Risk assessment in dolomitic terrain: a case history

The dewatering of dolomitic groundwater compartments in the Far West Rand in the Transvaal Province of South Africa has, in the past, resulted in ground movements in the form of subsidences and sinkholes. These have caused damage to various structures and in some instances loss of life. Dewatering of these compartments has taken place as a result of economic and safety considerations for the continued operation and development of deep-level gold mines in these areas. The dewatering of another groundwater compartment is currently underway. Consequently, a risk assessment, primarily to evaluate the potential for sinkhole development, was prepared for the main highways crossing the compartment.Risk assessment in dolomitic terrain is a much debated subject, the main reason being the subjectivity of the various approaches. It is generally accepted, however, that a number of factors affect subsurface stability, for example, the position of the watertable, the presence of weak, dolomitic residuum, bedrock characteristics and the ponding of surface-water. A combination of methods was utilized to produce a final assessment. These included a multivariate classification system and comparison with published data collated by the Geological Survey of South Africa. The methods and results of the risk assessment are discussed together with possible alternative solutions for maintaining the traffic routes and for ensuring the safety of road users.