A microstructure-based elastoplastic model to describe the behaviour of a compacted clayey silt in isotropic and triaxial compression

The paper focuses on the hydro-mechanical behaviour of an unsaturated compacted clayey silt, accounting for fabric changes induced by drying-wetting cycles occurring at low-stress levels. The response along isotropic compression and triaxial compression (shear) at constant water content was investigated by laboratory tests on both as compacted and dried-wetted samples. Compaction induces a micro-structural porosity pertinent to clay peds and a macro-structural porosity external to the peds. Drying-wetting cycles decrease the micro-porosity and increase the macro-porosity, which reduces the water retention capacity, increases the compressibility and promotes higher peak strengths with more brittle behaviour during triaxial compression. A coupled double porosity elastic-plastic model was formulated to simulate the experimental results. A non-associated flow rule was defined for the macrostructure, modifying a stress-dilatancy relationship for saturated granular soils to account for the increase in dilatancy with suction observed in the experiments. The average skeleton stress and suction were adopted as stress variables. Consistently with model predictions, the shear strength at critical state is not significantly influenced by the degree of saturation or by the hydraulic history. On the other contrary, the higher peak strength, brittleness and dilatancy of the dried wetted samples are mostly explained by their reduced water retention capacity.