PL566 Remedial Assistance Project

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Colorado
Case Study

• PL566 Remedial Assistance Project

Frank RiggleColorado Assistant State
ConservationistWater Resources
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The Dam

• Built with PL-566 in 1971
• Flood Control
• Crest to reservoir 30
• Structural Height 70
• Structure length 490

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Holes
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Intelligent Rodents???
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Accelerating Problem

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

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Breach Order

• 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

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OPTIONS ???

• Met with sponsors and State Engineer
• Discussed needs and strategy
• Initiated Formal Engineering Investigation

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Why is the dam cracking?

• Extended drought?
• Poor operation and maintenance?
• Poor construction?
• Engineering and design oversight?

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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,

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

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

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Emergency Action Plan

• Breach Inundation Study
• NOAA
• Monitor precipitation
• Monitor dam when rainfall gt2
• Reverse 9-1-1
• Brief County and City Emergency Staff

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How Many Engineers Does it Take??
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EMBANKMENT AS RESERVOIR FILLED
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Horizontal Crack
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Transverse Cracks
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Transverse Crack
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Transverse Crack
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The filter has not been contaminated with piped
soil material.
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Engineers 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

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RESTORED EMBANKMENT
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Current Activities

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

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Questions ???