress states in a corresponding elastic strain relaxation. In other words, viscosity is a sort of slow plasticity before the Coulomb criterion is reached. Creep settlement can be regarded as one of the viscous soil phenomena, but also the slow movement (a few mm or cm every year) of many natural slopes in mountainous area's. Therefore a viscous constitutive model is derived from the standard principles that the reduction rate of the relative shear stress (the viscosity) is a function of this relative shear stress and that the reduction rate approaches zero while the stress-state approaches the K0-state. As an example a User-Defined (Soft) Soil Model is developed in order to model the continuous moving slopes. Although the model works fine for viscous behavior and creep problems, it seems as if the slope movements do not depend on viscosity but on dilatancy.