A critical state constitutive model for unsaturated structured soils

This paper presents an advanced approach for interpreting the hydro-mechanical response of unsaturated structured soil, including the effect of bond strength degradation and the role of the degree of saturation in unsaturated conditions. The model relies on the definition of a “structure degradation law,” which accounts for the progressive breakage of inter-particle bonding during loading. The soil compression index is assumed to be a function of the degree of structure and the effective degree of saturation. A new volume change equation is proposed to better explain the nonlinear change of compressibility of structured soils under saturated and unsaturated conditions. The proposed volume change equation is generalised to three-dimensional stress states by incorporating the modified Cam-clay model. The basic properties and performance of the proposed constitutive model are then illustrated through numerical examples with various stress paths, degrees of structure, and degrees of saturation. The proposed model is verified using the experimental data available in the literature for both structured soils and reconstituted soils under saturated and unsaturated conditions. The results indicate that the proposed model simulates the mechanical response of structured and reconstituted soils reasonably well in saturated and unsaturated conditions.