Authors: Ford, NT; Silverton, TR; Cottrell, MG

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Ford, NT, Silverton, TR & Cottrell, MG 2008, 'Rock Mass Characterisation of a Prospective Base Metal Deposit Using a Combined FracMan/ELFEN Approach', in Y Potvin, J Carter, A Dyskin & R Jeffrey (eds), Proceedings of the First Southern Hemisphere International Rock Mechanics Symposium, Australian Centre for Geomechanics, Perth, pp. 605-617.

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This paper demonstrates how the combined use of a discrete fracture network tool (FracMan) and a combined continuum-discrete element tool (ELFEN) has resulted in significant improvements in the understanding of the fundamental mechanisms contributing to rock mass behaviour in poor ground allowing geotechnical risks to be better understood and managed. The Golder Associates’ FracMan combined with Rockfield Software’s ELFEN code has been applied to the problem of quantifying the geotechnical risk for a number of potential mining methods being considered for the recovery of a selected lead/zinc ore body in Algeria. FracMan enables the generation of a rock mass model based on geotechnical data obtained from boreholes, photographs and face mapping. It then permits the creation of integrated three-dimensional (3D) stochastic models using a discrete fracture network (DFN) description. The synthetic fractures in a DFN model can have any size and shape, and can be located deterministically, stochastically, or conditioned to field measurements. The stability of excavations (both 3D benched slopes and complex underground shapes) can be analysed using FracMan. This allows for the identification of completely defined rock blocks formed by finite fractures and allows for the factor of safety of these blocks to be determined in both supported and unsupported ground. The underlying Monte Carlo technique enables sensitivities to ground variability and the quantification of the geotechnical risk for an excavation to be determined. A FracMan model of the selected rock mass from a lead/zinc deposit in poor ground was created using the available geotechnical data. The resulting DFN formed the basis of a series of conceptual models, analysed using the combined continuum-discrete element capabilities of ELFEN. Various potential excavation shapes for underground mining were considered to assess the viability of the mining method and quantify the associated geotechnical risk. ELFEN has been used to examine the fundamental mechanisms contributing to rock fragmentation and the consequent flow of material, this approach enables the rock mass performance to be predicted with confidence. The use of FracMan modelling and ELFEN to determine the geotechnical risk is a relatively new approach and provides a more powerful tool than traditional methods of rock mass assessment, allowing greater confidence in calculating geotechnical risk during the process of mining method selection and design.

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