Tailings Dam Failures, ARD, and Reclamation Activities

1. Tailings Dam Failures, ARD, and Reclamation Activities John A Meech Professor of Mining Engineering The University of British Columbia Email:cerm3dir@mining.ubc.ca

2. Outline • Tailings Dam Construction Methods • Tailings Dam Failures • Reclamation of Dams, Waste Piles, and Sites • Britannia Beach and the Millennium Plug Project • Atmospheric Risks at the Sullivan Mine • Acid Rock Drainage – what is it? • ARD Control Methods • Microbiology of ARD

3. Issues Stability of dam structures • Use borrowed coarse material • Cyclone tailings to extract coarse fraction • Control pond water level so ground water does not enter the structure (phreatic surface) - Use barge/pump system - Use a tunnel/overflow tower system

4. Water-Retention Type Dam Steven G. Vick, 1983. Planning, Design, and Analysis of Tailings Dams, John Wiley & Sons, New York, pp. 369, ISBN 0-471-89829-5 [The textbook on the subject! A reprint was published in 1990 by BiTech Publishers Ltd., Richmond B.C., Canada (ISBN 0-921095-12-0)

5. Sequentially-built Tailings Dams Each lift requires more material – 1,3,5,7, etc.) Each lift requires more material – 1,2,3,4, etc.)

6. Sequentially-built Tailings Dams

7. Sequencing of Up-steam Tailing Dam Lifts

8. Phreatic Surface in Upstream Dams kL = permeability at the edge of the pond water at the slimes zonek0 = permeability at the spigot point (dam crest)kF = permeability of foundationkh / kv = anisotropy ratio (horizontal vs. vertical)

9. Ring Dike Construction - Kalgoorlie

10. Valley Deposit - HVC

11. Cross-Valley Plan View CROSS VALLEY IMPOUNDMENT - SINGLE AND MULTIPLE (Extracted from Vick, 1983. Planning, Design, and Analysis of Tailings Dams)

12. Side-Hill and Valley-Bottom Plan Views SIDE-HILL and VALLEY-BOTTOM IMPOUNDMENT - SINGLE AND MULTIPLE (Extracted from Vick, 1983. Planning, Design, and Analysis of Tailings Dams)

13. In-Pit Storage

14. Underground Storage • Hydraulic sand • Cycloned tailings sand (coarse fraction) • Cemented fill • Required to fill void space and create strength • Paste backfill • All tailings dewatered to 60-65% solids • Dry rock fill • With and without cement

15. Paste Backfill - Lisheen Mine, Ireland • Backfill plant with deep cone thickener

16. Hazards for Tailings Dam Stability Two Major Hazards: • Excessive increase in level of pond water on impoundment • Operational error during filling • Natural events (thunderstorms and/or flood inflow) • Beach width between the water and dam crest becomes too small • Phreatic surface rises in the dam and leads to collapse • Liquefaction during an earthquake • Tailings may change physical properties under seismic stress • Cyclic stresses can lead to liquefaction • Highly susceptible due to low bulk density and high saturation • Hazards are not theoretical • Many tailings dam failures prove the theories over and over again. • Recent example - Harmony gold mine tailings dam in South Africa (Feb. 1994) after heavy rainstorm - village completely buried - 17 people killed

17. Water Balance in a Tailings Dam

18. Up-steam Tailing Dam Typical Failure

19. Up-steam Tailing Dam Piping Failure

20. Up-steam Tailing Dam Failure too rapid rise - must be < 15 m/year

21. Up-steam Tailing Dam Failure over-topping

22. Up-steam Tailing Dam Failure liquifaction

23. Up-steam Tailing Dam Failure slope stability

24. Comparison of Surface Impoundment Types

25. Comparison of Surface Impoundment Types

26. Tailings Dam Failures • From 1968 to August 2009 - 149 documented failures worldwide • 3,500 tailings dams exist around the world 25,000 to 48,000 large water storage dams exist around the world. • Tailings dam failures closely match water storage dam failures So, failure frequency is far higher (an order of magnitude). • Since 2001, the failure rate is roughly one every 8 months. • 85% of incidents were Active tailings dams / 15% Abandoned dams • 76% of incidents were Upstream construction methods • 56% of incidents were dams greater than 30 m in height M. Rico, G. Benito, A.R. Salgueiro, A. Díez-Herrero, H.G. Pereira, 2010. Reported tailings dam failures. A review of the European incidents in a worldwide context.

