Case Study
Colorado
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???
Accelerating Problem
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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
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
OPTIONS ???
 Met with sponsors
and State Engineer
 Discussed needs and
strategy
 Initiated Formal
Engineering
Investigation
Why is the dam cracking?
 Extended drought?
 Poor operation and
maintenance?
 Poor construction?
 Engineering and
design oversight?
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,
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.
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
 Test pit dug over hole shows crack at depth
 Bucket width approximately two-feet
 Yellow soil is paint used to reveal crack while digging.
Emergency Action Plan
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Breach Inundation Study
NOAA
Monitor precipitation
Monitor dam when rainfall >2”
Reverse 9-1-1
Brief County and City Emergency Staff
How Many Engineers Does it
Take……??
EMBANKMENT AS RESERVOIR FILLED
Horizontal Crack
Transverse Cracks
Transverse Crack
Transverse Crack
The filter has not been
contaminated with piped soil
material.
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
RESTORED EMBANKMENT
Current Activities
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Draft environmental assessment
Interagency review
Copies of draft to affected landowners
Final draft designs to Fort Worth for review
Questions ???