A GROUND STABILITY RISK ASSESSMENT OF THE #4 GOLD MINING SHAFT OF THE DRIEFONTEIN DIVISION OF GOLD FIELDS, LTD., CARLETONVILLE, SOUTH AFRICA

The Carletonville area of South Africa is located upon both gold deposits and dolomite. Efforts to mine the gold bearing quartz conglomerates of the Witwatersrand Supergroup have caused dewatering of the underlying Malmani Subgroup of interbedded dolomite and chert. This dewatering, combined with high water use, has resulted in a elevated occurrence of sinkhole formation. The formation of a sinkhole at the Gold Fields, Limited, Driefontein division number four shaft refrigeration plant on March 16, 2004 prompted an investigation into the relative risk of sinkhole formation across the shaft area. The investigation was carried out primarily as a desk study and resulted in the creation of borehole descriptions and an isopach map for the Driefontein number four shaft area. In addition, previously collected datasets of gravity (Bouguer anomaly) data, geologic maps, and leveling data were compiled and analyzed in conjunction with the borehole and isopach data to create a map of relative risk of subsidence for the area.

While the relative risk map did not display any major trends within the data, maps of isopach data and gravity anomalies displayed a northwestern trend accompanied by a minor northeastern trend, consistent with the orientations of the major regional faults, the Bank Fault and the Master Bedding Fault.

Based upon the trends within the isopach and gravity data and relative risk scores created for the risk map, the Driefontein number four shaft is generally at moderate risk of sinkhole formation, with several areas of high risk. The number four shaft should follow stringent water controls and consider expanding the number of boreholes that reach bedrock across the site in order to make a more comprehensive risk assessment.