Rock engineering aspects of a modified mining sequence in a dip pillar layout at a deep gold mine

摘要

Scattered mining was practised on Kusasalethu Mine (previouslyElandsrand Gold Mine) prior to 1998, but at deeper mining levels it was nolonger feasible, since it would have resulted in unacceptably high stresslevels and energy release rates. Longwall mining was not adopted at thesedepths as the mine required a more flexible mining method owing to thehighly variable grade and the presence of geological structures. A miningmethod was developed that consisted of dip stabilizing pillars for regionalsupport, as well as bracket pillars to clamp geological structures. A strictsequence of extraction was followed and this, together with the particularlayout, was called the ‘sequential grid mining method’. This methodaddressed two key problems, namely negotiating adverse geology and theerratic grade of the Ventersdorp Contact Reef (VCR) orebody. However, asignificant drop in production rates resulted in the need for alternativesand improvements to the original mine design. Modifications to the designwere proposed in order to increase production rates, and an investigationto consider the rock engineering implications of these modifications wasconducted. The study indicated that the modified method, called the‘multi-raise mining method’, appears to be feasible and might addresssome of the production problems that were experienced with the originalsequential grid design. An analysis of actual seismic data showed nosignificant differences between the original sequential grid mining and theimplemented multi-raise mining. The numerical modelling of the mininglayouts showed slightly higher interim energy release rates (ERRs) andaverage pillar stress (APS) levels during the extraction process. The finalvalues are nevertheless identical to that of the original sequence. Thestudy also investigated the use of a modelled moment method to analysefuture seismic trends. The study illustrated that the expected seismictrends will be very similar for the multi-raise method compared to theoriginal sequential grid mining method. This study is nevertheless considered of a preliminary nature and ongoing monitoring and analysisof seismic data at the mine is required to verify the response of the rockmass to the modified sequence and increased extraction rate. In particular,future work needs to investigate the effect of mining rate (advance ratesin individual panels as well as volume of mining in particular raise lines)on the level of seismicity.