SAFETY OF TAILINGS DAMS

(last updated 7 Feb 2019)

Contents

• Introduction
• Tailings dam disasters
• Typical tailings dam failure modes
• Tailings slurry waves
• Tailings dam stability analysis
• Requirements for safe tailings disposal
• Bibliography

> See also:

• Current Issues - Tailings Dam Safety
• Tailings Dam Properties
• Slope Stability Calculator
• Tailings Flow Slide Calculator

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).

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:

• View Chronology of major tailings dam failures

• View Chronology of uranium tailings dam failures

• The Stava tailings dam failure (July 19, 1985, Trento, Italy)
• Preliminary Report on Technical Causation - Omai Tailings Dam Failure, Submitted to Guyana Geology and Mines Commission by Dam Review Team, November 16, 1995
• The Los Frailes tailings dam failure (Apr 1998, Aznalcóllar, Spain)
• The Aurul S.A. tailings dam failure (Jan 2000, Baia Mare, Romania)
• The Aitik tailings dam failure (Sep 2000, Gällivare, Sweden)
• The Inez coal tailings dam failure (Oct 2000, Kentucky, USA)
• The Sebastião das Águas Claras tailings dam failure (June 2001, Minas Gerais, Brazil)
• The San Marcelino tailings dam spill (Aug/Sep 2002, Zambales, Philippines)

TYPICAL TAILINGS DAM FAILURE MODES

Hazard from weak foundation

Hazard from seismic events

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.


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

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.

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.

Hazard from piping

Hazard from excessive dam rising rate

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 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.

> see Slope Stability Calculator

REQUIREMENTS FOR SAFE TAILINGS DISPOSAL

• Establish world-wide standards for tailings dam safety
  These must include:
  • Planning:
    • site assessment (geology, seismicity, climate, upstream catchment area,...)
    • hazard assessment (heavy rain, flood, earthquake, ...)
    • analysis of anticipated dam failure impact (travelling path of slurries, downstream land use and water use,...)
    • selection of embankment type
      Upstream-type tailings dams should be allowed under strict conditions only: experience shows that this type is very difficult to control in real situations. Downstream-type and water-retention type embankments provide for much better safety margins. Another option for a safer tailings management is paste disposal rather than the widely practiced slurry disposal.
  • Construction:
    • quality assurance program for the technology and materials used, and for the construction process
  • Surveillance:
    • regular monitoring of phreatic surface within embankment, of dam movements, ...

• Improve stability of existing tailings impoundments
  (for example, by construction of diversion dams to prevent flood inflow, smoothing of slopes, ...)

• Also consider long-term safety and failure modes other than complete embankment failure (such as seepage, dust, long-term erosion, bio-intrusion, ...)
  While embankment breaks are the most spectacular failure mode for tailings dams, the long-term hazards should not be neglected either: tailings retain their hazard potential for thousands of years; this requires efficient measures to contain these hazards in the long term (see Uranium mill tailings deposits).

BIBLIOGRAPHY

GENERAL

• Mine Tailings Storage: Safety is no Accident . A UNEP Rapid Response Assessment, by Roche, C., Thygesen, K., Baker, E. (Eds.), United Nations Environment Programme and GRID-Arendal, 2017

> See also 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

• Tailings Dams - Risk of Dangerous Occurrences, Lessons learnt from practical experiences, Bulletin 121, Published by United Nations Environmental Programme (UNEP) Division of Technology, Industry and Economics (DTIE) and International Commission on Large Dams (ICOLD), Paris 2001, 144 p. [compilation of 221 tailings dam incidents mainly from the next two publications, and examples of effective remedial measures]
  > Read review · Download full text: UNEP (1.2M PDF) , or: ICOLD (947k PDF)

• Tailings Dam Incidents, U.S. Committee on Large Dams - USCOLD, Denver, Colorado, ISBN 1-884575-03-X, 1994, 82 pages [compilation and analysis of 185 tailings dam incidents]

• Environmental and Safety Incidents concerning Tailings Dams at Mines: Results of a Survey for the years 1980-1996 by Mining Journal Research Services; a report prepared for United Nations Environment Programme, Industry and Environment . Paris, 1996, 129 pages [compilation of 37 tailings dam incidents]

• Damage Cases and Environmental Releases from Mines and Mineral Processing Sites, U.S. Environmental Protection Agency, Office of Solid Waste, Washington DC, 1997, 231 pages - [62 summaries illustrating recent mining and mineral processing damage cases in a variety of mineral commodity sectors and states, including several tailings dam failures]
  Download full text - (800k, PDF format)

CASE STUDIES

• Church Rock uranium tailings failure, New Mexico, USA, 1979

  • Uranium Milling and the Church Rock Disaster , chapter 9 of Killing Our Own, The Disaster of America's Experience with Atomic Radiation by Harvey Wasserman and Norman Solomon, New York 1982

