Abstract
The research described in this paper was done to confirm the Upper Group 2 (UG2) PlatMine peak pillar strength formula (Watson et al., 2007), which was determined from a back-analysis of failed and unfailed pillars. Underground measurements were made on a stable pillar that was loaded by firstly reducing it's length and then by mining the surrounding pillars until pillar failure took place. The pillar was instrumented with suitably positioned strain cells and closure meters, which allowed both the average pillar stress and strain to be determined. The paper describes the methodology applied to identify a suitable position for the instrumentation, as well as the results. A stress/strain curve is presented for a UG2 pillar with a w/h ratio of 2.0, at Booysendal Platinum Mine. The measured pillar strength was similar to the predicted strength using the PlatMine pillar strength formula for UG2 pillars. The PlatMine formula has been successfully implemented on Booysendal Platinum Mine, and about 3 670 pillars have been cut without a single failure. An additional revenue of US$1.3 billion was calculated for the 25-year life of the mine as a direct result of the improved pillar design, given the January 2020 platinum group metals basket price. An extended life of mine and better mining efficiencies will also be realized.
Highlights
Mechanization of the Bushveld Comlpex platinum mines has resulted in an increase in bord-and-pillar mining, in the newer, shallow-depth mines on the eastern side of the Complex
The Upper Group 2 (UG2) chromitite pillars have been designed for many years using the Hedley and Grant (1972) formula to determine peak pillar strength
Design of stress measurements The aim of the stress measurements was to determine the average pillar stress (APS) of the instrumented pillar from a stress measured at a point in a borehole somewhere above the pillar
Summary
Mechanization of the Bushveld Comlpex platinum mines has resulted in an increase in bord-and-pillar mining, in the newer, shallow-depth mines on the eastern side of the Complex. The PlatMine pillar strength formula (Watson et al, 2007) is based on a back-analysis of failed and unfailed pillars on the UG2 Reef. It suggests that pillar strengths are greater than predicted by Hedley and Grant (1972), if a downrated k-value of 35 MPa is assumed in the Hedley and Grant (1972) formula. The derivation of this formula is described in a separate paper (Watson, Lamos, and Roberts, 2021) It was based purely on a regression analysis of failed and unfailed pillars, where
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