Abstract:
Scattered mining was practised on Kusasalethu Mine (previously
Elandsrand Gold Mine) prior to 1998, but at deeper mining levels it was no
longer feasible, since it would have resulted in unacceptably high stress
levels and energy release rates. Longwall mining was not adopted at these
depths as the mine required a more flexible mining method owing to the
highly variable grade and the presence of geological structures. A mining
method was developed that consisted of dip stabilizing pillars for regional
support, as well as bracket pillars to clamp geological structures. A strict
sequence of extraction was followed and this, together with the particular
layout, was called the ‘sequential grid mining method’. This method
addressed two key problems, namely negotiating adverse geology and the
erratic grade of the Ventersdorp Contact Reef (VCR) orebody. However, a
significant drop in production rates resulted in the need for alternatives
and improvements to the original mine design. Modifications to the design
were proposed in order to increase production rates, and an investigation
to consider the rock engineering implications of these modifications was
conducted. The study indicated that the modified method, called the
‘multi-raise mining method’, appears to be feasible and might address
some of the production problems that were experienced with the original
sequential grid design. An analysis of actual seismic data showed no
significant differences between the original sequential grid mining and the
implemented multi-raise mining. The numerical modelling of the mining
layouts showed slightly higher interim energy release rates (ERRs) and
average pillar stress (APS) levels during the extraction process. The final
values are nevertheless identical to that of the original sequence. The
study also investigated the use of a modelled moment method to analyse
future seismic trends. The study illustrated that the expected seismic
trends will be very similar for the multi-raise method compared to the
original sequential grid mining method. This study is nevertheless considered of a preliminary nature and ongoing monitoring and analysis
of seismic data at the mine is required to verify the response of the rock
mass to the modified sequence and increased extraction rate. In particular,
future work needs to investigate the effect of mining rate (advance rates
in individual panels as well as volume of mining in particular raise lines)
on the level of seismicity.
