Investigation of long-hole stope performance at south deep gold mine

Abstract

South Deep Gold Mine is located 45km south west of Johannesburg within the gold-rich Witwatersrand Basin. The reefs at the mine occur within an economically significant stratigraphic bundle containing Ventersdorp Contact Reef (VCR) and Upper Elsburg conglomerate units. Long-hole stopping (LHS) is the primary method of mining and accounts for over 60% of the gold produced at the mine. It is, therefore, critical that the stopes meet the required production output for the designed mine lifespan at efficient stope turnaround times. However, mine data suggests that ore production from LHS has been erratic and has significantly affected the mine’s ability to produce to its installed capacity. The reduction in productivity and utilization is an area requiring research and this project investigated LHS performance at the mine to understand factors that influence stope turnaround time. Three research questions guided the study intending to improve LHS performance: • What is the influence of geological environments on LHS performance; • How do design aspects at South Deep affect LHS performance; and • How do current drilling and blasting practices affect LHS performance? To answer the research questions, the research adopted a stepwise process of categorising mining environments, analysing stope data files on Deswik software, developing predictive models on Python and MS Excel, informing excavation design, and finally reviewing drilling and blasting practices at the mine. These steps helped in leveraging the intersection that exists between geological environments, technical design aspects as well as operating tactics at the mine. iii The findings from the research questions led to the emergence of two themes. The first theme corresponding to the influence of geological environments and design aspects relates to the application of engineering principles in the mining environment, within the jurisdiction of the mining engineer / mine planner. The second theme, corresponding to drilling and blasting practices, relates to operational excellence, within the jurisdiction of the miner. The success of LHS performance hinges on the interplay and balance of these two themes. The major findings of the research indicated that deviation from set standards and malfunctioning components compromised drilling accuracy while wrongly charged holes (over-charged/under-charged) compromised blasting efficiency. They further showed that conventional and de-stress cut environment stopes were the most susceptible to over-break. Severe over-break was found on Secondary and Sequential stopes, predominantly on hanging walls and footwalls. Hanging walls and footwalls with hydraulic radius (HR) values above 6.0 (mine standard) also showed high susceptibility to over-break. In the process of analysing the results, the research developed a multi-variate model to predict the behaviour of the relationship between LHS performance and HR. It also created a matrix that informs how the mine may manage over-break. The research concluded by giving a set of recommendations for improving LHS performance, the chief of which emphasised the importance of strict supervision of drilling and blasting practices underground, measuring drilling against quality and quantity metrics as well as aiming for designing stopes with HR values below 6.0. It further recommended the adoption of the stoping scenario matrix developed, the improvement of the integrity of backfilling as well as the automation of the drilling and charging processes.