EFFECTS OF MINING-INDUCED SEISMIC EVENTS ON A DEEP UNDERGROUND MINE

The performance of an underground excavation at the Lucky Friday Mine that was subjected to repetitive episodes of mining-induced seismic ac tivities is assessed. Field measurements indicate that an underground excavation may respond to mining-induced seismicity with step changes in displacement of the rock mass around the excavation or step changes in excavation closures. However, step changes in displacement or clos ure were also observed without the presence of mine seismic events. Th is behavior may be explained using the concept of stick-slip on joints or bedding planes within the rock mass. The stick-slip mechanism is b elieved to be controlled by the state of stresses at a joint or beddin g plane. Through this mechanism, joint displacement is accumulated in a progressive fashion leading to a weakening of rock mass around an ex cavation. As a result, the excavation becomes less stable and therefor e is more susceptible to seismic impacts. The concept of using peak pa rticle velocity to assess damage threshold for underground excavations may not be conservative when considering the effect of rock mass fati gue.