Potential Consequences of Failing to Control Seepage and Decant Water Level in a Tailings Dam

Tailings dams are used to contain the waste material of mining operations and represent the greatest environmental footprint of a mine. Although the size of tailings dams vary they can be as large as 4 square kilometres and, on flat terrain, are built in the form of a ring dyke. Tailings dams are used to dry out the slurried waste material of a mine and subsequently capture excess water so it can be reused. A ring dyke tailings dam spigots out the thickened slurry from the dam perimeter, allowing it to drain along a sloping beach towards a low point.

Although the preferred engineering option for such dams are similar to a water retaining earth dam, tailing dam embankments are progressively built using readily available materials in either an upstream, downstream, or centreline orientation as the mining operation progresses (figure 1).

Figure 1: Conventional embankment design techniques for a tailings dam [Source: www.tailings.info/conventional]

From a geotechnical engineering perspective, the downstream embankment design is preferred as it does not rely on the tailings material for its strength. Often a combination of high moisture content, low density and low stresses combine to produce a low strength tailings material. However, the downstream option is rarely used by mine managers as more and more material (and land) is required as the embankment is progressively built.

In all dams the designer aims to control the amount of seepage, seepage induced pore pressures and internal erosion/piping to ensure dam stability. This is often achieved through the application of drains (and filters) within the embankment to ensure the phreatic surface does not daylight on the downstream face of the embankment. If daylighting of the water table does occur then the effect is two fold; a reduction in shear strength and increased erosion/internal piping which can lead to a catastrophic failure of the embankment. Such a failure can have dire consequences to life and the environment, such as a large mudflow and the release of toxic/acidic tailings. One such failure occured in an Italian fluorite mine which sent 180000 cubic metres of material flowing towards the town of Stava, killing 268 people and causing 155 million Euros of damage.

During the design of a tailings dam embankment it is also important to cater for larger than expected inflows by designing spillways and bypass channels. Flood erosion is a problem common to all surface water impoundments, but overtopping is particularly important for tailings dams. This is because overflow across unconsolidated tailings can cause rapid erosion, leading to drainage from the tailings impoundment.

Overtopping occured in the Aurul S.A. tailings dam located in Baia Mare, Romania sending 100000 cubic metres of cyanide contaminated liquid into the Lapus stream (Figure 2). Described as the greatest European environmental disaster since Chernobyl the cyanide waters killed tonnes of fish and poisoned Hungarian drinking water supplies.

Figure 2: The failure of the Aurul Tailings dam to account for overtopping caused a great environmental disaster [Source: http://www.bristolbayalliance.com/mines_and_fish.htm]

The operation in Baia Mare revolved around the reprocessing of gold tailings from an old, unlined mine using sodium cyanide to recover the residual gold. The tailings stored in the Aurul impoundment had a cyanide concentration of 400 mg/L, but the designers constructed an embankment of inadequate height to account for melting snow and a heavy rain event. The rise in water level caused the dam crest to wash away, releasing the poisionous tailings. Except for the crest, the embankment remained structurally intact.