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.

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The Ernest Henry crusher chamber is a large excavation located at a depth of approximately 1 km. During the pre-feasibility study, a single exploration drill hole was extended below the orebody which identified no adverse ground conditions and the crusher location was set. Additional diamond drilling into the planned crusher chamber location was conducted in 2010 and followed by a geotechnical assessment of the life of mine chamber stability. This analysis found that the planned chamber was located almost entirely within a large fault zone, and that the chamber was likely to be unstable. The chamber was relocated as far as the constraints allowed, which ensured that there was no known fault exposure in the chamber backs although the fault zone was still exposed in the walls. The alternate chamber location was found to be more stable, but it was understood that the poor ground conditions would require heavy ground support to ensure life of mine stability. A detailed geotechnical assessment of the revised chamber location was undertaken to assess stability and support requirements, comprising of core logging, structural mapping, rock mass characterisation, empirical and kinematic stability assessments, simple numerical models and benchmarking against crusher chambers at other mines. The excavation sequence was set to minimise damage from over-mining, promote stability via the ground support installation sequence and minimise impacts on the mining schedule. Three-dimensional discontinuum non-linear numerical modelling was used to confirm the sufficiency of the proposed excavation design, sequence and ground support system over the entire life of the mine to identify potential problems and additional support requirements before the chamber was mined. Chamber construction was managed like a civil engineering project with a stringent quality control programme for both mining and ground support. An instrumentation system and ongoing monitoring programme were designed to monitor rock mass behaviour and ground support response and compare actual behaviour to the model forecasts.

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Numerous studies have proved that blast design impacts gravity flow in sublevel cave mines. However, such designs are yet to be fully optimised owing to the small number of mines with published full-scale experimental studies specifically planned to measure the effect of blast design variables on fragmentation and recovery. This paper details the results of a three-year experimental program undertaken at the Ernest Henry sublevel cave mine. The program consisted of 162 production blast rings conducted over six experimental trials to quantify the impact of ring burden, explosive density and crosscut height on gravity flow and recovery. The results of the research program are split between this paper and another entitled 'Full-scale experiments to measure the effect of crosscut height on recovery in sublevel cave mines' (Campbell 2018). The impact of ring burden and explosive density on fragmentation and recovery was quantified using full-scale experiments. Electronic cave markers were used to measure recovery over multiple sublevels and the extraction zone evolution in real time. Fragmentation was measured during loading since the size of the fragmented rock is known to affect gravity flow behaviour and ore recovery in sublevel cave mines. The size distribution of the fragmented rock from blasting was measured using high-resolution 3D laser scanning technology. Results of the experimental programs prove that ring burden and explosive density have a significant impact on ore recovery. Cumulative secondary recovery was between 8 and 10% higher on average for the reduced explosive density and increased ring burden trials compared with the baseline measurements. The effect of blast design variables on fragmentation was found to be less than expected. The experimental outcomes differ from earlier theories and the research hypotheses, which predicted the opposite effect on recovery for changes in ring burden and explosive density. Underground observations and detailed analysis are used to explain the results. Although the results are affected by the mine design and rock mass conditions at the mine and therefore site-specific, the findings on the relative impact of design variables on fragmentation and recovery are potentially transferable to other sublevel cave (SLC) mines. Additional site-specific testing is required to confirm the impact of design variables in different conditions and is recommended prior to selecting or altering blast design parameters.