GEOTECHNICAL CLASSIFICATION OF DEEP AND ULTRA-DEEP WITWATERSRAND MINING AREAS, SOUTH-AFRICA

As depth of mining increases, so the production costs of the aurifereo us Witwatersrand deposits are continuously increasing, with some mines already mining at depths exceeding 3500 m. Health and safety aspects are, simultaneously, gaining in importance. Therefore, in order to min e deep orebodies in existing mines, as well as to develop new mining v entures safely and efficiently, novel approaches, including mining str ategies, layouts and support systems have to be adopted. Geological fe atures largely control the deformation mechanisms associated with Witw atersrand orebodies. These features are grouped into two major categor ies: primary and secondary features. Both impact on the rockmass behav iour associated with the excavations, and contribute to the definition of geotechnical areas. Primary geological features are defined by the various rock types, orebody geometry, and the frequency and mineralog ical characteristics of sedimentary structures (e.g. various kinds of bedding planes and lithological boundaries). The primary features also control rock engineering properties, closure rates, attitude and freq uency of mining-induced stress fracturing, and planes may be reactivat ed during seismic events. Secondary geological features are faults, dy kes and veins/joints, and associated metamorphism. These cause stabili ty problems and are often associated with seismic events. Compositiona l and textural characteristics of these features, and their controls o n the rockmass behaviour, are ill-defined, Primary and secondary geolo gical characteristics also play an important role in identifying the a ppropriate mining strategy, layout and support. Geological features ca n be predicted into deep unmined areas, and therefore contribute to th e safest and most efficient extraction of the orebody. Witwatersrand o rebodies are mined in complex geological environments, with the rockma ss behaviour differing from one orebody to the other. This is approach ed by employing a new methodology that attempts to quantify the proble ms encountered when mining the major Witwatersrand orebodies.