Small-strain stiffness of sand subjected to stress anisotropy

Stiffness of soils at small strains expressed through the small-strain shear modulus is critical for the evaluation of deformations of geo-structures subjected to a variety of stress states. While most of the previous studies of small-strain shear modulus of sands have focused on the isotropic stress state, there exist innumerable situations in geotechnical engineering in which the soil is under an anisotropic stress state. In this study, the influence of stress anisotropy on the small-strain shear modulus (G_max) of sands is evaluated using the results of a comprehensive set of bender element tests conducted on saturated sand samples under isotropic and anisotropic loading conditions. It is shown that the small-strain shear moduli of sands under anisotropic loading conditions are greater in magnitude than those subjected to isotropic stress states at a given mean effective stress. It is also shown that the influence of stress anisotropy on the small-strain shear modulus of sands is more pronounced for sands with irregular in shape grains and wider grain size distribution in comparison to uniform sands of relatively rounded and spherical grains. Based on the experimental results, a new G_max model is developed which incorporates the contribution of grain size characteristics and particle shape in the prediction of the small-strain shear modulus of sands subjected to stress anisotropy.