Strength of an Australian coal under low confinement

Experimental testing of brittle rocks has shown that both brittle and ductile behaviours can be observed, depending on the level of confinement applied to the specimen. In particular, brittle rocks fail in a brittle mode as long as the confining stress falls below the Mogi line (Mogi 1966). Spalling of rocks is associated with brittle failure and is known to occur under low confinement, i.e. in the vicinity of excavation walls (Stacey 1981; Martin et al. 1999; Cai and Kaiser 2013). Indeed, at low confinement, large tension cracks may develop parallel to the excavation boundary when the stress exceeds the crack initiation threshold, which may lead to rapidly propagating instabilities and formation of thin slabs. Such slabs can represent a significant hazard to the workforce in confined mining excavations. Increasing the level of confinement modifies the nature and propagation mechanism of the cracks that develop upon loading: at high confinement, short shear cracks develop a ...