The document discusses the undermining of spillway chutes in dams, including common failure modes, case histories, prevention, inspection, and rehabilitation methods. Some key failure modes are high velocity flow leading to structural collapse or hydraulic jacking. Case studies examine dams where undermining occurred due to poor design elements like inadequate cutoffs, filtering, or joint support. Prevention strategies include adding waterstops, cutoffs, dowels, and improving drainage systems. Inspection uses methods like coring and video to detect voids. Rehabilitation can include grouting, overlays, anchoring slabs, and improving joints and cutoffs.

1. UNDERMINING OF SPILLWAY CHUTES Greg Paxson, PE Jeremy Young, PE ASDSO DAM SAFETY 2010 SEATTLE, WASHINGTON September 22, 2010

5. Failure Modes - Trojanowski High Velocity Flow – Open Joint

6. Failure Modes - Trojanowski High Velocity Flow – Offset Joint

7. Other Failure Modes Inadequate Filtering

8. Other Failure Modes Inadequate Cutoff/ Excessive Seepage

9. Undermining Prevention Measures Staged Filter Waterstop Dowel Cutoff for Joint Support and Drain Isolation

11. Case Histories – PFBC Dams

13. Colyer Lake Dam – Cracked Spillway

14. Colyer Lake Dam – Loss of Drainfill

16. Colyer Lake Dam – Upstream Cutoff

17. Colyer Lake Dam – Joint Detail

18. Colyer Lake Dam - Drainage Detail

21. Speedwell Forge Lake Dam – Joint Detail

22. Speedwell Forge Lake Dam – Endsill Detail

25. Colyer Lake Dam – Weir Detail

26. Colyer Lake Dam – Joint Detail

27. Colyer Lake Dam – Drain Detail

29. PFBC Dams – Comparison Colyer Speedwell Leaser Year Constr. 1966 1966 1970 Upstream Cutoff No Yes No Joint Support Yes No Yes Waterstop No No No Cutoff No No Yes Adequate Filter No No No

32. Township Line Dam – Spillway Deterioration

33. Township Line Dam – Spalled/Offset Joint

34. Township Line Dam – Cutoff Detail Design As-built

37. Stoney Creek Dam – Spillway Deterioration

38. Stoney Creek Dam – Offset Joint

39. Stoney Creek Dam – Joint Detail

40. Stoney Creek Dam – Drain Detail

44. Youngman Dam – Joint Detail

45. Youngman Dam – Weir Detail

47. Youngman Dam – Ogee Rehab

49. Big Creek Lake Dam – 2005 Sinkhole

50. Big Creek Lake Dam – Loss of Drainfill 2005

51. Big Creek Lake Dam – Open Joint 2005

52. Big Creek Lake Dam – Undermining 2005

53. Big Creek Lake Dam – Joint Detail

55. Big Creek Lake Dam – Slab Rehab

56. Big Creek Lake Dam – Overview

63. Questions? UNDERMINING OF SPILLWAY CHUTES Greg Paxson, PE Jeremy Young, PE ASDSO DAM SAFETY 2010 SEATTLE, WASHINGTON September 22, 2010