Geotechnical classification of deep and ultra-deep Witwatersrand mining areas, South Africa

Abstract

As depth of mining increases, so the production costs of the aurifereous 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 mine deep orebodies in existing mines, as well as to develop new mining ventures safely and efficiently, novel approaches, including mining strategies, layouts and support systems have to be adopted. Geological features largely control the deformation mechanisms associated with Witwatersrand ore bodies. These features are grouped into two major categories: primary and secondary features. Both impact on the rock mass behaviour associated with the excavations, and contribute to the definition of geotechnical areas. Primary geological features are defined by the various rock types, ore body geometry, and the frequency and mineralogical 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 frequency of mining-induced stress fracturing, and planes may be reactivated during seismic events. Secondary geological features are faults, dykes and veins/joints, and associated metamorphism. These cause stability problems and are often associated with seismic events. Compositional and textural characteristics of these features, and their controls on the rock mass behaviour, are ill-defined. Primary and secondary geological characteristics also play an important role in identifying the appropriate mining strategy, layout and support. Geological features can be predicted into deep unmined areas, and therefore contribute to the safe stand most efficient extraction of the ore body. Witwatersrand ore bodies are mined in complex geological environments, with the rock mass behaviour differing from one ore body to the other. This is approached by employing a new methodology that attempts to quantify the problems encountered when mining the major Witwatersrand ore bodies.