Abstract
Established in 1896, the Sons of Gwalia Mine orebody was successfully extracted by Bewick Moreing up until its closure in 1963. The mine had reached a depth of 1075 m below surface and over 2·6 Moz of gold was mined during this time. The Lalor brothers purchased the mine in 1983 and open cut mining commenced one year later. The open pit was completed in 1999 reaching a depth of 285 m below surface. Underground mining began and reached a depth of 375 m below surface before the mine was once again closed in 2002 due to a low gold price and the requirement for capital to increase the depth of the mine past a low grade section. A further 1·3 Moz was mined during this time. St Barbara Ltd purchased the mine from Sons of Gwalia Ltd in 2005 and continued development of the decline beyond 375 m. The Gwalia Deeps Mine has been planned to 1600 metres below surface (mbs) where it was expected that the rock strength to stress ratio based on an extrapolation of the typical stress ratio of 3:1 would produce significant stress induced problems in the extraction horizons. An initial stress programme using acoustic emission appeared to confirm the expected trend. A programme of deformation rate analysis (DRA) testing completed in 2006, however, showed that the stress ratio at depth would be less significant with an expected stress ratio of 1·7:1. If correct this would have significant implications to the reserve which could then be pushed out to a depth of 2000 mbs and the mining development. Another significant aspect is the impact of the rock mass anisotropy. During underground development the stress levels have been checked by CSIRO hollow inclusion cell tests. These confirm the DRA testing to a remarkable degree. The paper will describe the approach, the results and show how the stress measurements with anisotropy explain the observations in the development drives.
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