Degradation of small-strain shear modulus of expansive soils under repeated wetting-drying cycles: experimental...

The study of the small-strain stiffness of soil is important for analyzing the deformation of ground and geotechnical structures. Nevertheless, the damage behavior and the predicted model of the small-strain shear modulus G_max for expansive soils subjected to multiple wetting-drying cycles have rarely been investigated. In this study, bender element tests were conducted on unsaturated expansive soils with different initial void ratios subjected to repeated wetting-drying cycles to elucidate the degradation behavior of the small-strain shear modulus. Subsequently, a new damage-based small-strain shear modulus prediction model for expansive soils was proposed and validated. The experimental findings demonstrate significant degradation in G_max of expansive soils under wetting-drying cycles, primarily following the first cycle. More importantly, a unified dependence of G_max on the evolution of matric suction is established across different cycles. X-ray CT analyses reveal the underlying microstructural shift from a particle-dominated fabric to a crack-connected network. Based on this, a damage-based model is proposed that links G_max degradation directly to the air-entry value evolution of the soil-water characteristic curve. The model is validated with a prediction determination coefficient exceeding 0.90. Its key strength lies in the ability to predict G_max without requiring prior knowledge of the number of wetting-drying cycles, providing a practical and reliable tool for engineering assessments under uncertain cyclic histories.