Stress-state dependency of small-strain shear modulus in silty sand and sandy silt of Ganga

This study explores the effect of stress state on small strain modulus (Gmax) in the naturally occurring Ganga sand and silt using bender elements during a series of carefully performed undrained triaxial tests. Unlike the previous studies with drained shearing or testing of dry samples, the undrained shearing is chosen here to decouple the effects of void ratio and stress state on the small strain stiffness. The results from undrained shearing of anisotropically and isotropically consolidated medium dense sand specimens are compared by keeping the same relative density and matching the initial mean effective pressure. The measured Gmax values show stress path dependency, i.e., it is a function of both mean effective stress and deviatoric stress. This behaviour of Gmax is captured well by a nonlinear isotropic model following energy conservation principle, which incorporates the effect of both mean effective stress and deviatoric stress. The stress strain response of soil shows some interesting observations such as the strain hardening occurs mostly at constant effective stress ratio and the stress ratio at phase transition varies with the effective confinement. Experiments with varying silt content in the sand indicate that higher fines content results into slightly lower Gmax values mainly due to the lower values of mean effective stress and deviatoric stress during undrained shearing.Amit Prashant, Debayan Bhattacharya and S. Gundlapall