Case History 29: Sinkholes, Far West Rand, South Africa

(Brink, 1979; 1984)

Numerous large sinkholes have formed in the Far West Rand, particularly since pumping for the gold mines intensified in the 1950s. These collapses characteristically occur suddenly and produce a deep crater. That at the West Driefontein mine in December 1962 was 55 m in diameter and at least 30 m deep. It claimed 29 lives. The largest crater still unfilled in the late 1950s formed in 1957 at the West Rand Gardens Estate; it is 100 m in diameter and 40 m deep. The ground consists of residual soils of the order of 100 or more metres thick overlying a highly irregular surface of cavernous early Proterozoic dolomites.

A mechanism of sinkhole development has been proposed by Brink (1979; 1984) (Fig. 5.6). In the initial equilibrium situation, vertical slots, plugged by residual soil, connect the floors of the buried "valleys" in the dolomite surface to flooded caverns located in this rock body at depths of around a kilometre. Dewatering of the caverns by the mine pumping, initiates a process of headward erosion, stimulated by water from leaking pipes, which first flushes out the slots and then leads to the formation of a progressively enlarging vaulted void within the overlying superficial layer. Finally the arched roof of this void collapses, usually as a result of the continued entry of water, earth tremors or vibrations, to form the sinkhole.

Associated models: Platform sediments and basins (Fig. 2.6); Shelf carbonates and evaporites (Fig. 3.6); Hot wet climate features (Fig. 3.23)