A unified constitutive model for unsaturated soils based on Generalized Plasticity Theory

This paper is focused on constitutive modelling of saturated and partially saturated granular soils from the state parameters point of view. The Generalized Plasticity constitutive equation has been extended to reproduce stress-strain behaviour of granular soils with a single set of intrinsic model constants for different densities, confining pressures and saturation conditions. The proposed constitutive model combines versatile and hierarchical formulation of Generalized Plasticity Theory with the Critical State framework. The "effective stress" principle has been used in order to introduce the effect of unsaturation into the stress variable. The state parameter concept has been incorporated to take into account the dependant of granular soil behaviour on density and confining pressure. A bonding parameter function of plastic modulus has been proposed to take into account hardening and softening effect caused by the change on saturation condition. The developed constitutive model has a hydro-mechanical formulation through the relation of saturation degree and normalized suction. The model also considered the void ratio and hydraulic hysteresis effect on hydraulic behaviour. The model has been calibrated by using experimental results reported in literature. The simulated tests are covering a wide range of densities, confining pressures and suctions for saturated and partially saturated soils.