A Modified sequential grid layout to increase production rates in deep level hard rock mines

Abstract

Scattered mining was practised on Kusasalethu Mine (previously Elandsrand Gold Mine) prior to 1998, but as mining proceeded deeper, it was no longer a feasible option. The scattered mining method would have resulted in unacceptably high stress levels and energy release rates on the active mining faces. Longwall mining was considered as it was practiced on neighbouring mines. This would have ensured that energy and stress levels remained within acceptable limits and avoided the formation of remnants at depth. Kusasalethu Mine required a more flexible mining method owing to the highly variable grade, the presence of multiple faults and dykes and the high production rate required. A mining method was therefore developed that consisted of dip stabilizing pillars for regional support as well as bracket pillars to support geological structures. This was called the Sequential Grid mining method. Sequential Grid mining addressed two main problems, namely, negotiating the adverse geology and the erratic grade of the VCR orebody. However, a recent drop in production resulted in the need for alternatives and improvements to the original mining layout. This involved modifications to the design in order to increase production rates without any compromise to safety. An investigation to modify for the Sequential Grid mining method was therefore conducted by the author in order to determine the consequences for layout stability. A few alternatives were investigated to determine the best possible solution for the Sequential Grid design. As a result, the modified Multi-raise mining method was introduced to address the problems that were experienced with the original design. This study compared the original mining method and the Multi-raise mining with regards to layout stability. Investigation of the seismic data showed no significant differences between the original Sequential Grid mining and the proposed Multi-raise mining. The numerical modelling of the mining layouts showed slightly higher interim Energy Release Rates (ERR) and Average Pillar Stress (APS) levels during the extraction process. The final values for these parameters are nevertheless similar to the original Sequential Grid mining method. It is therefore concluded that the Multi-raise mining method will not have adverse effects on the mine stability. It is concluded that changes to the original Sequential Grid mine design are possible without influencing the seismic hazard. In addition the anticipated interim increases in ERR and APS levels could be kept within acceptable levels through slight decreases to the extraction ratio. The concept of an average value for the stress distribution in a remaining portion of solid rock (pillars) surrounded by underground mining excavations is used as the rock engineering design parameter called average pillar stress (APS). Although averages are generally simple to calculate given the availability of appropriate data, average values for the stress distributed with pillars is somewhat more complex especially when working in MINSIM 2000 is proposed to ensure that the calculation of APS is done appropriately. The study also investigated the use of the Modelled magnitude 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.