Modeling of viscoplastic deformation in geomaterials with a polycrystalline approach

A micromechanical model is proposed to describe the viscoplastic deformation of cohesive frictional geomaterials with a granular texture. Inspired by the widely used polycrystalline theory, the macroscopic viscoplastic deformation is attributed to the subcritical sliding along a number of oriented weakness planes, which are assimilated to the crystallographic planes in single crystals. Main features of geomaterials, such as pressure sensitivity and plastic compressibility/dilatancy, are taken into account by introducing an appropriate yield criterion and a non-associated plastic potential for each weakness plane. A simple interaction law is adopted to relate the local stress and strain fields to the macroscopic ones of the representative elementary volume. Computational aspects about the implementation of constitutive equations of both single crystal and polycrystal are discussed in detail. The performance of proposed model is checked through the comparisons between numerical results and experimental data on both triaxial compression tests and creep tests performed on a typical rock - granite.