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

Abstract

Roof falls remain one of the greatest hazards facing underground coal miners (Barczak et al., 2000; Razani et al., 2013; Oraee et al., 2016). In 2017 there were 91 lost-time injuries from roof falls (in US underground coal mines). A further 48 roof falls were reported in US underground coal mines with no lost days (MSHA, 2018). These numbers have certainly decreased over the last century (MSHA, 2018), but the goal of zero injuries still remains. Assessing the likelihood of roof falls is therefore highly important and will have a direct effect on the prevention of accidents caused by them. One method developed to help assess roof instability in underground coal mines is the Coal Mine Roof Rating (CMRR). The CMRR is a field-based empirical method which is straightforward to use and gives a quantitative interpretation of coal mine roof geology. The CMRR classification system was developed by Molinda and Mark (1994) to quantify the geological description of mine roof into a single value which could be used in engineering design. It provides an excellent starting point, but it does not necessarily include all the factors that may influence roof stability, nor is it widely used in the Western US. This thesis research uses two underground longwall coal mines located in the Western US (Mine A and Mine B) as case studies to investigate which parameters are indicative of roof falls at these mines. It also evaluates whether the CMRR is applicable to them, and if not, why this might be. A data set was collected at 30 sites in each mine. This data set 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 correlation between roof stability and the non-CMRR parameters collected was also evaluated. To further evaluate how influential each parameter already included in the CMRR is at each mine, each constituent of the CMRR was removed in turn and a modified CMRR was calculated. This modified CMRR was then evaluated for correlation with roof stability. At Mine A, the correlation between the CMRR and roof stability was found to be statistically significant (significance threshold α = 0.05), with a p value of 0.0073. Logistic regression analyses showed the CMRR to be reasonably predictive of roof stability at Mine A. Faulting, along with depth of cover and slope angle of surface topography were found to be the most significant non-CMRR parameters to correlate with roof stability at Mine A. At Mine B, the correlation between the CMRR and roof stability was not found to be statistically significant (p value = 0.95) against a significance threshold of α=0.05. The logistic regression analyses also showed the CMRR to have little predictive capability on roof stability at Mine B. At Mine B, location at an intersection and depth of cover were found to be significantly correlated with roof stability. The CMRR is therefore moderately effective at Mine A but not effective at Mine B. This is thought to be 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 suggested 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, likely explains why the CMRR is much less effective at predicting roof stability at Mine B compared to Mine A. Mine A was also found to more closely match the geological conditions of the mines in the CMRR reference database than Mine B. The majority of the coal seams sampled in the CMRR reference database are located in the Appalachian or Illinois basins. The Appalachian Basin is a foreland basin with a complex geological structure and a high incidence of faulting. This is similar to the regional geology at Mine A. The Illinois basin as a whole more closely matches the geological setting at Mine B; both are located in broad, gentle structural depressions. However, the Illinois Basin coal seam most frequently sampled in the CMRR reference database is one with notable faulting and a roof geology which is complex and laterally inconsistent. This is the opposite of the geological conditions in the roof at Mine B, which are laterally uniform and continuous. It is likely that the CMRR is not applicable at Mine B because the geological conditions there are not captured in the CMRR reference database.