A state-dependent elasto-plastic model for saturated and unsaturated soils

Abstract

A state-dependent elasto-plastic constitutive model for both saturated and unsaturated soils is presented in this paper. The model, which is developed under an extended critical-state framework, uses two independent stress state variables: net normal stress and matric suction. In addition, the influence of a simplified non-linear soil-water characteristic curve on mechanical behaviour is incorporated in the model by using a bi-linear function. Most importantly, a state-dependent dilatancy formulation is introduced to account for the effects of stress level or stress ratio, internal state (density) and soil suction. In this paper, details of the model formulation and the determination of model parameters are described and reported. By using the new model, numerical simulations of triaxial tests have been carried out on both saturated and unsaturated fine-grained (sandy silt) and coarse-grained (gravelly sand) weathered soils. The simulated triaxial tests include undrained shear tests on saturated specimens, constant water content tests and wetting tests under constant deviator stress on unsaturated specimens. The computed results show that the stress–strain and volumetric-shear–strain relationships at both loose and dense and saturated and unsaturated states can be modelled properly with a single set of parameters. Moreover, the model is capable of capturing some key observed features, such as a sudden increase in shear strain and volumetric strain as suction is reduced under a constant deviator stress.