The mining of the tabular auriferous reefs on the rim of the Witwatersrand Basin in South Africa is very extensive, and even within individual mines, underground workplaces are often widespread. There is just cause not to evacuate an entire operation's workforce following the unanticipated occurrence of a large mining-induced seismic event. We present the rationale for the quick withdrawal of the possibly affected underground workforce following such seismic events and the calculation methodology based on the damaging peak particle velocities (PPV) of the past seismicity. A minimum PPV from the source at which observable damage to the underground excavations is no longer apparent was inferred. The radii from the source within which ground velocities exceed this amount are then calculated for all recorded events with local magnitudes ML1.0 and greater. Those workplaces located within these negatively affected volumes are deemed at risk and the workforce is withdrawn.
The old gold mines that have been extracting the rich tabular reefs on the Witwatersrand Basin have reached or are approaching the end of their lives after many decades of production. Those still in operation are mostly mature, with widespread active stopes. A non-mechanized method of reef extraction necessitates a large workforce. Such is the case with Sibanye Stillwater's Kloof and Driefontein gold mines. The distances between underground workplaces can be of the order of kilometres. The question arises about the size of the zone of negative influence following the occurrence of mining-induced seismicity and, specifically, what portion of the workforce should be evacuated when the elevated seismic risk demands it.
When and where possible, a record of damage due to seismicity is usually kept for rockmass re-support or rehabilitation efforts. This information is regularly updated in the mines’ Codes of Practice. The records are expected to be incomplete since access to older accessways and workings is restricted, and often impossible due to personnel barricades and ventilation seals. Given a significant number of large-event occurrences, the threshold distance of observable damage can nonetheless be ascertained. Having the magnitudes of causative events coupled to the calculated ground motions experienced at these distances, the baseline peak particle velocity (PPV) associated with damage can be determined. A further step sees the application of this knowledge, calculating the radius around the focus of all future events of ML1.0 and greater, where the baseline PPV is reached. Personnel whose workplaces are located within the volume defined by this radius will receive notification to move to a safe place and then return to the surface in an orderly fashion, or until such time as safe re-entry, post-inspection, is declared.
