Assessment of a Rockburst in the Shaft Pillar of Cooke 4 Gold Mine, South Africa

Abstract

ABSTRACT Rockbursts are the most serious and least understood hazard associated with deep underground excavations, typically involving violent energy release and the sudden ejection of rock fragments that may result in fatalities and damage to underground infrastructure. A ML2.8 seismic event with a source dimension of between 110 and 240 m affected two primary working levels at a depth of 1808 m (the 38 and 38A levels). The peak particle velocity (PPV) is generally used to estimate the likelihood of damage to the rock mass and structures. It is not an accurate measure of the PPV generated by a seismic event (particularly in the near field). Still, it represents a scaled-distance relationship, which can be correlated to the rockburst damage. The most severe rockburst damages were encountered on Level 38 of the cross-cut south, consisting mainly of sidewall and hanging wall ejections of rock and footwall heave. The damage extended over a 70-m length of the 38 cross-cut south. From the underground observations, it is estimated that the seismic event's hypocenter is located near the 38 cross-cut south, resulting in this excavation experiencing strong ground motion. Scattered falls of ground were experienced up to 70 m radially out from the hypocenter location. INTRODUCTION Cooke 4 Mine (previously known as Ezulwini Mine), operated by the Sibanye-Stillwater Group, is situated in the Witwatersrand Basin (Fig. 1), which is famous for hosting one of the world's largest gold deposits. Historically, Cooke 4 exploited the Ventersdorp Contact Reef (VCR) of the Venterspost Formation. The rock mass surrounding the stopes, tunnels and shaft consists of quartzites, pebbly quartzites, argillaceous quartzites, the VCR conglomerate, and medium-strong and weak volcanic rocks of the Klipriviersberg Group. The mine ceased to exploit the VCR due to difficulties supporting the weak hangingwall, a 20-m-thick zone of soft ultramafic lavas (48–138 MPa) of the Westonaria Formation (Harvey et al. (2006)). The mine was closed in 2001 and reopened in 2010 to exploit a secondary orebody, known locally as the Upper Elsburg Reefs, in the shaft pillar located 1-1.2 km below the surface.