The occurrence of sinkholes and subsidence depressions in the Far West Rand and Gauteng Province, South Africa, and their engineering implications

Dewatering associated with mining in the gold-bearing reefs of the Far West Rand, which underlie dolomite and unconsolidated deposits, led to the form ation of sinkholes and subsidence depressions. Hence, certain areas became unsafe for occupation and were evacuated. Although sinkholes were initially noticed in the 1950s, the seriousness of the situation was highlighted in December 1962 when a sinkhole engulfed a three-story crusher plant at West Driefontein Mine. Consequently, it became a matter of urgency that the area s at risk of subsidence and the occurrence of sinkholes were delineated. Si nkholes formed concurrently with the lowering of the water table in areas w hich formerly had been relatively free of sinkholes. In addition, subsidenc e occurred as a consequence of consolidation taking place in the unconsolid ated deposits as the water table was lowered. In the latter case, the degre e of subsidence which occurred reflected the thickness and original density of the unconsolidated deposits which were consolidated. These deposits var y laterally in thickness and thereby gave rise to differential subsidence. Subsidence also occurred due to the closure of dewatered voids at the rock- soil interface. The risk of sinkhole and subsidence occurrence is increased by urban develo pment, since interrupted natural surface drainage, increased runoff, and le akage from water-bearing utilities can result in the concentrated ingress o f water into the ground. Where the surficial deposits are less permeable, t he risk of instability is reduced. In the area underlain by dolomite, which extends around Johannesburg and Pretoria, these problem have been more not able in recent years because of housing development, both low-cost and up-m arket, and the growth of informal settlements. Residential densities may be very high, especially for low-cost housing, the development of which frequ ently has proceeded without recognition of the risk posed by karst-related ground instability. The appearance of significant numbers of small sinkhole s has been associated with dolomite at shallow depth, that is, occurring at less than 15 m beneath the ground surface. The vulnerability of an area ov erlying dolomite bedrock at shallow depth is largely dependent on the spaci ng, width and continuity of grikes. When dolomite is located at depths grea ter than 15 m, the sinkholes which appear at the surface usually are larger in diameter. The risk of sinkhole occurrence in areas of shallow dolomite in general, may be greater, although the hazard itself is less severe. A cl assification system for the evaluation of dolomitic land based on the risk of formation of certain sized sinkholes has enabled such land to be zoned f or appropriate development. Ongoing monitoring and maintenance of water bea ring services, and the implementation of precautionary measures relating to drainage and infiltration of surface water are regarded as essential in de veloped areas underlain by dolomite. Special types of foundation constructi on for structures are frequently necessary.