Rock instability and risk analyses in open stope mine design

Modern underground mining methods often call for the excavation of large, often unsupported voids called stopes. In many cases, these are nonentry stopes, with the removal of broken rock (mucking) performed by remote control. With the risk to human safety thus reduced, stability concerns within the stopes are primarily driven by the economics of unplanned overbreak and waste rock dilution of the mined ore. In this context, it is appropriate to speak of acceptable risks of instability and to optimize design by balancing risk costs with productivity potential. Larger individual stopes generally increase productivity while reducing stability. The modified stability graph is a popular empirical mine design tool that has been calibrated to provide discrete recommendations for maximum stope dimensions based on a rock quality index. It seems appropriate to extend the method to account for variability in the calibration data and also for variability in each new set of input parameters. A risk template and associated probabilistic techniques are proposed to supplement the discrete design limits currently established.