Mechanism of physicochemical effect on the shrinkage of expansive soil

Due to the fixed negative charges in the skeleton of expansive soil, there are exchangeable cations between the layers that balance the negative charges in expansive soil. That makes the soil exhibit strong expansion and shrinkage properties. The research results show that the expansion and shrinkage of expansive soil will be affected by the chemical composition of the pore solution. In this paper, based on the strong expansive soil in Guangxi, a series of tests are carried out to investigate the soil-water retention curve (SWRC) and the soil shrinkage curve (SSC) for the soil saturated with solutions of different NaCl concentrations. To address this issue, the concept of intergranular stress is introduced, which takes into account the effects of osmosis, capillary, and adsorption. The results show that pore saline solution affects the SWRC through osmotic suction, with a lesser effect on matric suction. The shrinkage deformation of soil samples during the drying process is controlled by intergranular stress, similar to the phenomenon of pressure-induced consolidation. The majority of shrinkage occurs during the capillary stage, exhibiting elastoplastic deformation; less shrinkage occurs during the adsorption stage, which is characterized by elastic deformation. A cutoff point on the compression curve is identified to distinguish between the regimes of capillarity and adsorption, consistent with the independently measured SWRCs at different compactness. The intergranular stress is shown to better describe the chemo-mechanical behavior of expansive soil, particularly at low water content.