EXCAVATION DESIGN FOR BENCH STOPING AT MOUNT-ISA MINE, QUEENSLAND, AUSTRALIA

Mount Isa mine is one of the largest underground mines in Australia an d a major world producer of copper, silver, lead and zinc. A descripti on is given of new developments related to mining methods, excavation design and rock mechanics practices in the silver-lead-zinc operations , which are locally referred to as the 'Lead mine'. Since early 1991 a ll cut-and-fill operations in the Lead mine have been replaced by benc h stoping. The method has been successful largely owing to advances in the understanding of unsupported hanging-wall behaviour, backfilling technology, ground support, drilling and blasting practices and the ap plication of remote-mucking technology. The work described is concerne d with assessment of the stability, performance and behaviour of unsup ported openings in bench stoping. An account is given of the developme nt and application of a new method of bench-stability assessment that links ground conditions and excavation geometry for each of the orebod ies. The method is used as a predictive tool by mine planning to optim ize bench-block designs. Greater bench heights are planned for stronge r orebodies, giving a significant reduction in sill development costs. Alternatively, if a particular bench height is fixed, the stability c hart that has been developed is very useful during bench extraction, w hen it can be used to calculate the maximum unsupported stable length. The 'hanging-wall stability rating' (HSR) method, as it has been term ed, assumes that geological discontinuities, induced stresses, blast d amage and excavation geometry are the main factors controlling hanging -wall stability. The most economical extraction sequences have been ac hieved in some weak orebodies, where the method is used in conjunction with continuous dry fill and narrow, downdip pillars.