ABSTRACT
Since 2000, Xstrata – Mount Isa Mines has mined two key underground access and infrastructure drives through backfilled stopes that had been mined and filled 10 years previously. The stopes were mined through similar types of cemented backfill in each case, and the condition of the fill in both cases was relatively good. Conventional ground control (rock bolts) was not anticipated to be the most effective and efficient ground control technique to use to maintain the stability of the fill mass around the opening. A thin shotcrete lining was determined to be the most effective solution. The lining was designed to support the expected failures that may take place as well as assist in preventing the backfill from dehydrating at the exposed surface, leading to decay and weakening. Both examples are a success and are recounted in this paper. One was mined by drill and blast techniques, leading to excessive overbreak and use of large volumes of shotcrete product and the other used a roadheader to excavate the fill mass which enabled far greater profile and shotcrete material controls to be exercised. The financial benefits and costs of the techniques employed, the fill stability issues that arose, the design and the performance of the thin shotcrete lining are discussed. Further areas of investigation and application are also discussed with regard to the use of shotcrete linings in new projects in different fill types and in other underground areas of study.
