Description of Inherent and Induced Anisotropy in Granular Media with Particles of High Sphericity

The primary objective of this study was to demonstrate that a significant degree of anisotropy may occur in particulate materials that have nearly spherical aggregates (i.e., are typically considered as isotropic) provided there is a bias in the distribution of pore space as a result of the initial densification process. The study included both an experimental and a theoretical component. An experimental program was conducted on dynamically compacted samples of Ottawa standard sand (C109) and incorporated a series of tests performed at different orientations of the specimens. Following the experimental part, a plasticity formulation based on the critical-plane approach was presented. A systematic procedure for identification of material parameters was outlined, and some numerical simulations of a series of triaxial tests were conducted. The last part of this work dealt with a much weaker load-induced anisotropy. Here, the mathematical framework was enhanced by an evolution law for the material fabric. Again, some numerical simulations were conducted, and it was demonstrated that the proposed framework can, at least in a qualitative manner, account for various manifestations of induced anisotropy.