Investigation of the failure mode of shale rocks in biaxial and triaxial compression tests

Abstract

High horizontal in-situ stress and weak sedimentary laminated roof rock can severely affect underground coal mine roof stability. These sedimentary rocks possess planes of weakness along the horizontal direction in a mine roof and delaminate easily when acted upon by high lateral stresses. Empirical studies have shown that the magnitude of these stresses can be as much as two to three times the local overburden stress. The resulting ground control problems (buckling, cutters, etc.) in such conditions are quite challenging for mining engineers. This paper describes the failure observed in coal mine shale rocks under biaxial and triaxial stress conditions. To carry out the investigation, special platens were fabricated that are capable of applying biaxial compressive stress on a cubic rock specimen when the entire arrangement is used inside a uniaxial compressive loading device. This experimental set-up was further modified to apply a pseudo-triaxial compressive stress. Laminated shale specimens tested under biaxial stress condition showed tensile failure along the laminations at macrolevel. Black shale specimens showed extreme brittle failure. Limestone specimens tested under similar conditions failed violently and it was concluded that failure observed in each rock type tested was unique and not an artifact of the experimental design. The pseudo-triaxial conditions reduced the influence of laminations on the failure mode. Laminated shale specimens were found to fail along multiple shear planes.