In order to improve the extraction ratio at a hard-rock mine, the partial post-extraction of strike stabilizing pillars was considered as an option at a concept level of study. An evaluation of the potential revenue based on a maximum extraction of up to 50% of the pillars was carried out, which suggested that the project would indeed be a viable way to increase the total potential revenue. This necessitated a study on the stability of the underground excavations during the pillar extraction process based on a conceptual design. The proposed mining layout and extraction sequence was simulated to determine whether this could be included in the mining plan, and if so, to determine the potential additional extraction that could be incorporated into the mining plan.

INTRODUCTION

A hard-rock mine in the Bushveld Complex in the Northwest Province of South Africa commissioned a study to review the potential for extraction of the regional stabilizing pillars, the function of which is to support the overburden, as well as to act as barriers (Zipf, 2001). The function of the barrier pillars is primarily to compartmentalize the mine. If a pillar collapse occurs, the barrier pillars need to prevent this collapse from spreading over a very large area.

The Bushveld Complex is a basin-like rock mass consisting of layers of igneous rock. The main rock types comprise norite, anorthosite, and pyroxenite. The upper Critical Zone of the Bushveld Complex hosts the platinum-bearing orebodies referred to as the Merensky Reef and the UG2 Reef. Although the Merensky Reef is generally regarded as uniform, large variations occur in reef thickness and orientation (rolling reef). Potholes (steep troughs within the stratigraphy that result in significant thinning of the reef) are generally common throughout the eastern and western limbs of the Complex. A conventional mining method is to be employed for stoping, i.e. drilling and blasting of stope panels, separated by crush pillars. Cleaning of stopes will be done by winches and scrapers, which will feed directly onto a conveyor belt in a permanent strike drive. The mining height was anticipated to range between approximately 1 m and 1.5 m.

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