Three dimensional discrete element modeling of granular media under cyclic constant volume loading: A micromechanical perspective

Abstract

Discrete element modeling is employed to investigate the micromechanics of two granular assemblies subjected to constant-volume cyclic loading. For this purpose, two assemblies of spherical particles are modeled at the same confining pressure but with two different void ratios. The cyclic behaviors of the assemblies are inspected and the micromechanical parameters and their variations during cyclic loading are carefully observed and analyzed. The evolution of contact force networks with the progression of the loading cycles confirms that the contact force networks are hysteretic and their formation depends on the previous strain conditions of the assemblies. The distributions of the contact normals and their normal forces are also investigated to obtain a quantitative insight of the changes in the contact force networks. The probability distributions of the normal and tangential forces during cyclic loading are similar to the results of previous experimental studies that were conducted on two-dimensional specimens of granular materials. In addition, variations of the fabric tensors, which were calculated for strong contacts, are studied to trace the changes of the structural anisotropy of the specimens. The results suggest that the structural anisotropy of the specimens increases dramatically when they approach the state of liquefaction and that the degree of anisotropy is more profound in the strong contacts. Finally, the displacements of the particles during specific loading cycles are calculated to determine the relation between the movements of the particles and the changes in the macro-scale behavior of the two assemblies. The results of this study elaborate the origin of liquefaction phenomena with respect to the microstructure of the granular soils, showing the role of different mode of contacts failure in micro-scale (sliding and rolling) on the overall observed behavior of granular soils with two different relative densities, moreover the importance of strong and weak contacts in cyclic constant-volume loading of the media. It also emphasizes on the variation of structural anisotropy in undrained cyclic loading of granular media and its relationship with common soil behavior in macro-scale during liquefaction failure.