Impact of Effective Stress on the Dynamic Shear Modulus of Unsaturated Sand

The dynamic shear modulus of soils is needed to predict soil behavior in response to cyclic loading. Even though the effective stress has been shown to have a significant impact on the dynamic modulus of water-saturated and dry soils, its effect on the dynamic shear modulus of unsaturated soils has not been evaluated. Specifically, studies on the dynamic response of unsaturated soils have characterized variations in small-strain shear modulus (G_max) as a function of the degree of saturation or matric suction alone. In contrast, this study evaluates the use of the suction stress characteristic curve to characterize the impact of mean effective stress (σ'_m) on the dynamic shear modulus of unsaturated sand. A fixed-free resonant column test device was adapted with a hanging column setup so that the small-strain dynamic shear modulus could be measured for sand specimens under different confining pressures and matric suction values. Trends between the small strain shear modulus and effective stress for unsaturated sand were found to be different from those reported in the literature, where G_max varied linearly with the square root of σ'_m.