Factors predictive of roof instability in addition to the existing CMRR criteria at two case study coal mines

This work uses two underground longwall coal mines located in the Western US (Mines A and B) as case studies to investigate which parameters are indicative of roof fall potential, and whether the Coal Mine Roof Rating (CMRR) is predictive of stability at these mines. Data were collected at 30 sites in each mine. These data sets included the CMRR, a record of the roof stability and a series of non-CMRR parameters thought to also be potentially indicative of roof stability but which are not included in the CMRR. These data were then statistically analyzed for correlation between CMRR and roof stability. The CMRR was found to be moderately effective at Mine A but not effective at Mine B. At Mine A, faulting was found to be the primary control on roof stability. Topographic curvature, slope angle and depth of cover were found to have reasonable predictive capability for assessment of roof stability category at Mine B based on a logistic regression analysis. These differences in roof stability controls are likely due, at least in part, to the unusual topography above Mine B, with differential erosion resulting in a landscape of flat plateaus and sharp river valleys. It is likely that these sudden changes in slope and topography lead to in-situ stress rotation and the development of shear stresses near the excavation at Mine B. This, combined with a lack of major discontinuities such as slickensides, which are central to the CMRR system, explains why the CMRR is much less effective at predicting roof stability at Mine B compared to Mine A.