A constitutive model for cemented unsaturated soils and weak rocks

The paper presents the formulation of a constitutive model for unsaturated cemented soils and weak rocks, based on principles of thermodynamics. The model gives some physically based description of the mechanics of unsaturated bonded soils and greatly simplifies their constitutive modelling. The main problems addressed in the model are reversibility and equilibrium (or mechanical instability) and the interactions between the mechanics of the granular skeleton and the bonding material. Destructuration implies the transformation of a continuous porous solid into granular matter. The exchange of mass between these two parts of the continuum (the 'intact' continuous porous solid and the granular matter) is studied as a thermodynamic process. The energy during mechanical destructuration is related to the chemical energy involved during diagenetic or artificial cementation. The model was first used to fit laboratory data for a fully saturated calcarenite and a natural clay. This allowed the study of the effects of mechanical bonding independently of the effects of partial saturation. The constitutive equation was then hierarchically extended to unsaturated conditions, reformulating the effective stress and modifying the hardening term. Ongoing research is applying the model to simulate novel laboratory data for unsaturated lime-treated UK clays.