The mechanical behaviour of pre-existing transverse cracks in lignite under uniaxial compression

Abstract

Intermediate geotechnical materials such as Victorian Brown Coal (VBC) often exhibit complex fracture characteristics due to the interplay of ductile and brittle behaviour. A diverse set of in situ joint orientations and fracture characteristics are found in the open-cut mines of the Latrobe Valley, Victoria, Australia. As such, the propagation of pre-existing discontinuities plays a significant role in the potential failure mechanisms of open-cut mines in the region. Although field assessment has yielded a range of Victorian Brown Coal in situ joint characteristics, insufficient data exists to determine the relationship between the fracture orientation of VBC and joint compressive strength. To supplement available site investigation data, experimental results of uniaxial compressive tests on pre-fractured Victorian Brown Coal specimens performed under laboratory conditions are presented for various crack angles to determine the relationship between fracture angle and joint compressive strength. Fractured VBC is found to exhibit a reduced uniaxial compressive strength, with appreciably varied fracture propagation characteristics depending on the fracture initiation angle. Laboratory tests were conducted on soaked VBC specimens containing predefined cracks (for angles of 0°, 15°, 30°, 45° and 60° to the axis of symmetry) as well as intact coal specimens, with fracture propagation paths photographed along the specimen surface. Intact coal specimens under compression experienced brittle failure with crack propagation parallel to the direction of loading. For specimens with pre-existing fractures, the crack angle was observed to significantly affect the failure mode and the direction of crack propagation. Obtained results suggest pre-existing crack angles alter the shape of the stress–strain curve, with a range of unconfined compressive strengths and crack initiation strengths presented. It was noted that pre-existing crack angles of 45° yielded the minimum observed failure stress. Given the presence of desiccation cracks, micro-cracks and large-scale jointing within Victorian Brown Coal, compressive strength parameters for a range of joint characteristics are vital to ensuring the ongoing stability of the large open-cut mine slopes within the region.