Shear strength and shear bands of anisotropic sand

Due to sedimentation, irregular particles of sand arrange anisotropically in nature. Anisotropy diversifies the internal friction angles between relative research planes at different directions in soil material. Thus, to analyze the mechanical mechanism of strength anisotropy, except for conventional inherent anisotropy of the material, stress distribution caused by external loads should also be considered in addition. The level and direction of the inherent strength anisotropy of geomaterials can be measured by a modified anisotropic variable, which is derived from the stress tensor and the fabric tensor. It is assumed that the overall shear strength of the material depends on the shear strength of the most mobilized plane, which is the spatially mobilized plane (SMP) and has a closely relationship with the stress state according to the SMP criterion. Therefore, the orientation of the most mobilized plane builds a bridge that connects the anisotropy of geomaterials and the magnitude and direction of stress. On this basis, a general description of anisotropic strength, considering both the magnitude and directions of the principal stresses, is given, of which the parameters can be easily obtained by triaxial tests. Besides, the variation of the sliding plane along with the direction of the major principal stress is approached. Furthermore, three types of tests, including the direct shear test, the true triaxial test and torsion shear test, were performed to prove the reliability of the new criterion. It shows that both the anisotropic shear strength and the orientation of the sliding plane can be predicted accurately by the newly built anisotropic strength criterion.