Soil Shrinkage: Underlying Mechanisms Revealed by Intergranular Stress

When drying from a fully saturated state, soil transits from saturation to unsaturation, activating the effects of capillarity, adsorption, and osmosis. Traditionally, these effects have been elusive in modeling soil shrinkage behavior. This paper addresses this gap of knowledge by reexamining the underlying mechanisms for the drying-induced shrinkage of soil. To this end, the concept of intergranular stress is introduced to lump all the effects of osmosis, capillarity, and adsorption into a unique effective stress tensor, and a simple constitutive model is developed based on the modified Cam-Clay (MCC) model. The proposed model inherits the simplicity of the MCC model while ensuring a smooth transition from saturation to unsaturation. It is shown that the drying-induced intergranular stress includes two components, accounting for capillary and adsorptive effects, respectively, intertwined with the osmotic effect. During a drying process, the adsorptive component of intergranular stress constantly increases and surpasses the capillary component at low water content, while the capillary component increases first and then decreases at the dry end. It is revealed that, for active soils, the effect of adsorption remains significant through the whole drying process, in contrast to nonactive soil where the effect of adsorption is significant only at low water content. As water content decreases, the deformation of soil is first elastic, then elastoplastic, and finally elastic. The elastic compression at the dry end can be attributed to the evaporation of the tightly adsorbed pore water. Comparison of simulations with experimental data shows that the proposed model captures very well the main features of the drying-induced shrinkage behavior of soil, implying that the intergranular stress tensor can be effectively used to address the hydro-chemo-mechanical behavior of soil under complex loading conditions.