Research paper DEM-enabled exploration of spatial inhomogeneities during the cyclic torsional shear of sand

Abstract

Cyclic torsional shear of a poorly graded sand was simulated with LIGGGHTS using three particle shape representations: spheres with rolling friction, ellipsoids and multi-sphere clumps. Templates describing the particle shapes were obtained from la-CT scans of the sand. The torsional shear apparatus was simulated in DEM with two meshes to represent the top and bottom end-caps. The radial confinement was imposed by a set of spherical particles, initially configured as a hollow cylinder. Each of these boundary particles moved independently along a constrained radial path to impose a specified stress. This emulated the flexible membrane used in the laboratory apparatus. That laboratory apparatus was used to carry out three sets of cyclic torsional shear tests at a confining pressure of 100 kPa, yielding material stiffness degradation and damping curves. Calibration of the DEM focused on the Young’s modulus of each shape representation; the Young’s moduli that were appropriate for direct shear tests at large displacements were not valid in this low-strain regime. The bulk behaviour after calibration was captured reasonably although the damping was overestimated at higher strains. Spheres with rolling friction accrued substantially more cumulative rotation than ellipsoidal or multi-spherical particles. Fabric changes were minimal due to the low strains imposed. Contrary to the continuum mechanics theory of linear elasticity, the particle displacements were much larger close to the moving upper boundary than elsewhere. The proportion of sliding contacts and energy dissipation were also much higher in this region where most of the reduction of the shear modulus occurred.