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Session 4: Ground Movements and Construction Operations |
McClure & Morton, 2 Elmwood Avenue, Belfast, UK
Van Wyk and Louw Inc., P.O. Box 905, Pretoria, S.A.
The Far West Rand in the Transvaal Province of South Africa is the location of some of the world’s largest gold mines. The area is underlain by dolomite which is sub-divided into a series of separate groundwater compartments by dykes. The inflow of water from the overlying dolomite into these deep-level gold-mines directly influences their continued development and operation. A policy of dewatering some compartments has therefore been adopted due to economic and safety considerations. However, such dewatering has in the past resulted in large-scale ground movements in the form of sinkholes and dolines, causing damage and collapse of structures and, in some cases, loss of life.
The dewatering of the Gemsbokfontein Compartment commenced in 1986. Prior to this a field trial was conducted to assess the potential of grouting as a preventative geotechnical treatment for protecting an important traffic route against the effects of sinkhole development.
During the field trial ground movements occurred as a result of the grouting. This paper describes the monitoring of these movements and the operational procedures adopted to minimize the induced settlements. On the basis of monitoring and analysis of the settlements it was concluded that the movements were the surface manifestation of sub-surface erosion of wad at depth due to the movement of grout across steeply inclined dolomite. Following the field trial, a major grouting contract along those sections of the highway identified as high risk was completed before the start of dewatering. During this operation, ground movements were again recorded and ascribed to erosion and disturbance of the wad due to grouting. The procedures established during the field trials were reintroduced and the contract was completed with minimal damage to the carriageways.
