Application of the strength reduction method in coal mine roof support design

Ground falls represent a significant proportion of injuries and fatalities in underground coal mines in the US. During 2013, ground falls were responsible for 4 of the 14 fatalities and 16.6% of the 1,577 reportable lost-time injuries. In addition, each year about 400 to 500 large roof falls are reported that can extend up to or above the bolted horizon. Support design for coal mine entries is largely based on past experience and a trial-and-error approach. A numerical model-based approach for support design is presented in which calibrated models are used to determine a stability factor for a supported entry. The stability factor is determined using the strength reduction method (SRM). Applying this technique, the relative merits of various support systems can be evaluated. The numerical models allow the contribution of individual support units to overall stability to be assessed. Two case histories are presented. In the first case the SRM approach is applied to assess the use of passive cable anchors as primary support in a room-andpillar coal mine. The second demonstrates how the SRM approach was used to evaluate the impact of angled bolts at the rib-roof corner on roof stability. It is concluded that the SRM approach provides useful information to assess the overall degree of stability achieved by a support system, and allows support elements to be optimized for particular geological and stress conditions.