Discrete element simulations of drained granular material response under multidirectional rotational shear

Abstract Multidirectional shearing tests on soils in the laboratory suffer from drawbacks such as non-uniformity of stress and unknown horizontal stress of the specimen. As a promising numerical technique, the discrete element method (DEM) enables investigation of the responses of virtual granular samples under general loading conditions and provides ideal numerical results to overcome the aforementioned limitations. In this study, three different drained cyclic tests, including conventional rotational shear, bidirectional circular rotational shear, and bidirectional elliptical rotational shear, were conducted on both isotropic and transversely isotropic samples under various stress ratios and density conditions. In all these tests, the magnitudes of the three principal stresses were kept constant during the entire loading process. Principal stress rotation occurred in one deviatoric stress plane in a conventional rotational shearing test but spatially (not necessarily in one plane) in the other two bidirectional shearing tests. The normalized deviatoric strain increment was decomposed into (1) coaxial and proportional, (2) coaxial and orthogonal, and (3) totally non-coaxial parts in reference to the deviatoric stress tensor. Their evolutions are presented and discussed in detail herein. The internal structure quantified by the contact-normal-based fabric tensor was tracked, and its correlation with the imposed stress path is illustrated.