Numerical analysis of dynamic strain localisation in initially water saturated dense sand with a modified generalised plasticity model

Shear band dominated process in fully and partially saturated sand samples is simulated by means of dynamic strain localisation analysis together with a multiphase material model. The partially saturated medium is viewed as a multiphase continuum consisting of a solid skeleton and pores filled by water and air (vapour) which, once it appears, is presumed to remain at the constant value of cavitation pressure (isothermal monospecies approach). The governing equations are based on the general framework of averaging theories. A modified generalised plasticity constitutive model for partially saturated soils, developed from the general Pastor-Zienkiewicz sand model, has been implemented in a finite element code and used in the computational process. This model takes into account the effects of suction in the stiffness of the porous medium (solid skeleton) in partially saturated state. A case of strain localisation, which has been tested in laboratory observing cavitation of the pore water, is studied. Negative water pressures, which are of importance in localisation phenomena of initially fully saturated undrained samples of dilatant geomaterials, are obtained similarly to those observed experimentally.