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. A micromechanical-based failure criterion is developed to predict the anisotropic shear strength of granular soils under direct shear loading. The approach decomposes the total shear strength into an isotropic frictional component, governed by interparticle sliding at the critical state, and two distinct anisotropic contributions: inherent and induced anisotropy. Inherent anisotropy-associated with the long-axis orientation of particles-is represented through a sinusoidal function defined by the angular deviation between the mobilized shear plane and the bedding plane. Induced anisotropy is captured by the intensity of contact normals, and the non-coaxiality between contact-normal orientation and the loading direction. The Spatially Mobilized Plane (SMP) concept is introduced to relate stress direction to fabric orientation and to determine the effective shear-plane angle governing strength. The resulting formulation provides a physically grounded, easily calibrated equation requiring only two additional strength conditions beyond the critical-state frictional parameters. The model is validated against experimental data for various sands exhibiting different fabrics, gradations, and initial void ratios. The predictions show strong agreement with measured shear strengths across a wide range of bedding angles, demonstrating that the proposed criterion successfully captures directional dependence of peak strength arising from both inherent and induced anisotropies. The framework provides a rigorous and practical basis for integrating micromechanical fabric descriptors into constitutive modeling of granular soils.