Micromechanical behavior of granular materials in direct shear modeling

Direct shear testing is among the most frequently performed laboratory tests for determining the engineering properties of soil, particularly dry granular soils. This study employed the discrete element method to simulate a direct shear test to examine the microstructure and micromechanical behaviors of granular materials in dense and loose states. The analysis results show that a relationship exists between the microscopic and macroscopic properties of the simulated dense granular assemblies. Dilation of the dense granular material resulted from an increase in volume in the area along the shear plane. By contrast, contraction of the loose granular material resulted from a decrease in volume decrease throughout the granular sample. The dense materials exhibited axial compression and lateral extension stress paths in the middle of the shear plane. The loose materials exhibited stress path variations that were markedly more complex and varied. For the dense materials, the initial orientation of the major principal stress plane was approximately horizontal. During the test, the formation of a contact force chain caused the major plane to rotate counterclockwise until it was at approximately 45°-55° to the original, horizontal position.