Authors: Edmondson, P; Rogers, S

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Edmondson, P & Rogers, S 2023, 'A hybrid deterministic–stochastic discrete fracture network to evaluate potential inter-ramp instabilities', in PM Dight (ed.), SSIM 2023: Third International Slope Stability in Mining Conference, Australian Centre for Geomechanics, Perth, pp. 617-626,

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The rock mass at Batu Hijau is characterised by the presence of major and intermediate scale faulting that impacts slope stability at the inter-ramp scale. This was identified as the key element for any anticipated failure mechanism during the next phase of the open pit development. Traditional kinematic analyses of planar (one sliding face) and wedge (two sliding faces) failures are carried out at the inter-ramp scale to assess stability based on prescribed Factor of Safety (FoS) and Probability of Failure (PoF) measures. This case study used a full 3D discrete fracture network (DFN) modelling approach, carried out within the FracMan® code, to fully evaluate inter-ramp scaled instabilities. The analysis technique uses a limit equilibrium approach to consider the stability of complex polyhedral wedges formed within the slope from both explicit wireframed structures and stochastically generated intermediate scale structures. The explicit wireframe structures are based on the deterministic fault model from mapping over life of mine. The stochastic faults were developed based primarily upon parameter inputs from acoustic televiewer (ATV) surveys obtained when drilling in the walls for the next stage of mining. For each realisation, the deterministic (explicit) major faults were combined with the stochastically generated intermediate scale faults to create a far more realistic description of the slope rock mass fabric in the form of a DFN model. The DFN is then searched for potential wedges, with both planar and wedge failing blocks being identified. A composite mechanism was explored by checking stability on non-daylighting wedges using the limit equilibrium tool. Key model input variables were calibrated to match failure volumes recorded from the previous stage of mining. The same variables were then projected onto the future mining stage to predict future failure volumes. Based on the anticipated structure, comparable results were obtained for blocks greater than 1,000 m³, considering the larger size of the next mining stage. Results provided a more realistic prediction of how the rock mass fabric contributed to instability and gave confidence to optimising slopes at the mine.

Keywords: discrete fracture network, complex kinematics, limit equilibrium tool, Probability of Failure, Factor of Safety

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