Shallow hard-rock bord-and-pillar mine designs are typically characterized by solid pillar systems and stable span designs. These spans are optimized to maintain stability, which permits safe and economic extraction of the orebody. Reef-parallel structures are known to affect the strength of pillars. In areas where such structures exist, careful considerations need to be given to rock mass properties with respect to pillar design, bord span and type, and support design, depending on the placement of the structure. Part of Unki mine has a reef-parallel structure referred to as the Footwall Fault (FwF), which is located either within or below the mining cut. In addition to the FwF, mining in the upper sections of the mine encountered poor ground conditions, characterized by low-angle joints trending N-S and truncated by E-W high-angle faults, joints, and dykes. These features resulted in frequent falls of ground, which were attributed to incorrect span design, inadequate support design, pillar fracturing, and sidewall failures. This paper presents the bord span review and the corresponding support optimization process, as well as the resultant improvements and benefits to the mining process as applied to this top section of the mine. The results presented here have led to successful mining through the challenging ground conditions. The same conditions had earlier resulted in the closure of one of the uppermost sections of the mine. Crucial to this research is the inclusion of rock mass properties in the bord span re-design, modifications to the design formulae to suit the downrated conditions, and probabilistic numerical modelling for support design.

INTRODUCTION

Mining in the upper portions of the Unki East shaft has been faced with ground stability challenges, leading to decommissioning of one of the half-levels (1 South). These upper half-levels contain a reefparallel structure referred to as the Footwall Fault (FwF) which is located either within or below the mining cut. Reef-parallel structures are known to affect pillar strength and have been cited as a major contributor to mine collapses. Bimha mine in Zimbabwe and Everest mine in South Africa are typical examples. A combination of the splay faults from the FwF, N-S low-angle joints intercalated by E-W high angle faults, dykes, and sympathetic joints resulted in frequent large falls of ground, which were ascribed to inadequacy of the span and support designs for the prevailing poor ground conditions, which were not envisaged at the mine design phase. Work presented in this paper has been carried out to mitigate the impact of this combination of geological structures and avoid prematurely decommissioning half-levels. The work was conducted when 1 North had intersected poor ground, entailing a high possibility of half-level decommissioning.

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