The influence of subsidence on planning and development in Johannesburg, South Africa

Gold mining in the Johannesburg area commenced in the late nineteenth centu ry. The early mines, now abandoned, were at shallow depth and their presenc e may give rise to subsidence, Consequently, they impose limitations on dev elopment in the old reef area which runs east to west through the city of J ohannesburg, Indeed, the erection of buildings on the undermined land has b een controlled by the Government Mining Engineer. These controls determined whether or not building took place, as well as the permissible heights of buildings in relation to the depth at which mining occurred. More recently, however, due to the pressure on available space in the central business di strict, the controls have been relaxed when development proposals have been accompanied by sound methods of mine stabilization, The extent and amount of subsidence which occurs varies, and can be difficult to predict, dependi ng on the depth and height of the workings, the number of reefs worked, the local dip, the presence of dykes and faults, the type and extent of any pa cking, and the age of the stopes, Various types of subsidence have occurred and include the development of sinkholes, and subsidence associated with c avern development and stope closure. Several methods of mine stabilization have been employed from dynamic compaction to concrete dip pillars to provi de rigid in-stope support, to plugging stopes and grouting above the plug. Some case histories are provided to illustrate the subsidence problem and h ow it has been dealt with.