Sand Behavior under Stress States Involving Principal Stress Rotation

The behavior of sands exhibiting both unstable and stable response, in their loose deposited state, under axial-torsional shearing involving continuous principal stress rotation is investigated using the hollow cylinder apparatus. This paper examines the parameters affecting the major principal stress direction attained at instability and/or phase transformation during torsional shearing following anisotropic consolidation. It is shown that constant stress ratio ($t/p^{\prime }$) lines, including the instability and phase transformation lines, are associated with the same major principal stress rotation with respect to the vertical within a wide range of initial mean effective stresses along the same consolidation stress ratio, $K_c$. In sands exhibiting instability, smaller principal stress rotations are required for the mobilization of the effective stress ratio at the onset of instability as the initial shear stress level increases ($K_c$ decreases). In sands exhibiting stable response, principal stress rotation at phase transformation increases with increasing dilatancy tendencies. The dependence of the angle of shearing resistance, ${\phi }^{\prime }$, mobilized at instability (IL), phase transformation (PTL), and failure (FL) lines on principal stress rotation and the intermediate stress parameter, $b$, is examined to verify whether the mobilized angle of shearing resistance can be considered as a material property. In continuous rotation tests, contrary to fixed principal stress direction and $b$ tests, the angle of shearing resistance at IL can be considered as material property. However, the angle of shearing resistance at PTL depends on $b$ and the direction of the principal stress. Moreover, phase transformation takes place at lower stress ratios as density increases.