Authors: Kabuya, JM; Simon, R; Carvalho, J; Haviland, D

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Kabuya, JM, Simon, R, Carvalho, J & Haviland, D 2020, 'Numerical back-analysis of highwall instability in an open pit: a case study', 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. 937-952,

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In this paper, a numerical modelling study to back analyse a highwall instability event in an open pit has been completed. The unstable highwall was over 125 m high and 200 m wide. The overall slope angle was 44°. The highwall instability was detected by slope stability radar and resulted in a multi-bench failure consisting of approximately 3,000,000 t of rock. This highwall’s failure provided the opportunity to develop an understanding of the mechanisms and key sensitivities involved with a failure of this magnitude. Using a combination of pre- and post-failure pit scans, the failure scarp was approximated and numerical models were developed and calibrated to simulate the event using SLIDE3 as the primary tool. The first model simulated isotropic rock mass strengths, without incorporation of directional weakness along the predominant orientations of discontinuities. We introduced successively directionally-dependent strength of the jointed rock mass and elevation-dependent foliation dip to confirm the failure surface. Once the failure surface was confirmed, the strength of the structures was reduced, and groundwater pressures were introduced to reach a Factor of Safety approaching the critical target of 1.0. To effectively simulate the highwall’s failure, the sub-horizontal joint set was weakened to initiate toe breakout. Subsequently, a section through the failure mass was analysed using RS2 finite element modelling software to explore the possibility of failure mechanisms not simulated in SLIDE3’s limit equilibrium calculation. While similar Factors of Safety (or shear strength reduction factors) were produced in the calibrated RS2 and SLIDE3 models using similar inputs assumptions, the significant differences in the two approaches must be considered. While significant uncertainties and opportunities for further study exist, the results of this simple calibration study were judged to reproduce the observed failure mechanism satisfactorily and may be referenced for future geotechnical design analyses for the open pit studied and other pits at the mine site where similar geotechnical conditions or features exist. The back-analysis highlights that understanding the geological variability associated with complex structural environments requires an excellent understanding of the orebody genesis and the regional geologic environment. Local orientations of discontinuity sets can have a significant effect on slope design and are often difficult to predict. The structural sets can be broadly defined but sensitivity analyses on the critical structural orientations should be completed, and continuously monitored as more data become available and during the mine development.

Keywords: open pit, back-analysis, limit equilibrium method, finite element method, slope stability radar

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