Viscous Soft-Soil Modelling

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.