Mining in the Carletonville area of the Witwatersrand Basin predates 1934. Owing to the depletion of higher grade ore, the current activities focus increasingly on the extraction of lower grade secondary reefs as well as remnant and pillar extraction. Of particular interest is multi-reef remnant extraction. Numerical modelling was conducted to investigate the rock engineering aspects of remnants being understoped on a secondary reef horizon. An analysis of the stress evolution in the middling between the two reef horizons indicated that a zone of high major and low minor principal stress develops between the two reefs. This indicates a high risk of violent shear failure. Some pillars were nevertheless successfully understoped in the past, and a study was conducted to understand this phenomenon. An ‘extended’ energy release rate concept introduced by Napier and Malan1 proved to be useful for investigating this problem. It was found that bedding planes and lithology play an important part in the stable dissipation of energy in multi-reef remnant geometries. The study indicated that stope convergence and the various energy components are affected by the presence, position, and properties of a bedding plane. The energy solutions are complex and sometimes counterintuitive, and. care should be exercised when modelling specific cases. The modelling was nevertheless valuable for indicating that energy dissipated on weak layers may reduce the risk of violent failure in a multi-reef mining scenario. This was confirmed by an actual case study.


To extend the lives of the Witwatersrand gold mines, work is being conducted to include a larger number of remnants and pillars in the reserve base of the mines. According to Jager and Ryder (1999), a remnant is an unmined block of ground surrounded by extensive mining. Remnants are typically left because of adverse or difficult mining conditions caused by geological structures, poor ground conditions, low grade, or rockburst damage (Durrheim, 1997). In contrast, pillars are typically the result of rock engineering designs to control energy release rate (ERR) and seismicity. During the early 1900s, remnants formed by the practice of scattered mining layouts had a significant effect on seismicity. Numerous task teams were appointed during this period. As the industry expanded, the severity of rockbursting increased and the research was formalized after the 1960s. Various recommendations were made and many of these are still used as best practice today (e.g. Jager and Ryder, 1999).

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