Structural mechanisms contributing to large-scale hangingwall instabilities on the UG2 reef horizon

Abstract

Structural complexities, including regional geologic structures, low-angle structures, ramp structures and alteration zones contribute towards large-scale hangingwall or structural (pillar) instabilities experienced in many UG2 chromitite mines. The associated anomalous behaviour of the rock mass caused by these structures may result in significant ore reserve write-offs. The inability of technical and underground mining personnel to pro-actively identify and treat these geologic structures and associated failure mechanisms has resulted in ongoing instabilities experienced in many mines. Anomalous behaviour is mostly as a result of the exposure of numerous low-angle structures on various scales, commonly known as ‘doming’. These low-angle structures are treated simplistically or go unnoticed. Also, the presence of pegmatite veins, which is a common joint characteristic, causes problems and is generally ignored as the potential for instability. Furthermore, the presence of alteration zones is typically unnoticed as it is not common. The exposure of these prominent structures or a combination of these structures can impact on the exposed hangingwall conditions, panel span, support- and pillar behaviour. In some instances the impact have resulted in multiple fatalities and total mine closure. Case studies were conducted by the author at the Lonmin Marikana Operations where large-scale instabilities have been experienced. These findings were related to other similar study sites in the Bushveld Complex and the Great Dyke in Zimbabwe. The investigations confirmed some of the findings made by previous studies. However, new information gained from this study provided an improved understanding of the formation, interaction and potential instabilities if these structures are exposed by mining. The formation of the Bushveld Complex and geologic structures contributed to the anomalous conditions experienced in some underground mine workings. With the application of a new technology by the author (a sub-surface profiler), for the first time, the presence and location of these anomalous structures could be verified in the hangingwall. This contributes to an improvement in the spatial interpretation of these structures and confirmed that it should be considered in the mining strategy and support design processes. The learnings from the study will assist with the early detection of specific structural conditions which may contribute to the mitigation of potentially unstable conditions. Suitable remedial strategies were developed by the author from the site investigations and are discussed in detail. This includes the application of preferred mining layouts, mining direction, spans and support strategies where these structures are present. This work may significantly reduce the risk of large-scale instabilities and is therefore considered a significant contribution towards improving safety and the understanding of these anomalous structures at the mines in the Bushveld Complex.