Abstract
The Upper Group 2 (UG2) chromitite reef is a shallow-dipping stratiform tabular orebody in the South African Bushveld Complex, which strikes for hundreds of kilometres. Mining is extensive, with depths ranging from close-to-surface to 2 500 m. Pillars are widely used to support the open stopes and bords. Little work has been done in the past to determine the strength of pillars on the UG2 Reef and design was done using formulae developed for other hard-rock mines. This has led to oversized pillars with consequent sterilization of ore. In this paper we describe a back-analysis of stable and failed UG2 pillars on the Bushveld platinum mines, and provides a strength formula for UG2 pillars. The formula may be used cautiously on all Bushveld platinum mines with similar geotechnical, geometrical, and geomechanical conditions to the pillars in the database.
Highlights
The Bushveld Complex in the northern part of South Africa is a large layered igneous intrusion, which spans about 350 km from east to west (Figure 1)
In this paper we describe a back-analysis of stable and failed Upper Group 2 (UG2) pillars on the Bushveld platinum mines, and provides a strength formula for UG2 pillars
The Merensky Reef overlies the UG2 Reef, and the width of both ranges between 0.7 m and 1.5 m
Summary
The Bushveld Complex in the northern part of South Africa is a large layered igneous intrusion, which spans about 350 km from east to west (Figure 1). The platinum group metals are concentrated in two planar, shallow-dipping orebodies (reefs):. ➤ The Upper Group 2 (UG2), chromitite seams ➤ The Merensky Reef, a mineralized pegmatoidal pyroxenite. The Merensky Reef overlies the UG2 Reef, and the width of both ranges between 0.7 m and 1.5 m. The strata generally dip at 8° to 15° toward the centre of the Complex, with a horizontal to vertical stress ratio (k-ratio) ranging from about 0.5 to over 2.5. The depth of mining ranges from outcrop to 2 500 m. The lower k-ratios are a feature of deeper level reefs
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