Experimental investigations on mechanical and failure behaviors of transversely isotropic shale containing twin fissures under true triaxial stresses

Carbonaceous shale is the common carrier of underground engineering, and its strength assessment and failure behavior prediction are essential for the analysis of engineering stability. Carbonaceous shale naturally contains defects in different forms such as bedding plane and fissures, acting as the potential sources for the crack development and failure. To investigate the shale behaviors in underground engineering, true triaxial compression experiments are performed on transversely isotropic shale samples containing twin fissures, and the mechanical properties and failure characteristics are the main concern. Results show that the strength difference of shales with the same bedding angle under different s(3) directions, respectively, reaches 33.34 MPa and 9.63 MPa, and the strength difference with the same s(3) direction under different bedding angles attains 43.8 MPa and 20.09 MPa. A new mechanism of the "effect of s(1)-defect-coupling-induced fracture" is proposed to interpret the s(1)-dominated crack damage, the fissure-dominated crack damage, and bedding plane effect. For the shale, the shear mechanism is dominated during the initial loading stage, while the tensile mechanism is progressively enhanced as the loading proceeds toward the failure. Moreover, nine crack types and five crack coalescence modes are found in current true triaxial experiments. This study provides the experimental basis for the evaluation of the shale behaviors in response to a truly three-dimensional stress circumstance.