Authors: Dueñas, J; Becerra, G; Ordoñez, R; Andrews, PG

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Dueñas, J, Becerra, G, Ordoñez, R & Andrews, PG 2020, 'Geotechnical evaluation of the east wall of the Cerro Corona Pit', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 473-486,

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This document describes the methodology applied to evaluate the anomalous deformation that began to develop in the east wall of the Cerro Corona pit; between levels 3,760 to 3,840 (masl). The main causes of the increase in deformation were associated with the presence of large structures that cross the slope (geological faults), the effect of groundwater, surface water (runoff), and the predominant lithology at these levels (mainly argillic alteration). The deformation began to increase from May 2017 until March 2019, where accumulated deformation rates of up to 1.60 m have been recorded in the robotic station, and up to 1.40 m with radar. A comprehensive geotechnical study was undertaken which has identified two steeply dipping structural systems (sub-vertical structures) that cross the east wall creating structural blocks (compartmentalised structures) that have facilitated the accumulation of both ground and surface water. In the formation of these structures, shear zones associated with the argillic alteration have been generated; allowing the formation of very low permeability aquitards which form fault bounded blocks. These blocks have generated high levels of groundwater compared to the levels outside the faults, and, consequently, have triggered the increase in deformation (due to anomalous increases in pore pressure). Using historical groundwater conditions, the stability analyses indicated that the slope was marginally stable, where the Safety Factors (or Factor of Safety, FoS) obtained (through numerical modelling) were between 1.1 and 1.2, which are below the Gold Fields acceptability criteria. To ensure that the slope design meets the established acceptability criteria, a depressurisation program was developed in this sector, where approximately 1,000 m of sub-horizontal drains have been drilled to depressurise the east wall, focusing on structural blocks and lithological contacts. The results of depressurisation programmes were very good, and consequently, the deformation rate has decreased to return to its normal deformation values. The results obtained from the stability analysis (current and life-of-mine (LOM)) under depressurisation conditions are well above the FoS acceptability criteria. Finally, the detailed geotechnical study and depressurisation has allowed the increase of the inter-ramp angle by 2° in the current LOM plan, and to add more ore reserves in ~ 1.4 Mtonnes.

Keywords: rock mass characterisation, slope stability optimisation, shear strength reduction, weak rock mass

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