Effects of coal mining on surface topography in South Africa — Updates and extensions

Abstract

Surface subsidence over coal mine workings received a great deal of attention in South Africa in the 1990s with the increased prevalence of high-extraction methods. This paper updates and further develops previously published material, based on new data and better insights. This can be used for mine planning and for the evaluation of the long-term stability of the surface overlying coal workings. Instead of just a single profile of subsidence in the case of high-extraction mining, upper and lower limits based on re-analysis of existing data are given. An improved method to predict the expected amount of subsidence in the case of pillar extraction is presented. The predictions of the maximum amounts of subsidence and induced tilt in the case of pillar failure are updated by additional information that was obtained after the previous method was published. A more realistic method for evaluating the possibility of sinkhole formation, based on a truncated cone model of the cavity, is presented. With the new model, two mechanisms of cavity arrest are evaluated, namely choking of the cavity by bulking of the collapsed material and wedging out of the cone before it reaches the surface. A method to estimate the maximum width of subsurface erosion cavities, which takes account of the maximum volume of the reservior for the eroded soil as well as the strength of the overlying ‘lid’ of the cavity, is presented. These updates illustrate the need for periodic review of all empirically derived methods. Suggested default input values are presented but they should only be used as a last resort.