Effect of Anisotropic Stress State on Elastic Shear Stiffness of Sand-Silt Mixture

Soil stiffness is a critical property in the evaluation of earth structure deformation and its accurate estimation is an essential part of any geotechnical investigation. In this study, the significant contribution of non-plastic silica fines content in the relationship between small-strain shear modulus and stress anisotropy of silty sands is investigated through a comprehensive set of bender element tests. All the samples of the study were created in the laboratory and tested in a stress path triaxial apparatus applying isotropic and anisotropic stress paths. The mixtures had variable contents of sand and a non-plastic silica silt. Data analysis demonstrated that the exclusive effect of fines content on the changing rate of elastic shear modulus due to anisotropic stress state turns out to be considerable and increasing for all the tested parent sands up to a specific silt portion, about 20%, after which this effect becomes negligible. Using the experimental results, the previously proposed expression for the prediction of the small-strain shear modulus under stress anisotropy for granular materials is extended to sand-silt mixtures. A comparison of the measured against the predicted elastic shear moduli under stress anisotropy, demonstrated that the concept of the skeleton (granular) void ratio could not be applied effectively in the evaluation of the dynamic behaviour of sand-silt mixtures.