Evolution of granular media under constant-volume multidirectional cyclic shearing

By means of the three-dimensional discrete element method, we study the long-time evolution toward liquefaction state in granular materials composed of spherical particles under multidirectional cyclic shearing at constant volume. Extensive simulations were carried out along 1-D linear, 2-D linear, circular/oval, and 8-like shear paths, and the evolution of the system was analyzed in terms of pore pressure, shear strain, and granular texture. The macroscopic stress path and stress–strain response agree well with laboratory experiments. We find that the liquefaction resistance, i.e., the number of cycles necessary to reach the liquefaction state, is generally lower under multidirectional loading as compared to unidirectional loading. As the transient vanishing of mean stress does not occur for all stress paths, we introduce a shear strain-based liquefaction criterion that can be consistently applied to all strain paths. The granular texture is monitored through the coordination number, particle connectivity, force and fabric anisotropies, and friction mobilization. In particular, a particle-void descriptor, named centroid distance, is found to be closely related to the shear strain accumulation. We show that the force anisotropy tensors become almost proportional to the deviatoric stress tensor more quickly than the fabric anisotropy tensor, which takes most of the pre-liquefaction period to follow the external loading. The relationship between deviatoric stress ratio and the force and fabric anisotropies, known to hold in monotonic triaxial loading, also holds with high accuracy in the studied multidirectional cyclic shearing paths; the contributing weights of the anisotropies level off in the post-liquefaction period and do not depend on the shear path.