Shear band formation in ideal dense sand in direct shear test by discrete element analysis

In order to analyze the shear behavior of sands, a numerical investigation was carried out on the shear band of ideal dense sand in direct shear test using PFC2D. The traditional band observation method is replaced by a new method, by which the sample is painted into square grid to observe sample deformation pattern. The discrete elements commercial software PFC2D is improved to visualize principal stresses and their inclinations in the sample. Averaged pure rotation rate(APR) and particle velocity are observed to describe the sample with aim to analyze the micro mechanism of the strain localization in dense sand. The study shows that in direct shear test, the strain is concentrated in a band near the shear plane, whose thickness is 10 to 15 times of d(50). The particles in the shear band demonstrate significant rotation. Higher gradients are found in velocity and rotation fields on the edge of the shear band. The observation on the contact fabric and contact force shows that principal directions of contact points and contact forces rotate in the similar way with their inclination direction about 60 degrees to the horizontal direction.