Modelling of overconsolidated unsaturated soils

Constitutive models for unsaturated soils are most commonly based on conventional critical state type models for saturated soils. The effect of partial saturation is taken into account through the introduction of an additional stress variable which depends on soil suction. The modified Cam-clay surface is usually assumed for the formulation of the yield and the plastic potential surfaces in the mean stress-deviatoric stress space. In this paper an existing model for unsaturated soils is used to simulate drained triaxial tests performed on highly overconsolidated unsaturated silt. Even though the model adopts an improved expression for the yield surface that allows a variety of shapes to be introduced, none of them can accurately simulate the observed soil behaviour in the laboratory tests. A new version of the model is, therefore, proposed and presented here. Following the formulation and implementation of these alterations, analyses of laboratory experiments on artificial silt are presented demonstrating the improved simulation of soil behaviour.