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Colorado

Colorado. Case Study. PL566 Remedial Assistance Project. Frank Riggle Colorado Assistant State Conservationist Water Resources. The Dam. Built with PL-566 in 1971 Flood Control Crest to reservoir: 30’ Structural Height: 70’ Structure length: 490’. Holes. Intelligent Rodents???.

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Colorado

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  1. Colorado CaseStudy PL566RemedialAssistanceProject Frank RiggleColorado Assistant State ConservationistWater Resources

  2. The Dam • Built with PL-566 in 1971 • Flood Control • Crest to reservoir: 30’ • Structural Height: 70’ • Structure length: 490’

  3. Holes

  4. Intelligent Rodents???

  5. Accelerating Problem • Holes along crest increased in the past 4-5 years • Colorado State Engineer’s inspection started in 1991 • NRCS State Engineer’s review mid 1990s • Northern Plains Engineering Team Review 1/2004

  6. Because of the worsening condition of the dam and a concern with dam safety: The Colorado State Engineer issued the sponsors a breach order in March 2004 Breach Order

  7. OPTIONS ??? • Met with sponsors and State Engineer • Discussed needs and strategy • Initiated Formal Engineering Investigation

  8. Why is the dam cracking? • Extended drought? • Poor operation and maintenance? • Poor construction? • Engineering and design oversight?

  9. Investigation • Investigation Engineering Team included members from Colorado, Texas, and Nebraska • Site Visit - Visual Examination • Excavated Exploration Trenches • Interviews with NRCS State Engineer, former NRCS District Conservationist, and Colorado State Dam Safety Engineer about involvement during construction • Examination of construction drawings, job diaries, soil engineering report, geology report, drill logs, profile and cross section drawings,

  10. ½” – 3” diameter holes became evident in early 1980’s, and are considerably larger today View of embankment with sink hole in foreground, shows developing pattern of holes along crest.

  11. Site Survey/Excavation • Holes, cavities, and cracks of significant size observed • Located in a line along the crest • Lined up in three distinct areas perpendicular to axis of dam • Lines of holes were on either side of principal spillway, and the third close to auxiliary spillway • Virtually all holes located on upstream half of embankment

  12. Test pit dug over hole shows crack at depth • Bucket width approximately two-feet • Yellow soil is paint used to reveal crack while digging.

  13. Emergency Action Plan • Breach Inundation Study • NOAA • Monitor precipitation • Monitor dam when rainfall >2” • Reverse 9-1-1 • Brief County and City Emergency Staff

  14. How Many Engineers Does it Take……??

  15. EMBANKMENT AS RESERVOIR FILLED

  16. HorizontalCrack

  17. Transverse Cracks

  18. TransverseCrack

  19. TransverseCrack

  20. The filter has not been contaminated with piped soil material.

  21. Engineer’s Report • Cause of cracking/cavities differential settlement of upstream embankment and at the steep abutments and principal spillway along upstream edge of cutoff trench • Holes and cavities are surface expressions of cracks in compacted earthfill • Transverse cracks represent the most serious concern regarding safety of dam • Responsibility for problem can be attributed primarily oversights and/or misjudgments during design, and construction of original embankment • Underestimating collapse potential of sandy alluvium • Decision to locate cutoff trench on dam centerline instead of near the upstream toe

  22. RESTORED EMBANKMENT

  23. Current Activities • Draft environmental assessment • Interagency review • Copies of draft to affected landowners • Final draft designs to Fort Worth for review

  24. Questions ???

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