Investigation of Indirect Shear Strength of Black Shale for Urban Deep Excavation

This study thoroughly investigated the compressive and tensile strength characteristics of black shale using both experimental and analytical approaches. Uniaxial compression tests were conducted to determine the elastic constants of black shale modeled as idealized, linear elastic, homogeneous, and transversely isotropic. Additionally, Brazilian tests were carried out on shale, considering it a transversely isotropic material. Strain measurements were recorded at the center of disc specimens subjected to diametric loading. By placing strain gages at the disc centers, the five elastic constants were accurately estimated. The effects of experimental methods and diametric loading on the elastic constant determination were evaluated and analyzed, and the indirect shear strength of the black shale, considering anisotropy, was determined using the estimated stress concentration coefficient. This study revealed that the indirect tensile strength of black shale is significantly influenced by the angle between the anisotropic planes and the diametric loading direction. Moreover, it was revealed that the stress concentration coefficients for anisotropic rocks vary from those of isotropic rocks, depending on the inclination angle of the bedding planes. This study confirms that the shear (tensile) strength of anisotropic black shale is not constant but varies with the orientation of the anisotropic planes in relation to the applied load.