  • Failure of the Church Rock tailings dam, by John D.Nelson, Joseph D.Kane. In: Geotechnical Engineering Program, Civil Engineering Department, Colorado State University (Ed.), Uranium Mill Tailings Management. Proceedings of the Third Symposium, November 24-25, 1980, Fort Collins, Colorado 1980, p.505-511

  • Survey of radionuclide distributions resulting from the Church Rock, New Mexico, uranium mill tailings pond dam failure, by W.C.Weimer, R.R.Kinnison, J.H.Reeves. NUREG/CR-2449, 1981, 164 p.
    Order address: NTIS

  • An independent review of the environmental health aspects of the Church Rock, N.M. tailings spill, by Chester A.Sautter. In: Geotechnical Engineering Program, Civil Engineering Department, Colorado State University (Ed.), Management of Uranium Mill Tailings, Low Level Waste and Hazardous Waste. Proceedings of the Sixth Symposium, February 1- 3, 1984, Fort Collins, Colorado 1984, p.377-386

  • Effects of uranium mining, Puerco River, New Mexico, by Thomas J.Lopes. In: ASCE (Ed.), Proceedings of the 1991 National Conference on Irrigation and Drainage, Honolulu, HI, USA, Jul 22-26, 1991 , ISBN 0-87262-811-6, New York 1991, p.508-515
    [impacts of Church Rock tailings pond dam failure]

  • Contaminant Loading On The Puerco River - A Historical Overview , by Chris Shuey, Southwest Research and Information Center, Albuquerque, New Mexico, October 14, 1992

  • Effects of uranium-mining releases on ground-water quality in the Puerco River Basin, Arizona and New Mexico , by P. C. Van Metre, L. Wirt, T. J. Lopes, S. A. Ferguson; U.S. Geological Survey, USGS Water-Supply Paper 2476, 1997, 73 p.

• Arcturus gold tailings failure, Zimbabwe, 1978

  • Failure of a Mine Waste Dump in Zimbabwe: Causes and Consequences, by Richard A. Shakesby and J.Richard Whitlow. In: Environmental Geology and Water Sciences Vol.18, No.2, 1991, p.143-153
    [on the Arcturus gold mine tailings dam failure]

• Stava fluorite tailings failure, Italy, 1985

  • Rassegna dei contributi scientifici sul disastro della Val di Stava (Provincia di Trento), 19 luglio 1985 / A review of scientific contributions on the Stava Valley disaster (Eastern Italian Alps), 19th July 1985. a cura di/edited by Giovanni Tosatti, Volume speciale del GNDCI-CNR, pp. 480, Pitagora Editrice , Bologna, 2003, ISBN 88-371-01405-2 [comprehensive compilation of 24 papers on the causes of the Stava tailings dam failure, most of which first appeared in various scientific journals; all papers are in original language (Italian, English, Japanese, German)]

  • Stava perché , La genesi, le cause, le responsibiltà della catastrofe di Stava negli atti dell'inchiesta ministeriale e nelle sentenze del procedimento penale, a cura di Graziano Lucchi, 254 p., ISBN 88-87534-37-3, Seconda edizione, Trento (Italy), gennaio 2002 [in Italian]

  • The Stava tailings dams failure, Italy, July 1985, by R.J.Chandler and G.Tosatti. In: Proceedings of the Institution of Civil Engineers Geotechnical Engineering, Vol.113, No.2, April 1995, p.67-79

• Omai gold tailings failure, Guyana, 1995

  • Report of Commission of Inquiry into discharge of cyanide and other noxious substances into the Omai and Essequibo rivers, 5th January, 1996, 61 p.
    Order address:
    PIRA Communications
    901, rue de Bleury, Bureau 380
    Montréal, Québec H2Z 1M3, Canada
    Tel. +1-514-393-3883, Fax: +1-514-393-3398, Email: pira@axess.com

  • Preliminary Report on Technical Causation - Omai Tailings Dam Failure, Submitted to Guyana Geology and Mines Commission by Dam Review Team, November 16, 1995: HTML text (22k)

  • Omai Tailings Dam Failure - Final Report on Technical Causation, Guyana Geology and Mines Commission, 1996; 200 pp; BiTech Publishers Ltd. , Richmond B.C., Canada

  • Omai aftermath - a case study, by Kate Harcourt and Sheranne Wickham. In: Minerals, Metals and the Environment II, Prague 3-6 Sep 1996, The Institution of Mining and Metallurgy, London 1996, ISBN 1- 870706-31-5, p.435-451

• Sullivan lead-zinc tailings dam failure, Kimberley, British Columbia, Canada, 1991

  • Static liquefaction slump of mine tailings - a case history, by M.P.Davies, B.G.Chin, and B.B.Dawson, Proceedings of the 51st Canadian Geotechnical Conference, Edmonton, Canadian Geotechnical Society , 1998

  • Stanford University class work, Fall 1996 *)
  • H.Thrains, Stanford University *)
  • Robert M.Bigler, Stanford University *)
  *) these pages are temporarily unavailable for legal reasons

• National Technical Information Service NTIS

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