Theoretical and experimental investigation of a shear failure model for anisotropic rocks using direct shear test

This study proposes a theoretical model for predicting the shear strength of anisotropic rocks in which failure is dependent on the inclination of the inherent structural weak planes. For this purpose, triaxial and direct shear tests were conducted on dried and saturated shale samples. Samples were cored at different orientations (β) to the inherent structural weak planes, i.e., bedding planes. To verify the anisotropic behavior of the samples, triaxial tests were performed on samples with various inclinations of the bedding planes. The experimental results demonstrated a U-shape relationship between the compressive strengths and the inclination of the bedding planes, which is in accordance with Jaeger's model of the plane of weakness (JPW). Accordingly, apparent strength and elastic properties, such as Young's modulus, Poisson's ratio, cohesion, and internal friction angle were all observed to be dependent on β, which verified the mechanical anisotropy of the shale samples. Many of the existing failure criteria consider the impact of bedding inclination on failure strength based on uniaxial and triaxial conditions. However, it is also important that the failure modes of anisotropic rocks are evaluated during shearing. For this purpose, a theoretical model was derived to relate shear strength to β following the Coulomb-Navier criterion. The relationship between shear strength and β was plotted and illustrated in an Inverted-U shape. Direct shear tests were conducted to validate the proposed criterion. Plotted experimental data confirmed the shape of the geometry determined by theoretical analysis. From the results, the maximum shear strength occurred when β was between 30° and 60° in direct shear tests. To obtain a more comprehensive understanding, changes in the shear stress-displacement curve and dependence of the friction angle, to the bedding plane orientation, β, were evaluated and indicated shear anisotropy behavior.