An elasto-viscoplastic model for partially saturated geomaterials

Abstract

Rainfall and reservoir water level changes lead to changes in pore water pressures and in turn to changes in effective stresses and thus to landslide deformations. This is also a time dependent process and creep may occur on top of it. To explore their creep mechanics, four groups of isotropic compression creep tests on unsaturated soils have been conducted. We found that the volumetric strain had a linear relationship with logarithmic time under a constant net confining pressure or suction. The gradient of the straight line is independent of the net confining pressure and has an exponential relationship with suction, indicating that the creep formula proposed by Yin and Graham can be employed to describe the strain in unsaturated soil; however, the creep parameters must be expressed by a function that considers suction. In this study, we established a creep model for unsaturated materials based on plastic and rheological theories. The hardening parameters of the yield stress include the plastic volumetric strain and viscous volumetric strain. We established a time-dependent yield surface equation for unsaturated soils by introducing a conversion time variable into the yield surface equation of the Barcelona Basic Model. Subsequently, based on the associated flow rule and the Perzyna theory, we derived a viscoplastic strain rate equation for unsaturated soil. To verify the model, a set of unsaturated shear and creep tests have been performed and recorded the volumetric, axial strains and stress over time. The values predicted by the model agreed with our experimental results, indicating the reliability of the new creep model for unsaturated soil.