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SAFETY OF TAILINGS DAMS

(last updated 21 Aug 2004)

Contents

> See also:


INTRODUCTION


image (127k) : "Denison Uranium Tailings Basin, Elliot Lake, Ontario" (2.8 km2, 63.3 million t) - CDA

Thousands of tailings dams worldwide contain billions of tonnes of mineral processing industry wastes. Most tailings dams are raised sequentially using the "upstream" construction method, following the level of the impounded tailings during the filling process (see Tailings Dam Properties).

Upstream tailings dam

This method, while available at low cost, implies a number of specific hazards for dam stability.
These hazards require a thorough assessment and continuous monitoring and control during siting, construction, and operation of the dam. Experience shows that these conditions often are not maintained.

 


TAILINGS DAM DISASTERS


Violation of the safety requirements has led to hundreds of tailings dam failures worldwide, some of which with catastrophic consequences:

Detailed descriptions of selected tailings dam failures:

 


TYPICAL TAILINGS DAM FAILURE MODES


Hazard from weak foundation

If the soil or rock at shallow depth below the dam is too weak to support the dam, movement along a failure plane will occur. This may result in partial or complete failure of the dam.

Dam failure from foundation failure (Aznalcóllar, Spain)

Section of Los Frailes dam failure

 

Hazard from seismic events

Upstream tailings dams are known to have very poor properties during seismic events. During cyclic mechanical stress, as experienced during seismic events, the tailings slurries (including the material used for the dam) may liquefy.

Dam failure from seismic event
(Click image to view animation)

Dam failure from seismic event (Click image to view animation)

As a consequence, large parts of the impounded tailings may be released in a slurry wave (see details), causing catastrophic devastation in the downstream area.

In Chile, where several such failures have occured with tailings dams located in steep mountain valleys in areas of high seismic activity, the upstream technique is no longer regarded an acceptable option for tailings dam construction. (Supreme Decree 86: Regulation on the Construction and Operation of Tailings Dams, 1970)

In case of marginal dam stability, liquefaction even may occur from vibrations caused from heavy equipment (for example scrapers travelling along the dyke crest or the dam toe), from nearby mine blasting, or the like.

The very large uranium mill tailings dams in Thuringia - Culmitzsch (90 million t - image 13k) and Trünzig (19 million t) - are built on geological faults and are located close to the center of seismic activity in the Eastern part of Germany. They are therefore at a specific risk during earthquakes.

The main dam of the 50 million tonnes Helmsdorf uranium mill tailings deposit (image 24k) near Zwickau, Saxony, with its length of 1800 m and height of 59 m, in 1992 did not even meet the safety margins of the German dam safety standards.

Section of Helmsdorf tailings main dam
In the case of a complete failure of the dam, 6 million m3 of ponding water and 15 - 30 million m3 of slurries would flow downstream, containing 80 tonnes of uranium and 600 tonnes of arsenic, among others. The slurry wave would threaten 1000 inhabitants immediately, and 6500 others by the subsequent damming up of the Mulde river. A total area of 1000 hectares would be devastated. Therefore, the dam is considered a "highly explosive structure", according to the speaker of the Saxonian Ministry of Environment.

 

Hazard from excessive water level rise

Excessive rises in the level of the water ponding on the slurries in the impoundment can also cause failures of upstream dams - even if no overtopping occurs. This level rise can be caused by inflow from heavy precipitation events or by inappropriate water management of the mill operator.

In May 1994, the water level in the Helmsdorf dam (see above) approached the limit with 6 cm to spare. For this reason, an additional protection dam was built on top of the deposit in Spring 1995, to prevent water from reaching the main dam. Meanwhile, significant amounts of the ponding water could be removed from the impoundment, since a water treatment plant has been taken into operation, allowing for the necessary treatment of the heavily contaminated water before release.

If the exposed beach width becomes too small, the phreatic surface within the embankment rises and causes the toe of the dam to become unstable: The whole dam can collapse, starting from the toe of the embankment.

Dam failure from water level rise
(Click image to view animation)

Dam failure from water level rise (Click image to view animation)

 

If, however, the water level rise results in water overtopping the dam crest, complete breaching of the embankment is very likely. The overtopping water erodes the embankment within a very short time and can lead to a failure of the overall impoundment within minutes.

Dam failure from overtopping
(Click image to view animation)

Dam failure from overtopping (Click image to view animation)

 

Hazard from piping

Piping occurs, if seepage within or beneath the embankment causes erosion along its flowpath. Excessive piping may result in local or general failure of the embankment.

Dam failure from piping
(Click image to view animation)

Dam failure from piping (Click image to view animation)

 

Hazard from excessive dam rising rate

If an upstream dam is raised too fast, dam failure can occur from excessive pore pressure within the dam.

Dam failure from excessive dam rising rate
(Click image to view animation)

Dam failure from excessive dam rising rate (Click image to view animation)

 


TAILINGS SLURRY WAVES


Upon a tailings dam failure, large parts of the impounded tailings may be released in a slurry wave, causing catastrophic devastation in the downstream area. Typically, such slurry waves can travel at speeds as high as 8 - 40 km/h.

> See: Aerial views and animations of slurry wave from Stava tailings dam failure

The extent of the flow slide is a matter of the dam size and height, the tailings properties, and the ground slope, among others. (see also Tailings Flow Slide Calculator)

Tailings Flow Slide
(Click image to view animation)

Tailings Flow Slide (Click image to view animation)
(image and animation produced with Tailings Flow Slide Calculator)

 


TAILINGS DAM STABILITY ANALYSIS


For a stability assessment of a soil slope (such as a tailings dam), the balance of forces and moments along potential failure surfaces is analyzed (Limit Equilibrium Method). The slope is stable, if the available shear strength of the soil exceeds that required to keep the slope stable.
Factors affecting the slope stability are the height and the angle of the slope, the soil properties, the pore pressure within the slope, and external forces, such as seismic ground acceleration.
Since some of these factors can only be determined at great uncertainties, a Factor of Safety is used to describe the safety margin.

Slope failure schematic

> see Slope Stability Calculator

 


REQUIREMENTS FOR SAFE TAILINGS DISPOSAL


 


BIBLIOGRAPHY


GENERAL

> See Tailings Dam Properties - Bibliography

> See also bibliography on Decommissioning & Tailings Management !

UNEP MRF - Mining and Environment - Technical Issues: Tailings

InfoMine TailingsMine

Tailsafe: Sustainable Improvement in Safety of Tailings Facilities - A European Research Project


SURVEYS OF TAILINGS DAM INCIDENTS


CASE STUDIES


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