Impacts of Drying-Wetting and Loading-Unloading Cycles on Small Strain Shear Modulus of Unsaturated Soils

The small strain shear modulus (Gmax) is an important parameter in geodynamic problems. To predict the Gmax of unsaturated soils that are normally subjected to complex drying and wetting processes, the effect of hydraulic hysteresis needs to be evaluated. Although several equations have been proposed in recent years, limitations still exist, requiring more research studies in this field. In this study, Gmax was investigated in a multistage test during several drying-wetting cycles and a loading-unloading cycle of net stress. The results revealed four key factors that directly influence the magnitude of Gmax: the void ratio, net stress, matric suction, and degree of saturation. Although variations of the void ratio, net stress, and matric suction cause persistent responses of Gmax (i.e., if all other factors remain unchanged, Gmax would then be reversely proportional to the void ratio and directly proportional to the net stress and matric suction), variations in the degree of saturation result in different responses. A decrease in the degree of saturation may induce a reduction or growth of Gmax because, on the one hand, it reduces the effect of matric suction, whereas on the other hand, it increases the total effect of van der Waals attractions and electric double-layer repulsions. At the same stress state, a reverse trend, induced by an increase in the degree of saturation, will occur with a growth in the effect of matric suction and a reduction in the combined effect of van der Waals attractions and electric double-layer repulsions. An analysis of the results showed that hydraulic hysteresis occurred in all the stress loops, and it directly influenced the response of Gmax. The effect of hydraulic hysteresis can only be captured if the van der Waals attractions and electric double-layer repulsions are considered. A model to estimate Gmax while incorporating the van der Waals attractions and electric double-layer repulsions was developed, and it was found to provide a good agreement with the experimental measurements.