Authors: Lucas, DS; Vakili, A; Hutchison, BJ

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Lucas, DS, Vakili, A & Hutchison, BJ 2020, 'Three-dimensional numerical modelling for successful design of steep slopes at the Kanmantoo copper mine', in PM Dight (ed.), Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 1083-1096,

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Hillgrove Resources Limited (HGO) operated the Kanmantoo copper open pit mine in South Australia from 2010, until its successful completion in May 2019. The mine was a drill and blast, truck and shovel operation, and included the Kavanagh pit which was completed in 2014 at a depth of 240 m. It was extended and deepened to create the Giant Pit, which was recently completed at more than 360 m deep. Structures control the stability of all pit walls, but the west wall is dominated by a set of widely spaced but extensive joints that dip toward the east, denoted the J1 joint set, and it proved to be the most challenging to design and to manage hazards. Lessons learned in mining of Kavanagh pit led to the west wall of Giant Pit being designed with steeper batters and inter-ramp slope angles, which are undercut by widely spaced, highly persistent, east-dipping joints (set J1). There were concerns about the possibility of large-scale, deep-seated failure due to sliding on J1 structures and through intact rock. The depth, steep walls and confined lower benches also raised concerns about the potential influence of mining-induced stresses on larger-scale stability. Several phases of three-dimensional numerical modelling were conducted to assess the larger-scale stability. Modelling indicated that the steeper design could be achieved with an acceptable Factor of Safety against larger-scale instability, but that crest failures and rockfalls should be expected. In early 2018, when the Giant Pit was two thirds completed, HGO identified some increased movement that had been developing gradually for some time. Modelling was carried out using FLAC3D with structures represented as explicit discontinuities, generated by means of a discrete fracture network (DFN), and the rock mass represented as a finite difference mesh. The improved unified constitutive model was used in FLAC3D analysis as this model offers a more reliable prediction of the rock mass behaviour in higher stress conditions. A back-analysis model showed a good match to the observed magnitude and distribution of displacement, relationship to the structures, and the interpreted mechanism of the movement. The forward analysis showed that the Giant Pit could be completed with an acceptable Factor of Safety against larger-scale wall failure. The good ongoing agreement between the modelled and observed displacements provided confidence in the forward predictive abilities of the modelling method. Successful completion of the Giant Pit essentially verified the modelling predictions that the large-scale Factor of Safety would exceed the mine’s design criterion of 1.30, and that mining-induced stress due to the threedimensional shape of the pit would not induce stress-driven failure. The three-dimensional shape and complex structures could not have been modelled as successfully using more traditional slope stability analysis methods.

Keywords: slope stability, slope design, numerical modelling, steep mining

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