ABSTRACT
Triaxial experiments from some unconsolidated reservoir sands demonstrate the material to be essentially cohesionless. Nevertheless the deformation curves (axial stress versus axial strain) exhibit an apparent hardening regime followed by plateau indicating developed plastic flow. This stress-strain behavior can be modeled using an increasing apparent internal friction angle and a simultaneously decreasing dilatancy angle through the hardening regime leading to the plastic deformation. Approximate relations for the friction and dilatancy angles variation with axial stress and strain are derived from the experimental data, and also for the shear modulus and Poisson’s ratio with axial stress. These relations enable the FLAC simulation of the triaxial (and uniaxial compaction tests under constant-radial-strain conditions) experiments within the framework of the Mohr-Coulomb model. The range of simulation results encompasses the experimental data. Theoretical model development shows that in the case of variable internal friction a linear relation is needed between the increments of the internal friction coefficient and the plastic volume strain, and the plastic hardening modulus depends on the effective average pressure and dilatancy. For certain conditions this leads to material instability and localization of irreversible deformation when the critical value of plastic hardening modulus is exceeded.
