Risk and Hazard assessment when leaving large pillars at depth, in a high stress environment, was based on a case study at Kusasalethu Gold Mine, operated by Harmony Gold, located on the West Rand in Gauteng, South Africa. A narrow tabular gold package is mined, at a depth of 3400 m below surface. A tabular hard rock mine which adopts the Sequential Grid mining method. Excessive on-reef displacements are limited by, systematically placed dip pillars (30 m wide), which are designed as non-yielding and the placement of backfill. Where panels are to be re-established via re-raising, it is separated from the mined area by crush pillars, designed to fail when cut. Pillars are designed, to prevent stress build up and violent failure. In August 2017, an incident resulted in the loss of life to five employees. Extensive rock burst damage was as a result of pillars exceeding their designed dimensions.
Following an incident that occurred on the 25 August 2017, which claimed the lives of five employees due to a rock burst, the need arose to assess and evaluate the effects of leaving pillars at depth, especially those that have not necessarily complied with the design criteria. This may not necessarily be intentional, but as a result of a myriad of factors. The aforementioned incident is used as a case study for the purposes of this paper. Methods entailed the replication of the scene and the analysis of potential problems and outcomes. Finally, the evaluation of risk and hazard associated when large pillars are left at depth in a high stress environment.
Kusasalethu Gold Mine, operated by Harmony Gold, is located 90 km from Johannesburg, Gauteng, South Africa and is situated on the far southern section of the West Rand goldfields. It forms part of the central portion of the greater Witwatersrand basin and mining is focused on the Ventersdorp Contact Reef (VCR). This is a conglomerate reef band with a strike of north 65° east and a dip of 23° to the south. The reef consists of various terraces separated by slopes, all of which can be structurally deformed by duplicated reef zones. Grade is highly variable with un-pay zones typically occupying sand-filled channels. The reef is characterized by a relatively large amount of faulting with throws of less than 10 meters.
