Anisotropic Strength of granular soils in Direct Shear Test by Microstructure Analysis

Granular soils deposited under gravity naturally develop an anisotropic particle arrangement. Variations in loading direction relative to the particle bedding plane intensify the anisotropy of the shear strength. This paper presents a micro-mechanically based anisotropic strength criterion for granular soils under direct shear conditions. The anisotropic resistance, arising from the internal structure of particles relative to the weakest plane or shear band, is captured using physically meaningful and easily measurable parameters. The anisotropic component of shear resistance is shown to depend on the deviation between the bedding plane and the most mobilized plane, referred to as the Spatially Mobilized Plane (SMP). By introducing a frequency distribution function for contact normals, relative to the mobilized plane, a relationship is established between shear resistance,, the inter-particle friction angle, , and anisotropic parameters, based on a microscopic analysis of grain behavior. Two types of anisotropies are considered: one associated with contact normal, and the other with the orientation of particles long axes. Differences in shear resistance are attributed to both the degree of anisotropy, and the angle of deposition relative to the SMP. All parameters in the proposed criterion can be readily determined through laboratory testing. The validity of the formulation is confirmed through experimental results on four different types of sands.