Authors: Fournier, M; Mercer, R; Yang, D; Miller, J

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Fournier, M, Mercer, R, Yang, D & Miller, J 2013, 'Characterisation and stability modelling in weak rock masses of the Robinson Mine', in PM Dight (ed.), Slope Stability 2013: Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 595-609,

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The Robinson Mine is a large porphyry copper deposit located approximately 400 km north of Las Vegas in central Nevada. Large scale open pit mining operations started in 1908, but ceased in 1978. The property was re-opened in 2008 and the mine currently consists of five open pits. Since 2012, the mine has been operated by KGHM Inc. with a focus on expanding and deepening the Ruth pit. The pit walls were largely developed with 15 m high benches and an inter-ramp angle of 36°. This wall configuration was largely based on a pit geotechnical study in the early 1990s. In 2011, excessive slope deformation and wall failures were observed along the 300 m high north wall. The deformations were primarily within the Chainman Shale, which forms the lower half of the wall. Initial deformation control and risk mitigation efforts focused on surficial flattening of the deforming materials and on the installation of a slope monitoring radar system. By December, the mine was forced to discontinue mining in the area, which has had a significant impact on the mine plan. Preliminary back−analyses in early 2012, suggested that a circular-type failure was occurring within the highly altered and weak shale and that this unit appeared to be degrading over time The back-analyses also suggested considerably lower strengths for this unit than what was being utilised in the existing pit slope design. A geotechnical drilling program was undertaken to better understand the rock mass characteristics at these depths and to support updated slope recommendations. The results of the drilling program suggested high core recoveries in the shale and the rhyolite, even though many sections of the drill core could be crushed by hand. The rock mass was classified using the RMR system, with low ratings assigned to the low quality intervals via adjustments derived from GSI mapping of available surface exposures. Triaxial and direct shear laboratory testing was also taken on the soil-like materials. The results of this work suggested that the strategically employed RMR classification, coupled with a Hoek–Brown model, may not adequately capture the strength condition for a rock mass that varies spatially between regions of intact rock and soil-like materials. In general, the back-analysis models show considerable deviation from what would be considered as the expected condition, given the rock mass characterisation and laboratory testing available. The incorporation of a disturbance zone into the slope stability analyses overcame some of these limitations and resulted in a better match to observed slope performance.

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