Managing seismicity at the Tasmania Mine

Abstract

Mining induced seismicity was anticipated at the Tasmania Mine (formerly the Beaconsfield Gold Mine) before the commencement of modern production in 1999. Consideration of available stress measurement data from throughout Tasmania, coupled with rock material property data gathered at the Tasmania Mine during project feasibility investigations, suggested that the onset of mining induced seismicity could be expected at depth around 700 m below surface. That proved to be the case, with mining induced seismicity regularly observed from 760 m and below from 2002. Before 2006, management of seismicity was strictly controlled through geotechnical investigation and application of geotechnical control of all mining activities. The local stress regime was measured, understood and modelled in 2003. A temporary seismic monitoring system was operational from April 2004 and it was replaced with a permanent array in mid 2005. In late 2005, mining methods at the narrow vein Tasmania Mine were typical of mining methods employed at similar operations the world over. A substantial difference was the degree to which external opinion was sought, innovative techniques and methods were implemented, and a peer review of the entire process was undertaken. It was not enough: a seismically induced rockfall on 25 April (Anzac Day) 2006 took the life of a colleague. That the entire world was appraised of the accident was of secondary importance. An extensive coronial investigation found that no corporation or individual erred in respect of the Anzac Day accident. The clear message from that finding was that generally acceptable techniques for the management of mining induced seismicity, while at the limit of extant engineering experience, were below the expectations of the wider community. Management of seismicity at the Tasmania Mine post Anzac Day is a multidisciplinary activity that involves the entire workforce at every turn. Extensive seismic monitoring, saturation level instrumentation, development of innovative mining (and in particular stoping) methods, detailed ‘first principals’ ground support and reinforcement designs, exclusion and re-entry protocols surrounding all stope blasts, and the continued professional peer review of all geotechnical aspects of the operation, are part and parcel of the continued operation of the Tasmania Mine. This paper describes the latest developments in that process.