Authors: Zhou, H; Amodio, A; Boylan, N

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Zhou, H, Amodio, A & Boylan, N 2019, 'Informed mine closure by multi-dimensional modelling of tailings deposition and consolidation', in AB Fourie & M Tibbett (eds), Proceedings of the 13th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 287-302.

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Abstract:
This paper introduces an advanced geotechnical numerical modelling approach, which can be used to simulate the gradual deposition and large-strain consolidation of tailings in a multi-dimensional space. The findings from the modelling can be used not only to inform planning and design at mine closure but also help the management of tailings impoundments. For example, the results can be used to determine the settlement of tailings with time and thus inform backfilling planning; or inform on the tailings settlement and thereby assist with the design of an effective drainage network to divert surface water. Following a state-of-practice review of tailings consolidation modelling, the recently developed multidimensional modelling technique (the Norwegian Geotechnical Institute [NGI] model) is described in the paper, with validations against available analytical solutions and comparisons with commonly used predictions presented by Townsend & McVay (1990). An example of the application of the NGI model is presented to demonstrate its capability and performance in modelling a full-scale scenario. The NGI modelling approach is built on commercially available specialised geotechnical modelling software, FLAC (and FLAC3D), through its embedded programming language, FISH. The NGI model extends FLAC’s existing capability of large-strain consolidation calculation to simulate the gradual deposition process of the tailings. The deposition of the tailings slurry is divided into many discontinuous layers, and these layers are activated one after another from the bottom up. Activation of each new layer (on top of the existing tailings surface) is followed by a large-strain consolidation stage, with the consolidation time being determined as a function according to the volume of the layer and discharge history. Rock backfilling can be modelled in a similar fashion or can be customised. A user-defined constitutive model (as part of the NGI model) has been developed to reproduce the key characteristics of the tailings during consolidation, including the variation of compressibility and permeability with reducing voids ratio. The consolidation of the tailings is modelled in a large-strain mode (i.e. the coordinates of the grid are updated frequently) in order to capture its effect on the consolidation behaviour and the deformation occurred prior to addition of a new layer. The NGI model is also capable of performing complex three-dimensional problems accounting for varying consolidation boundary conditions, non-uniformity of the tailings material, and irregular pit geometries. As illustrated by the example application, this approach can be used to predict the development of tailing consolidation settlement with time, the amount of water expressed during consolidation, the capacity of the pit for tailings storage and the required amount of rock for backfilling. Further development is ongoing in order to expand its modelling capability, such as prediction of increase in tailings strength with consolidation, modelling drying and consolidation of tailings, simulation of tailings dam construction process to improve prediction of tailings dam stability, and so forth.

Keywords: numerical modelling, tailings, large strain, consolidation

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