Abstract

Lonmin Platinum has experienced several cases of large-scale hangingwall instabilities during the extraction of the Upper Group 2 Chromitite Layer (UG2) at a number of the Marikana operations along the Western Limb of the Bushveld Complex. These instabilities were all attributed to the presence and interaction of prominent geological features and highlighted that assumptions made in the design of the stable pillars and stable span were potentially overly simplistic in these areas. Investigations into the various case studies over the past ten years have resulted in the identification and classification of the rock mass characteristics of the geological structures causing the instabilities.

In order to get a better understanding of these instabilities, it is important to understand the structural geology in general, and the sequence of events that led to the formation of the Bushveld Complex in particular. The tectonics involved in the formation of the various layers around the economic mineralized layers has been investigated, and the influence of these tectonic events on the stability of the UG2 stopes hypothesized.

Following this investigation, the behaviour of the various layers in the hangingwall of the UG2 is better understood. This paper presents a description of the ground conditions being experienced in the underground stoping environment, the possible source of the associated structures and the resultant behavioural mechanism of these structures causing the rock mass instabilities.

Introduction

The Lonmin shafts located on the eastern side of the Elandsdrift Fault Zone (EFZ) are intersected by geological features that cause hangingwall instabilities resulting in the formation of problematic ground conditions. Lineaments, including faults, dykes and joints as well as reef-parallel layering and clay-like, undulating planes of weakness are the prominent structural features exposed during development and stoping operations along the Upper Group 2 Chromitite Layer (UG2). This paper presents a description of the rock mass instabilities associated with the intersected geological structures. The intersection of these problematic structures is linked to the mechanism driving instabilities.

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