1. 01/2012 The Shikwamkwa Replacement Dam Five Years Later C. Richard Donnelly, Hatch ● Bryan Tatone, University of Toronto ● Warren Hoyle, Noel Boucher, Sean Hinchberger, Hatch ● Nina Godbout, Brookfield ● Darren Protulipac, Ontario Power Generation

2. 2 01/2012 The Original Shikwamkwa Dam A Dam in Severe Distress

3. 3 01/2012 Why do Dams Fail? 1. Overtopping 38% (including failure of gate to open) 2.Internal Stability 33% (Piping) 3.Foundation 23% (instability, bearing capacity, etc) 4.Other 8% (eg. Gate failure)

4. 4 01/2012 When do Dams Fail?

5. 5 01/2012 ‘Stable’ and ‘Unstable’ Steady State Timing of Failure related to achieving Stable Steady State Original Shikwamkwa –Inadequate seepage defenses –series of remedial measures extended the life of the dam by 50 years –Stable Steady State conditions could not be achieved The Challenge –A design that fully accommodates the adverse foundation conditions

6. 6 01/2012 The Nature of the Problem High Pressure Silt Upper Aquifer Lower Aquifer

7. 7 01/2012 The Solution Manage the Seepage Flows

8. 8 01/2012 Fast Track Embankment Dam Construction Over 1 million m 3 of fill placed Fast Track Embankment Dam Construction OVER 1 MILLION M 3 OF FILL PLACED

9. 9 01/2012 One of the World’s Deepest Cutoff Walls

10. 10 01/2012 Slurry Ponds (Mixing pond, live storage, spare) Slurry Ponds (Mixing pond, live storage, spare) Plastic Concrete Batch Plant Fine Aggregate Coarse Aggregate Working Platform ~30m wide (Typical) Working Platform ~30m wide (Typical) QC/QA Laboratory COW Equipment Maintenance Area AgitatorAgitator Bentonite hydrator Slurry supply lines ChiselChisel Active COW excavations Stop-endStop-end De-sanderDe-sander Service crane GrabGrab Tremie pipe On Time, Under Budget and No Lost Time Accidents

11. 11 01/2012 Cut-off Wall Construction Challenges Boulder and cobble nests Hard, dense glacial till at bedrock interface Steep/irregular bedrock topography Difficulty extending excavation to bedrock in some locations resulting in windows

12. 12 01/2012 The Impact Continued higher pressures Concentrated flows though local windows Recharge of Lower aquifer through active sinkholes

13. 13 01/2012 The Result Boils in the tailpond immediately after construction

14. 14 01/2012 Monitoring Head Pond Lower Aquifer Upstream Lower Aquifer Downstream

15. 15 01/2012 The End Result Reduced pressure and seepage velocity allows infilling of Sinkholes Gradual sealing of windows and reduced pressures/seepage velocities Tailpond boils eliminated Pressures gradually reducing in lower aquifer

16. 16 01/2012 Environmental Enhancements Low level flow compensation –cold water from the bottom of the reservoir –simulated spring and fall freshets and up-wellings in fish spawning areas tail pond thermal refuge Flow modification structures – placement of coniferous trees anchored along the margins of the tail pond Rock vortex weirs, –grade control and diversity of flow velocities using a series of large boulders.

17. 17 01/2012 Environmental Results Mercury Concentrations in Fish –Slight incraese for some species Surface Water Quality –minor changes for about a year. Methyl Mercury Concentrations –slight increase in shallower portion of the new reservoir Shoreline Erosion –By Year 5 all erosion had ceased. Fish Community –continuous increase with a healthy distribution of year classes

18. 18 01/2012 Lessons Learned Despite thorough investigative efforts, problematic geology can be encountered during construction –Experienced site supervision and ability to make change essential for dam construction Dam design continues until the dam and its foundation reach stable steady state In the case of the Shikwamkwa dam stable steady state reached after a period of five years –Today seepage and piezometric pressures have decreased above and beyond expectations –No signs of concentrated seepage or downstream boils Properly designed, dams can provide environmental benefits

19. 19 01/2012 Shikwamkwa Dam Job Complete

20. 20 01/2012 For more information, please visit www.hatch.ca