Full-scale experiments to measure the effect of crosscut height on recovery in sublevel cave mines

Authors: Campbell, AD

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DOI https://doi.org/10.36487/ACG_rep/1815_34_Campbell

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Campbell, AD 2018, 'Full-scale experiments to measure the effect of crosscut height on recovery in sublevel cave mines', in Y Potvin & J Jakubec (eds), Caving 2018: Proceedings of the Fourth International Symposium on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 443-456, https://doi.org/10.36487/ACG_rep/1815_34_Campbell

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Abstract:
A major disadvantage of the sublevel cave (SLC) mining method is the potential for substantial dilution and ore loss. Scale-model experiments demonstrate that gravity flow and recovery in SLC mines is affected by crosscut geometry. Several authors also suggest that crosscut height affects the digging depth of loading equipment and therefore influences recovery, particularly from the back of a blasted ring. However, specifically designed full-scale experiments that directly compare the effect of different crosscut heights on recovery have not been published to date. This paper details the results of experiments conducted in an operating SLC mine to measure the effect of crosscut height on recovery and depth of draw. Two trial programs, each consisting of six blast rings, and each with approximately 285 electronic cave markers, were used to measure recovery in real time. One trial measured recovery for the standard 5 m crosscut height and the other for a crosscut height of 4 m; the smallest practical dimension for loaders used at the mine. The effect of reducing the crosscut height is a reduction in the distance a muck pile extends from the brow. The literature suggests that a shorter muck pile will enable loading equipment to dig deeper relative to the drawpoint brow. This was hypothesised to increase ore recovery from the back of the blast ring in a mining method in which shallow draw is common. The experimental program showed that reducing the crosscut height did not achieve the desired effect of improving primary recovery. It is proposed that drawpoint flow is controlled by the outflow depth at the drawpoint brow rather than the dig depth or length of the muck pile under the test conditions. Observations identified that the bucket of the loader did not penetrate the muck pile beyond the drawpoint brow, despite the crosscut height being reduced as far as practical. The experimental results do not justify SLC mines reducing the crosscut height to increase ore recovery. In terms of mine design guidelines, it is recommended that crosscut height be based solely on geotechnical, operational and equipment requirements. An accompanying paper in the proceedings (Campbell 2018) details the results of experiments conducted using different ring burdens and explosive densities to determine the effect on fragmentation and recovery.

Keywords: gravity flow, mine design, ore recovery, sublevel caving

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