Particle elongation and deposition effect to macroscopic and microscopic responses of numerical direct shear tests

Abstract

In this study, a series of numerical direct shear tests is carried out by the three-dimensional discrete element method. The box is filled by either spherical or elongated particles of mono-size. Particles of three different aspect ratios (defined as length/width of a particle), 1 (i.e., spherical), 1.5 and 2, are modeled. Elongated particles are created by joining primary spherical balls together, and no particle breakage is allowed. The granular specimen is prepared by either depositional method or by random generation of particles inside the box. By controlling the interparticle friction coefficient, number of particles and deposited direction, particle assemblies with very close initial density but different packing or micro-structure can be obtained. Various measurement spheres are defined at different locations of the box to reveal the local stresses by considering interparticle interaction forces. The results show a significant spatial variations of the stresses, which deviate noticeably from the global measurements recorded at the box boundaries. Furthermore, global measurements appreciably conclude higher ultimate strength of the assemblage as compared to the local ones from the measurement spheres, regardless the particles' aspect ratio and packing. The ultimate shear strength increases with particles' aspect ratio. Initial fabric affects the ultimate shear strength such that the assemblage having more particles aligning parallel to the shear direction (DepS) yields the lowest strength. On the other hand randomly packed assemblage exhibits the highest strength. Furthermore, DepS specimen shows the least amount of dilation. Particle orientation is described by a tensorial parameter, and its evolution during shear is discussed. Analysis shows that only particles close to the shear plane exhibit significant rotation and thus a noticeable change in the fabric. It is found that the evolution of fabric tensor is closely linked to the macroscopic response of an assemblage. Fabric analysis helps to explain the macroscopic responses from a microscopic particle rearrangement perspective. Copyright © 2010 by ASTM International.