Leinster cave seismic risk management: a block cave solution

On 31 October 2013, production at Leinster’s sublevel caving (SLC) operations were suspended following a significant mining-related seismic event that caused unforeseen levels of damage to underground workings. The event occurred when mining was taking place in an area known as the 11 Level Fold, approximately 1,100 m below surface. An assessment of the seismic risk following the 2013 event determined that it was too significant to continue with the SLC operations. Therefore, the SLC operations were not restarted and alternate mining methods would need to be investigated to address the exposure of personnel to the effects of the seismic concentration associated with the 11 Level Fold area. The proposed solution avoids the zones of highest seismic activity by transitioning to a block cave mining method and locating a production footprint at a level where interaction with the previous SLC and the 11 Level Fold can be avoided. This design is based on an advance undercut and excavation of 22 drawbells. Due to a significant regional shear that vertically bisects the orebody, it is expected that a ‘chimney’ cave will develop, thereby connecting with the previous sublevel cave column. The design will maintain critical infrastructure, such as the existing shaft, and use the current decline infrastructure to access the new production level. This work shows how high seismic risk can be managed by defining seismic exclusion zones and employing a mine design and mining method that best accounts for the geotechnical environment. Keywords: seismic risk, sublevel caving, block caving, mine design