The ‘Bermuda Triangle’ at Saffy Shaft is located in a structurally complex area. It is defined by a graben structure contained between two antithetic faults, where structurally driven instabilities have caused underground mining challenges. Subsidiary fault and joint structures which splay off the major faults are exposed in the interior of the graben, forming extensional duplexes of variable dimensions. This area comprises faults with displacements of 30 cm to 1.8 m, multiple joints, and alteration in the hangingwall, leading to the hangingwall rock mass unravelling between the support units. The alteration and ramp structures, in conjunction with the related major fault structures, have presented significant ground control challenges in the ‘Bermuda Triangle’. In many cases, the poor ground conditions encountered by mining operations result in blocks of ‘un-mineable’ ground being left in-situ, which adversely affects the extraction ratio. It was therefore necessary to conduct an in-depth site investigation to identify and determine the deformation events and sources contributing to these conditions. This paper describes the in-depth study of the deformation events contributing to the unstable hangingwall conditions experienced within the ‘Bermuda Triangle’. The investigations identify the mechanisms driving specific structural behaviours. This will greatly assist in identifying the precursors to timeously detect the specific structural conditions that could contribute to instabilities and enabling appropriate risk mitigating strategies to be implemented proactively.

INTRODUCTION

Saffy Shaft is one of Lonmin's vertical shafts extracting the UG2 Chromitite Reef. The damage zone referred to as the ‘Bermuda Triangle’ represents a portion of ground along the eastern half-levels of Saffy Shaft which is bounded by the Saffy East Fault to the west and the Turffontein West Fault to the east. Site investigations have identified unstable hangingwall conditions related to the occurrence of complex geological structures along the majority of this block. Altered hangingwall conditions and unravelling between support units and around grout packs (Figure 1) pose an increased fall-of-ground risk. The adverse hangingwall conditions influence the potential mineable ore reserves, rate of mining, and support strategy. This paper will describe the site investigations, identification of the geological precursors, structural behaviours, and the resulting mechanisms. With prior knowledge, appropriate strategies can be implemented proactively.

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