ABSTRACT:

In South Africa, the Bushveld platinum group metal deposits are unique in that they are the largest platinum group metal deposits in the world. These deposits occur as two distinct stratiform tabular ore bodies and strike for many hundreds of kilometers. Mining is extensive, with depths ranging from close-to-surface to 2000 m. The mining method is a variation of planar open stoping. Pillars are widely used to support the open stopes. Crush pillars are commonly used in this role, where the residual pillar strength provides the required support resistance to stabilize the stoping excavations. This paper describes the direct measurement of stress within these crush pillars, and describes the stress ? strain behavior of these pillars. These findings indicate that the measured pillar failure stress is too high. The implications of these findings are that roof damage and pillar bursting could occur, and some examples of this type of damage are shown. The paper concludes that this problem could be mitigated if the three dimensional pillar geometry was modified and the pillars were cut smaller.

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 8o to 15o. The k ratio varies from about 0.8 to over 2.5 and locally can cause severe strata control problems, particularly in tunnels. The depth of mining ranges from outcrop to 2300 m. In the mining depth range from surface to about 1400 m there is a vertical tensile zone in the stope hangingwall. If a sufficiently large mining span is achieved, or the stope abuts a geological feature, a large volume of hangingwall rock can become unstable, resulting in a stope collapse, or colloquially, a backbreak. 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.

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