Johnson, TM, Pere, V, Dixon, R, de Graaf, P, Wines, DR & Hebert, Y 2016, 'Geotechnical optimisation of Southern Ridge Cutback 3 at Tom Price mining operations', in PM Dight (ed.), Proceedings of the First Asia Pacific Slope Stability in Mining Conference
, Australian Centre for Geomechanics, Perth, pp. 183-199, https://doi.org/10.36487/ACG_rep/1604_08_Johnson
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
Aquino, TVC, Figueiredo, RP & Franca, P 2009, ‘Geological and geotechnical characterization and failure mechanism of Friable Hematitic Ore in Vale iron ore mines. West of Iron Ore Quadrangle, MG – Brazil’, in Proceedings: Slope Stability 2009, Santiago de Chile, v. Único.
Barton, NR & Bandis, S, ‘Review of predictive capabilities of JRC-JCS model in engineering practice’, in N Barton & O Stephansson (eds), Rock joints, Proceedings of the International Symposium on Rock Joints, Loen, Norway, Balkema, Rotterdam, pp. 603–610.
Bitencourt, R, Mackenzie, P, Gordon, J & Morey, B 2002, ‘High-grade optimisation and improved grade control practices at Mount Tom Price’, in Proceedings of the Iron Ore Conference, AUSIMM, Melbourne, pp. 261–277.
Cai, M, Kaiser PK, Tasaka, Y & Minami, M 2007, ‘Determination of residual strength parameters of jointed rock masses using the GSI system’, International Journal of Rock Mechanics and Mining Sciences, vol. 44, pp. 247–265.
Dalstra, HJ 2014, ‘Structural evolution of the Mount Wall region in the Hamersley province, Western Australia and its control on hydrothermal alteration and formation of high-grade iron deposits’, Journal of Structural Geology, vol. 67, pp. 268–292.
Hoek, E & Karzulovic, A 2000, ‘Rock mass properties for surface mines’, in WA Hustrulid, MK McCarter & DJA Van Zyl (eds), Slope Stability in Surface Mining, Littleton, Colorado, SME, pp. 59–70.
Hoek, E, Carranza-Torres, C & Corkum, B 2002, ‘Hoek-Brown Failure Criterion – 2002 edition’, in R Hammah, W Bawden, J Curran & M Telesnicki (eds), NARMS-TAC2002: Mining and Tunnelling Innovation and Opportunity, University of Toronto Press, Toronto, pp. 267–273.
Itasca 2012, FLAC3D, Fast Lagrangian Analysis of Continua in 3 Dimensions, software, version 5.0, Itasca Consulting Group, Minneapolis.
Itasca 2013a, 3DEC, Three-Dimensional Distinct Element Code, software, version 5.0, Itasca Consulting Group, Minneapolis.
Itasca 2013b, Kubrix Geo Automatic Grid Generator, software, version 12, Itasca Consulting Group, Minneapolis.
Maptek 2013, VulcanTM, software, version 8.2.3, Maptek Pty Ltd, Adelaide.
Marinos, P & Hoek, E 2001, ‘Estimating the geotechnical properties of heterogeneous rock masses such as flysch’, Bulletin of Engineering Geological Environments, vol. 60, no. 2, pp. 85–92.
Morris, RC 2002, ‘Genesis of high-grade hematite orebodies of the Hamersley Province, Western Australia—A discussion’, Economic Geology, vol. 97, pp. 177–181.
RITO (Rio Tinto Iron Ore) 2010, ‘Geology and mineralogy of the Hamersley Province ores: Year 2010 update’, unpublished internal report, Rio Tinto Iron Ore Pty Ltd.
Rocscience 2010, Slide Version 6.0 – 2D Limit Equilibrium Slope Stability Analysis, Rocscience Inc., Toronto, www.rocscience.com
Rocscience 2015a, Dips Version 6.0 – Graphical and Statistical Analysis of Orientation Data, Rocscience Inc., Toronto, www.rocscience.com
Rocscience 2015b, Swedge Version 6.0 – 3D Surface Wedge Analysis for Slopes, Rocscience Inc., Toronto, www.rocscience.com
Rocscience 2015c, RocFall Version 5.0 – Statistical Analysis of Rockfalls, Rocscience Inc., Toronto, www.rocscience.com
Stacey, PF 2009, ‘Fundamentals of slope design’, in J Read & P Stacey (eds), Guidelines for Open Pit Slope Design, CSIRO Publishing, Collingwood, pp. 1–14.
Stacey, PF 2013, ‘Review of Slope Design Studies for the STR3 Cut’, unpublished letter report to RTIO, Stacey Mining Geotechnical Ltd.
Taylor, D, Dalstra, HJ, Harding, AE, Broadbent, G & Barley, ME 2001, ‘Genesis of high-grade hematite orebodies of the Hamersley Province, Western Australia’, Economic Geology, vol. 96, pp. 837–873.
Tien, YM, Kuo, MC & Juang, CH 2006, ‘An experimental investigation of the failure mechanism of simulated transversely isotropic rocks’, International Journal of Rock Mechanics and Mining Sciences, vol. 43, pp. 1163–1181.
Trendall, AF & Blockley, JG 1970, ‘The Iron Formations of the Precambrian Hamersley Group, Western Australia with Special reference to the associated Crocidolite’, Bulletin 119, Geological Survey of Western Australia, Perth.
Tyler, IM 1991, ‘The Geology of the Sylvania Inlier and Southeast Hamersley Basin’, Bulletin 138, Geological Survey of Western Australia, Perth.
Villaescusa, E, Windsor, CR & Machuca, L 2012, ‘Stress measurements from oriented core - a decade of results’, MassMin2012, Sudbury.
Wines, DR 2015, ‘A Comparison of Slope Stability Analyses in Two and Three Dimensions’, in Proceedings of Slope Stability 2015: International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, SAIMM.
WSMP (World Stress Map Project) 2013, World Stress Map Project,