Authors: Johnson, TM; Pere, V; Dixon, R; de Graaf, P; Wines, DR; Hebert, Y
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The Southern Ridge Cutback 3 (STR3) at the Tom Price mine site will be the highest and steepest slope in Rio Tinto Iron Ore’s Pilbara operation. Initial geotechnical assessment of the STR3 western slope using two dimensional limit equilibrium methods recommended a substantial flattening of the design. This would have resulted in the deferral of 3.2 Mt of high grade ore. Given the good performance of the preceding STR2 cutback, it was considered that the two-dimensional (2D) analysis results were not representative of the expected stability and were overly conservative. Structures constraining the dominant mode of instability strike oblique to the slope. This aspect and the effects of 3D lateral confinement are not considered by 2D analysis. In order to address this, a 3D modelling project was initiated with development of a 3D model by Itasca Australia Pty Ltd. The 3D model method utilises both 3DEC™ (Itasca 2013a) and FLAC3D™ (Itasca 2012) software to develop a constitutive model that takes into account the dominant bedding anisotropy within the slope. As this was the first such model developed for Rio Tinto Iron Ore (RTIO), an external review board was appointed to provide technical guidance during the project. Sensitivities were carried out to address questions regarding in situ stress regime, pervasive joint orientations relative to bedding, potential for large scale wedges, and ore friability. Three-dimensional modelling results were favourable and indicated that the existing design exceeded stability acceptance criteria. In addition, further optimisation of the slope was possible and would realise an additional 1 Mt of high grade recovery. In order to achieve this, revision of the slope design configuration was required. Work supporting this included assessment of the viability of 90 degree batter face angles and a re-routed haulage design. Batter-berm configurations and placement of wide geotechnical berms were tested for inherent stability and rockfall risk management effectiveness. Overall stability of the revised slope design was confirmed by the Itasca modelling. Some areas of potential local instability were identified and have been addressed by detailed design changes. This project demonstrates the potential value add that can be realised by 3D analysis, when compared with traditional 2D methods. Due to the high value of the STR3 ore as a blending material, this slope is being mined at a relatively high strip-ratio when compared with other RTIO Pilbara pits. This emphasises the potential impact of the protect plan and optimisation outcomes.

Keywords: STR3, 3DEC, FLAC3D, anisotropy

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