Coal mine roof rating (CMRR), rock mass rating (RMR) and strata control: Carborough Downs Mine, Bowen Basin, Australia

Abstract

The rock mass rating (RMR) has been used across the geotechnical industry for half a century. In contrast, the coal mine roof rating (CMRR) was specifically introduced to underground coal mines two decades ago to link geological characterization with geotechnical risk mitigation. The premise of CMRR is that strength properties of mine roof rock are influenced by defects typical of coal measures stratigraphy. The CMRR has been used in longwall pillar design, roof support methods, and evaluation of extended cuts, but is rarely evaluated. Here, the RMR and CMRR are applied to a longwall coal mine. Roof rock mass classifications were undertaken at 67 locations across the mine. Both classifications showed marked spatial variability in terms of roof conditions. Normal and reverse faulting occur across the mine, and while no clear relationships exist between rock mass character and faulting, a central graben zone showed heterogeneous rock mass properties, and divergence between CMRR and RMR. Overall, the CMRR data fell within the broad envelope of results reported for extended cuts at Australian and U.S. coal mines. The corollary is that the CMRR is useful, and should not be used in isolation, but rather as a component of a strata control programme.