Abstract

Crush pillars designed to crush whilst formed at the mining face are commonly used in the intermediate depth platinum mines in South Africa. These pillars generally provide stable conditions by preventing "back breaks" whilst allowing for higher extraction ratios.

An analytical derived model was used to calculate the residual average pillar stress values of crush pillars. The values predicted by this model were compared to numerical simulations. The aim of this paper is to use the models to investigate the impact of pillar width on crush pillar behaviour.

1. Introduction

Crush pillars are utilized in the intermediate depth platinum mines of South Africa. The mechanism of pillar crushing whilst the pillar is being formed at the mining face fulfills a support function. The residual state of the pillar typically provides the required support resistance to support the highest prominent parting plane above the reef and prevents potential back breaks. On the Merensky Reef this contact can be situated between 5 m–45m above the reef. Crush pillars allow for higher extraction ratios due to the slender nature of the pillars. The pillars must, however, be used in conjunction with a barrier pillar system. Where significant geological losses are present, these structures can be incorporated as part of the regional support strategy. Crush pillars are typically 2 m–3m wide and 4 m–6m long. The inter-pillar span between rows of crush pillars are approximately 30 m–33mwith a mining height of between 0.9m and 2m being applied on most conventional narrow tabular mines (reef dip 9–15 degrees).

The pillars are normally cut at a width to height ratio (w:h) of two. However, underground, these pillars are typically cut too big or are over-mined, resulting in different pillar dimensions. This can result in these pillars triggering damaging seismicity. On some mining operations crush pillar layouts are implemented at 400 meters below surface (mbs).At this shallow depth, pillar crushing is typically not achieved.

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