Elasto-plastic Modelling of Unsaturated Soils : an Overview

This paper presents an overview of elasto-plastic modelling of unsaturated soils. Alternative stress and strain variables that have been used in constitutive models for unsaturated soils are discussed. The use of an effective stress equation may simplify constitutive relations, but often result in a stress space that depends on the material state. In this case, the constitutive behaviour is embodied both in the stress definition and in the constitutive relation. Alternative volumetric models in the literature are then discussed. The focus is placed on the comparison of a new model with other existing models. This new model is formulated in terms of independent stress variables and features a smooth transition between saturated and unsaturated states. The implications of the volumetric model for yield stress and shear strength functions are discussed. The paper also shows how hysteresis in soil-water characteristic curves can be incorporated into the elasto-plastic framework, leading to a coupled hydro-mechanical model. Finally, the paper demonstrates the derivation of the incremental stress-strain relations for unsaturated soils and discusses briefly the implementation of these relations into the finite element method.