A coupled model of mechanical behaviour and water retention for unsaturated soils with double porosity

Many natural soils and engineering geomaterials, such as aggregated soils and compacted clay pallets, exhibit two levels of porosity corresponding to the inter- and intraaggregate pores within their hierarchical structure. Mechanical behavior of these materials, in particular when unsaturated, is an issue of added complexity which should be described an appropriate constitutive framework. A coupled water retention–mechanical constitutive model for unsaturated soils with double porosity is presented here. Based on the multi-scale experimental results, the model incorporates the inter-particle bonding, fabric and partial saturation effects in a single framework. It is formulated within the framework of hardening elasto-plasticity and is based on the critical state concept. The mechanical model is coupled with the water retention law which itself takes into account the two levels of porosity. The coupling is made through the expression of the effective stress and the evolution of the preconsolidation pressure with suction. On the other hand, the mechanical model at the macro-scale is also coupled with the pore-scale behavior of the materials through an internal variable which accounts for the evolution of the soil structure. The model is used for numerical simulation of the behavior of aggregated and bonded soils. Comparison of numerical simulations and the experimental results show that the model can successfully address the main features in the behavior of unsaturated soils with double porosity