27. 20th Century Tailings Dam Failures

28. Ten Causes of Failure ________________________________________________ Type of Failure Number % ________________________________________________ Unusual Rainfall 36 24.5 Seismic Liquefaction 21 14.3 Poor Management Operation 15 10.2 Structural Failure 13 8.8 Piping/Seepage 10 6.8 Foundation Failure 9 6.1 Overtopping 9 6.1 Slope Instability 7 4.8 Mine Subsidence 3 2.0 Snow melt 2 1.4 Unknown 22 15.0 _________________________________________________ TOTAL 147 100.0 _________________________________________________

29. Dam Failures due to Management Issues • Poor beach management • Faulty maintenance of drainage structures • Inappropriate dam procedures • rapid dam growth • Heavy machinery on top of unstable dam

30. Real-Time Monitoring of Tailings Dams • Piezo-electric gauges • Pore pressures at depth • Both horizontal and vertical directions • Control of barge pumps • Controllable CCD cameras • On top of dam structure • Along all diversion ditches • Water levels in all collection ditches/drains

31. Spigot Discharge

32. Other Methods

33. Submarine Tailings Disposal • Alpine lake disposal • High alpine regions (no fish) • Riverine disposal • Banned except in Indonesia • Deep Ocean disposal • Kitsault and Island Copper

34. Sub-aqueousTailings DisposalOptions • Impoundment • Covered Dam • Pit Filling • Submarine

35. Factors affecting Submarine Disposal

36. Island Copper Site Reclamation After 20 years of operation, the Island Copper Mine began reclaiming its waste dumps in 1996. Tailings were discharged deep into the adjacent fjord known as Rupert Inlet.

37. Island Copper Pit Flooding Pit was flooded with sea water to create a Meromictic lake – 3 layers: Top – clean water; Middle – a reactor for surface ARD; Bottom – retain precipitated solids.

38. Island Copper Pit Flooding Pit was flooded with sea water to create a 3-layer meromictic lake: Top – clean water; Middle – a reactor for surface ARD; Bottom – retain precipitated solids.

39. Mill EZD – Euphotic depth UWD – Upwelling depth MLD – Mixed Layer depth Deep Sea Disposal of Tailings Mill EZD – Euphotic depth UWD – Upwelling depth MLD – Mixed Layer depth

40. Thickened Discharge • Water drainage management is key http://technology.infomine.com/articles/1/1507/tailings.paste.thickened/paste.and.thickened.aspx

41. Dry Stack Tailings • Anglo-American's La Coipa Mine in Chile http://www.tailings.info/index.htm

42. Dry Stack Tailings • Anglo-American's La Coipa Mine in Chile

43. Dry Stack Tailings • Deposition by trucking

44. Dry Stack Tailings • Anglo-American's La Coipa Mine in Chile • Dewatering tailings to a filtered wet (saturated) or dry (unsaturated) cake • Must be transported by conveyor or truck • Material is deposited, spread and compacted as unsaturated tailings pile • Produces a stable deposit requiring no retention dam • Typical moisture content is below 20% - several percent below saturation • Combination of belt, drum, horizontal and vertical pressure plates and vacuum filtration systems

45. Dry Stack Tailings • Advantages • Dewatering tailings to a filtered wet (saturated) or dry (unsaturated) cake • Must be transported by conveyor or truck • Material is deposited, spread and compacted as unsaturated tailings pile • Produces a stable deposit requiring no retention dam • Typical moisture content is below 20% - several percent below saturation • Combination of belt, drum, horizontal and vertical pressure plates and vacuum filtration systems

46. Dry Stack Tailings • Disadvantages • High capital and operating costs due to filtration • Limited to low throughput operations (~20,000 tpd) • Diversion systems to prevent inundation of stack • Surface contour management to handle surface water • Must prevent ponding and erosion of the stack • No option to store water within a dry stack facility • Sulfide oxidation creates high metal levels, low volumes • Dust generation is problematic in arid climates • Not suitable in high rainfall environment • Seasonal fluctuations are important considerations

47. Co-Disposal of Waste & Tailings • Co-mingling • Tailings and coarse waste rock material transported independently • Mixed together mechanically in storage facility or slurry-pumped • Mixing promotes voids filling (mingling) to maximise density • Co-placement • Tailings and coarse waste rock material transported independently • Not mixed to form a single discharge stream • Waste rock end dumped into tailings facility • Waste rock used to create internal berms or retaining walls (sometimes) • Co-deposition • Similar to co-placement, but waste streams placed in layers • Deposited tailings naturally enters voids in underlying rock • End-dumping waste rock with tailings deposition down face prior to further end dumping

48. Dam Remediation Efforts By today's standards this dam is just too high for its design water flow and material properties. Built over many decades, a second dam was required to be built in the late 1990s to prevent water release (high As content).

49. Main dam of the Helmsdorf uranium mill tailings deposit, Oberrothenbach (Saxony)

50. Reparation Work