The experience of seismic activity associated with stabilizing pillars at intermediate depth, approximately 2000 metres below surface on the Ventersdorp Contact Reef horizon, is summarized. One particular case study is used to describe certain aspects in detail. The evaluation is based On seismic data, closure-ride measurements and the extent and intensity of underground damage. Experience from pillar associated seismicity allows a review of stability criteria of stabilizing pillars at intermediate depth which seem to be dominated by the composition of the footwall strata.
Stabilizing pillars have proved to be the focus of large seismic events which resulted in considerable damage at Western Deep Levels South (WDLS). The gold mine is situated in the Western Transvaal, about 70 km west of Johannesburg. The mine forms part of the West Wits Line of mines which work two major conglomerate formations, the Ventersdorp Contact Reef (VCR) and the Carbon Leader Reef (CLR).
At present, at WDLS, only the VCR is mined between 2000 and 2400 m below surface. The reef dips approximately 21 degrees towards South with a varying channel width of 0,05 to 4,50 m. It is intersected by dykes and some faults of minor throw « 50 m). Both the VCR up dip to the North and the CLR some 900 m stratigraphically below are mined by the adjacent WDL-West and WDL-East mines. WDL has adopted longwall-mining as a general concept which allows a face-advance between 10 and 12 m per month.
The mine experiences a relative high incidence of seismicity. Stabilizing pillars became part of Western Deep Levels general mining layout to reduce stress concentrations at the stope faces and hence the incidence of seismic activity. Some major seismic events have occurred with catastrophic damage along these pillars but the circumstances which lead to them have until recently been poorly understood.
To shed light on this phenomenon, a study was carried out on pillars and their seismic activity, involving mainly the Lower Carbon Leader Reef at WDL-East and West mines (Lenhardt and Hagan, 1990). General results of this study and from Jantzon et al. (1990) can be summarized as follows:
The width of pillars between 20 and GO m has no bearing on the magnitude of pillar related seismic events.
Pillar associated events tend to recur some distance behind or close to the advancing stope face, depending on the mining geometry.
Pillars of20 m width and greater (the widest pillars on WDL are 60 m) do not crush in a hard rock geological environment such as at WDL but the footwall heaves violently in adjacent excavations.
Pillar associated seismicity increases with depth.
During investigations of pillar related seismicity, which concentrated on the deeper, CLR, horizon of WDL, it became apparent that pillars at intermediate depth on the VCR also experience stability problems. The results of a special project which was initiated to analyze the nature of this phenomenon is described in the following paragraphs.
(Figure in full paper)
