ABSTRACT:

The exploitation of gold bearing reefs at depths greater than 3500 m has become an economic necessity for South African mining. A research project aimed at establishing the probable response of the rock mass to mining at such great depths and its impact on stope support was undertaken as a part of the DEEPMINE Collaborative Research Programme. Several subtasks were defined: an estimation of the state of stress, a compilation of relevant rock mass properties, an estimation of the level of seismicity, an estimation of the extent and nature of the fracture envelope around excavations, the expected stope closure and ride (including time dependency), and the height of the hangingwall to be supported in respective geotechnical environments. Furthermore the impact of these findings on the behaviour of stope support is being determined.

INTRODUCTION

As the Witwatersrand orebodies at depths shallower than 3000 m become depleted, the survival of the gold mining industry becomes more dependent on exploiting reserves at greater depths. As a consequence the industry as a whole has decided to position itself in order to confront this challenge. It is forecast that by the year 2015, some 40 per cent of South Africa's gold production will be sourced from depths greater than 3000 m, assuming a favourable economic environment (Willis et al., 1997). Currently, some mines, already mine at depths exceeding three kilometres and ore resources have been indicated to reach depths of up to five kilometres. The feasibility of mining at those depths however, demands a careful assessment of the expected rock mass behaviour. An understanding of the rock mass response to the high stresses expected to be encountered at ultra-deep levels would facilitate the design and adoption of appropriate rock engineering strategies to ensure safe mining conditions.

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