Smithyman, M, Puebla, H, Chance, A, Beddoes, R & Creighton, A 2013, 'Three-dimensional numerical stability analysis of the Oyu Tolgoi open pit', in PM Dight (ed.), Slope Stability 2013: Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering
, Australian Centre for Geomechanics, Perth, pp. 669-682, https://doi.org/10.36487/ACG_rep/1308_44_Smithyman
Three-dimensional (3D), stress-deformation models are useful for analysing the stability of complex 3D problems in open pit mine slopes. Stress-deformation models are very powerful because in addition to the Factor of Safety, the states of stress and strain throughout the pit are computed in a single model where a large number of potential failure mechanisms may be considered simultaneously. However, the massive quantities of information provided by these models make them difficult to interpret.
This paper presents and discusses a FLAC3D, version 4.0 (Itasca, 2009) numerical model that was developed to assess the stability of the proposed Oyu Tolgoi open pit, which will be approximately 2,900 × 2,100 m wide, and 600 m deep once it is completed. This large pit will include two distinct sub-pits that will have numerous fault intersections within the pit walls that cannot be adequately represented and assessed by 2D slope stability analysis models.
The FLAC3D model of the proposed Oyu Tolgoi open pit focused on assessing 3D complex failure mechanisms that involve one or more of the steeply dipping faults within the pit walls at two critical stages over the life-of-mine. This model allowed consideration of the pit stability from a stress-deformation perspective. While this approach can be used to assess pit stability in cases where very large displacements associated with well-defined failure mechanisms are computed (e.g. catastrophic failure), assessing the large scale structurally controlled stability is more challenging if a catastrophic failure is not predicted.
In this analysis, a number of methods were considered to determine the overall and local stability of the pit slopes. The following characteristics were considered: i) displacements, ii) strains, iii) velocities, iv) the yielding state of the model elements, and v) FLAC3D’s internal Factor of Safety calculation.
The automated Factor of Safety (FS) calculation within FLAC3D provides only the single most critical failure in the model. In the case of the Oyu Tolgoi open pit, the computed FS was relatively large and not related to 3D complex failure mechanisms involving the faults within the pit walls. Had the model been be set up to focus only in specific mechanisms of interest, the computed FS would have been larger, but nothing more than another number. From this perspective, the FS calculation was found of limited use in this case and hence, other information from the stress-deformation analysis was used to assess the stability of the pit slopes beyond the FS calculation. Examination of the patterns of displacement, strain, velocity, and yielding elements were found to be far more useful in determining the stability state of the pit. These were used to identify areas of the pit which showed the highest potential for movement or failure.
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