Abstract
The stability of open-pit brown-coal mines is affected by the manner in which water is transmitted or retained within their slopes. This in turn is a function of the in-situ fracture network at those mines. Fracture networks in real mines exhibit significant degrees of heterogeneity; encompassing a wide range of apertures, inter-fracture separations, and orientations. While each of these factors plays a role in determining fluid movement, over the scale of a mine it is often impractical to precisely measure, let alone simulate, the behaviour of each fracture. Accordingly, effective continuum models capable of representing the bulk effects of the fracture network are needed to understand the movement of fluid within these slopes. This article presents an analysis of the fracture distribution within the slopes of a brown coal mine and outlines a model to capture the effects on the bulk permeability. A stress-dependent effective-fracture-permeability model is introduced that captures the effects of the fracture apertures, spacing, and orientation. We discuss how this model captures the fracture heterogeneity and the effects of changing stress conditions on fluid flow. The fracture network data and the results from the effective permeability model demonstrate that in many cases slope permeability is dominated by highly permeable but low-probability fractures. These results highlight the need for models capable of capturing the effects of heterogeneity and uncertainty on the slope behaviour.
Highlights
The stability of an open-cut mine is strongly influenced by water flow within the in-situ fracture network
This is true of brown coal mines: while solid coal is relatively impermeable e.g., [1,2], it contains numerous fractures that permit water to flow into the mine slopes
To illustrate the effects of the different fracture-permeability distributions on the fluid flow, the numerical model was implemented within the Multiphysics Object-Oriented
Summary
The stability of an open-cut mine is strongly influenced by water flow within the in-situ fracture network. The effects of water can be represented as an overall reduction of shear strength in the rock [4,5,6] This fails to account for the effects of fractures and its heterogeneity in the slope. These discrete fracture models often require extensive knowledge and data of the fracture network to explicitly simulate the fracture [12,13,14] They are too numerically intensive to accurately represent large-scale simulations over long time frames, as required for mine slope stability [15,16,17]. By incorporating the fracture distribution in the numerical model, we are able to capture the effects of fracture heterogeneity This model can be applied in other contexts such as slope stability to highlight areas of high fluid retention that may cause slope failure e.g., [30]. The method used here to capture fracture heterogeneity in open-cut mines can be applied in other fields such as underground tunnelling and shale gas where heterogeneous fractures often dictate the fluid or gas flow [38,39,40,41]
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