Abstract: Bedding structural planes play a dominated role in the failure process of carbonaceous slate under loading/unloading conditions. In this work, specimens with the clay mineral rich carbonaceous slate were sampled from Muzhailing tunnel of Shaanxi province in China. A series of Brazilian split tests for the rock specimens was conducted and the corresponding numerical simulations were performed using the cohesive zone model (CZM), in which different bedding angles in the rock were taken into account. This study indicates that the load-displacement relations of the rock specimens with different bedding angles under loads are similar to each other. All of them demonstrates identical basic characters that includes the initial stage, the rock specimens compaction, nearly linear elasticity, and finally complete collapse. The tensile strength of different specimens with bedding angles of 0o, 30o, 45o, 60o, 90o and divider are 1.59, 1.12, 0.89, 0.76, 0.66, 1.65 MPa, respectively. The anisotropy coefficients are 2.41 and 2.50 (divider), and the deterioration of such tensile strength is significantly related to the intrinsic defects and damage due to the water-rock effects. Furthermore, the failure modes and patterns of the rock specimens strongly depend on bedding structures, which can be classified as approximately pure tension failure, shear failure, and mixed tension-shear failure. Additionally, the three-dimensional numerical results show that the predicted split failure processes of carbonaceous slate by the CZM model are in a fair agreement with the results of laboratory tests. Therefore, the fracture mechanical parameters of CZM are valuable for similar tunneling engineering as a reference.

Key words: carbonaceous slate, bedding, tensile strength, failure mode, cohesive zone model