Anisotropic index strength behavior and failure mode validation of weathered shale

The failure mode limitations presented in the International Society for Rock Mechanics (ISRM) were recognised as an obvious issue by invalidating most of the failure modes along the bedding plane. In this study, the anisotropic behaviour of index strength and failure mode of weathered shale are evaluated by utilising the point load test (PLT) and the block punch index (BPI). The outcomes elaborate that the degree of anisotropy of PLT is four times the BPI degree of anisotropy. The failure mode of weathered shale is observed to be controlled by the bedding plane orientation (β). Particularly, when the point loading is not perpendicular to the bedding plane orientation, it is necessary to consider the inclined failure mode along bedding since eliminating these results may cause a significant over-prediction of index strength. Meanwhile, the presence of secondary failures along a bedding in weathered shale under BPI is because of the normal stress distribution. Consequently, the very low bonding between the bedding planes is not able to resist the tensile stresses developed at the specimen faces during punching. The study also highlighted the effect of the horizontal angle of bedding orientation (θ) with respect to the puncher.