The geotechnical evolution of deep level mechanised destress mining at South Deep

The South Deep mine is located approximately 45 km south-west of Johannesburg in the Far West Rand goldfield of the Witwatersrand Basin. It is a deep level mine that is actively mining between 2600 m and 3000 m below surface with expectations to mine to 3400 m depth. South Deep is situated in the geologically unique and renowned Witwatersrand Basin, which is the world’s premier gold region. The South Deep ore body gradually increases in thickness to the west, from approximately two metres at the sub-crop to approximately 120 metres in thickness. The geometry of the Upper Elsburg Reef package, which is the primary economic target, lends it to a fully mechanised mining method. The main geotechnical challenges to successfully mine the South Deep orebody were to introduce a mechanised massive mining method at depth to destress and then extract the extensive orebody. The destressing method then had to allow a productive method for economic extraction of an essentially low-grade bulk volume orebody. Several variations of different mining methods have been used to date but all rely on a destressing method to reduce the in situ stresses. Originally the destressing was done conventionally (traditional South African narrow reef gold mining methods). Since Gold Fields acquired South Deep in 2007, the push for further mechanisation has seen four mining method changes, including: mechanised, low profile, apparent dip destress mining; the introduction of a low profile, horizontal destress method with backfill. The year 2011 saw the introduction of low profile, horizontal destress with 2m wide crush pillars And in 2015, the mine moved to high profile (5.5m high) horizontal destress development with mechanised installation of ground support. Crush pillars were replaced with yield pillars.