Discrete Element Method Simulations of the Critical State of a Granular Material

Abstract

The critical state of a granular material was investigated by performing cubical triaxial simulations using the discrete element method. These samples, assemblages of ellipsoids of two kinds, were prepared either by settling the randomly generated particles under gravity or by compressing these particles without gravity. Gravity was set to zero and the samples were further consolidated isotropically. Then, numerical drained tests were carried out until the critical state was reached. Macroscopic and microscopic data in the critical state were examined. The result shows a unique critical state line in the void ratio-mean stress space regardless of the initial conditions. The obliquity (ratio of major and minor principal stresses) as a function of strain is very similar for these samples at different confining pressures. The most interesting result is the linear critical state line in the void ratio-mean stress space. Three microscopic parameters including particle orientation, branch vector, and contact normal force were examined. At critical state, the long axes of most ellipsoids are perpendicular to the major principal stress direction, the distribution of branch vectors is random, and the distribution of contact normal forces shows a concentration along the major principal stress direction. The micromechanical descriptor based on contact normal forces is closely related to the macroscopic variable, obliquity.