Impact of hydraulic hysteresis on the small strain shear modulus of unsaturated sand

The results of previous studies on silt and clay indicated that variations in the small strain shear modulus, Gmax, during hydraulic hysteresis had a non-linear increasing trend with matric suction, with greater values upon wetting. However, due to differences in material properties and inter-particle forces, a different behavior is expected for the Gmax of unsaturated sand. Although considerable research has been devoted in recent years to characterizing the behavior of the Gmax of sand during drying, less attention has been paid to the effect of hydraulic hysteresis on Gmax and its variations during wetting. In the study presented herein, an effective stress-based semi-empirical model was developed to predict the variations in the Gmax of unsaturated sand during hydraulic hysteresis. The proposed model incorporated the impact of the possible changes in volume through an empirical void ratio function as well as the effect of the degree of saturation through the use of suction stress. The effective stress was also defined using the concept of suction stress. The efficiency of the proposed model was evaluated by comparing the model predictions with the results of an experimental testing program involving the measurement of the Gmax of sand with different grain size distributions during hydraulic hysteresis. Specifically, a suction-controlled triaxial testing device, equipped with a pair of bender elements, was used to define the hysteretic trends in Gmax for different values of mean net stress. The model was found to provide satisfactory predictions of the trends in Gmax with matric suction, as well as its peak value and the suction corresponding to the occurrence of the peak Gmax. It also provided satisfactory predictions of the variations in Gmax upon subsequent wetting.