Closure estimations by the timber pack assessment method were conducted at Mponeng gold mine at four different sites covering various rock formations and a depth range between 2382 m and 2747 m. The method assumes that the packs are built with timber units having standard thicknesses, allowing the estimation of original stoping width. The estimated closures were calculated by subtracting the current stoping widths from the estimated original stoping widths. An elastic numerical modelling program was used to evaluate the elastic convergences at the same positions where the closures were estimated. It was found that the estimated closure is 56% higher than the modelled convergence. It would be possible to estimate the underground closure from numerical modelling results by using this percentage difference. A limited number of underground observations could then be conducted to support the closure estimations further.

INTRODUCTION

To exploit the remaining gold reserves in the Witwatersrand Basin, innovative methods must be developed to address the challenges arising with increasing depth. The depths being planned for mining in future, if South Africa still desires to be a major role player in gold production, could not be imagined five decades ago.

Mponeng gold mine currently holds the world record of being the deepest mine (AngloGold Ashanti, 2013). The longwall mining method without backfill was employed at moderate depths to 89 level at 2 700 m depth. The mining method was then changed to sequential grid with backfill from 89 level to the lowest level 120 at 3 400 m depth. As the depth increased, the seismic activity on the mine also increased.

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