Noncoaxial Behavior of Sand under Various Stress Paths

In this paper, the results of three series of drained tests carried out on sands using hollow cylinder apparatus are presented. The noncoaxiality, defined as the difference between the major principal stress direction and the corresponding principal strain increment direction, is investigated. In the first series of tests, the sand was isotropically consolidated before being sheared with the fixed principal stress direction. In the other two series of tests, the sand specimens were isotropically consolidated and then sheared by rotating the major principal stress axes while the deviator stress level was either fixed (pure stress rotation) or increased continuously (combined shear loading). The experimental results provide clear evidence for material noncoaxiality when the rotation of principal stress direction is involved. The results from these series of tests show that the degree of noncoaxiality depends on the level of deviatoric stress and the stress increment direction. It tends to decrease when the specimen is approaching a failure state. It was also observed that the effect of soil density on noncoaxiality is more significant at lower shear stress levels. Test results on two different materials, Portaway sand and Leighton Buzzard sand, were also compared with the study of the influence of material anisotropy associated with sand particle characteristics. Numerical simulations of granular materials using the discrete element method (DEM) were conducted to understand the effect of the initial material anisotropy produced during sample preparation. The DEM results were consistent with those obtained from the hollow cylinder tests.