ABSTRACT:

The Bushveld platinum group metal deposits in South Africa are the largest in the world. These deposits occur as two distinct stratiform tabular ore bodies and strike for many hundreds of kilometres. Mining is extensive, with depths ranging from close-to-surface to 2300 m. The mining method is a variation of planar open stoping. Crush pillars are widely used to support the open stopes. These pillars are designed to fail and the residual strength provides the required support resistance to stabilize the stoping excavations. This paper describes the in situ measurement, of stress within a pillar, and provides a stress-strain curve including pre and post failure behaviour. 2D FLAC modelling, with strain softening, was done to show how damage expands into the hanging- and/or footwall. Although the so-called ?squat? effect is still present at larger width to height ratios, the modelling suggests that it does not dominate the pillar system behaviour. The paper investigates the behaviour of a system that incorporates the immediate hanging- and footwall, as well as the pillar itself.

1. INTRODUCTION

The Bushveld Complex is a large layered igneous intrusion which spans about 350 km from east to west. This region is situated north of the city of Pretoria in the northern part of South Africa (see Fig. 1). This remarkable geological phenomenon hosts not only the majority of the world?s platinum group metals but also contains nickel and gold. There are also vast quantities of chromium and vanadium in seams parallel to the platinum ore bodies some hundreds of metres in the footwall and hangingwall respectively. The platinum group metals are concentrated in two dipping planar ore bodies known as the Merensky Reef, a mineralised pegmatoidal pyroxenite 0.7 m to 1.4 m thick, and, underlying this, the UG2 Reef comprising one or more chromitite seams of similar thickness. The strata generally dip toward the centre of the complex at 8° to 15°. The k ratio varies from about 0.8 to over 2.5 and locally the relatively high horizontal stress can cause severe strata control problems, particularly in tunnels. The depth of mining ranges from outcrop to 2300 m. If a sufficiently large mining span is achieved, or the stope abuts a geological feature, a large volume hangingwall rock can become unstable, resulting in a stope collapse, or colloquially, a "backbreak."

Fig. 1 The extent of the Bushveld platinum exposure. (available in full paper)

In order to prevent these backbreaks a high support resistance support system is required. This is universally achieved by the use of small in-stope chain pillars oriented either on strike for breast mining (see Fig. 2) or on dip for up or down dip mining.

Fig. 2 Plan view of a typical stope on one of the planar platinum ore bodies [1](available in full paper)

The pillars are known as crush pillars and they are required to fail in a stable manner soon after being cut. The residual strength of the pillars provides the required support resistance to prevent backbreaks and keep the stope hangingwall stable. These pillars provide an ideal opportunity to study the in situ behaviour of small pillars.

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