Liquefaction resistance of anisotropically consolidated sand under monotonic and cyclic undrained torsional shear

The stress-induced anisotropy of sand significantly affects the liquefaction susceptibility. This study systematically investigates the undrained behavior of saturated sand under both monotonic and cyclic loading through a series of torsional shear tests, considering the effects of initial anisotropic consolidation states, including extensional and compressional consolidation states. Special attention is paid to the evolution of effective stress paths, deformation pattern, pore water pressure generation, and the rotation of the principal stress direction during the torsional shear process. The experimental results show that specimens under different initial stress states and loading conditions exhibit two typical failure modes, i.e., residual strain accumulation and cyclic mobility. The evolutions of principal stress direction under both monotonic and cyclic loading are presented, which can provide useful insights into the underlying mechanism of the occurrence of different failure modes. Based on the experimental results, a new index, unified cyclic stress ratio (USR), is proposed by correlating the number of cycles required for failure (Nf) with the initial anisotropic stress state and the shear strength at the phase transformation point. The proposed index USR can serve as a unified criterion for the evaluation of liquefaction resistance of sand considering anisotropic consolidation conditions.