PL566 Remedial Assistance Project Colorado Case Study Frank Riggle Colorado Assistant State Conservationist Water Resources
Jan 06, 2016
PL566 Remedial Assistance ProjectColorado
Case Study
Frank RiggleColorado Assistant State ConservationistWater 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 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
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
Draft environmental assessment Interagency review Copies of draft to affected landowners Final draft designs to Fort Worth for review
Questions ???