Mine Tailings Storage: Safety Is No Accident
Causes of tailing dams failures 1915-2016
30 Slope instability - static failure A constant load that causes deformation, to the point at which a dam partially or completely fails. Often caused by partial saturation of areas of the dam that are designed to remain dry.
52 Unknown Many of the older dam failures that were not sufficiently documented may fall into this category.
15 Foundation - structural and foundation conditions, foundations with insufficient investigations Failure related to building the dam on a surface that does not provide sufficient support for the weight of the dam. An example is a layer of clay under a dam. 16 Structural - structural inadequacies, inadequate or failed decants Design errors or failure of a designed component to function as designed. Failed decants (which drain water from the impound- ments) are a common cause.
27 Earthquake - seismic instability Dams are designed to withstand earthquakes, but if the earthquake is larger than that which was anticipated, the structure can be destroyed by the shaking.
7 Erosion - external erosion Simple erosion of a dam face, typically due to precipitation run-off that is not repaired
44 Overtopping Water flowing over the top of a dam. Tailings dams are made of erodible material, and overtopping will cause erosion.
17 Seepage - seepage and internal erosion Erosion of dam material due to water passing through areas of the dam that are designed to remain dry.
1 Mine subsidence If the dam or impoundment is built above an underground mine, collapse of the underground mine workings can lead to release of the impounded tailings.
Source: ICOLD 2001, Chambers 2017
Figure 10. Reported causes of tailings dam failures
instability from seismic loading compared to the downstream method (Liang and Elias 2010).
in the process. If no water were retained on top of a tailings storage facility, overtopping would be a vastly reduced risk.
The presence of water on tailings exacerbates the consequences of a tailings dam failure. The Mount Polley expert panel noted: “Without exception, dam breaches produce tailings releases. This is why best practices can only go so far in improving the safety of tailings technology that has not fundamentally changed in the past hundred years. Improving technology to ensure against failures requires eliminating water both on and in the tailings: water on the surface, and water contained in the interparticle voids.” (IEEIRP 2015, pp. 119–120). An initial breach of the retaining embankment of a tailings storage facility – for example, due to overtopping – can result in erosion of the embankment to the extent that retained water begins flowing from the tailings storage facility. Once this begins, it invariably escalates, with the entire volume of retained water flowing through the breach, usually mobilizing a large volume of fluid tailings
Unfortunately, getting rid of all free water on top of existing tailings storage facilities depends on the storage method employed. Many tailings storage facilities deposit wet tailings. When the solids settle, large volumes of free water separate out and must be stored and managed (in most cases, water is returned to the mill), although in wet climates, a “wet cover” is a common closure strategy to prevent acidic and metallic drainage. This requires the surface of the tailings storage facility to be kept submerged, potentially for decades or even centuries. In light of the Mount Polley failure, there will undoubtedly be a focus on assessing the risk of using wet covers as long-term closure options.
In order to avoid, or at least minimize the storage of water on top of a tailings storage facility, what are the realistic options
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