Safety Modification Study: Nolin Lake Dam
Safety Modification Study Overview:
We provided geotechnical and geophysical subsurface exploration services to assist AMEC with its dam safety modification study of the Nolin River Dam. The team performed drilling, sampling and laboratory testing as well as multiple geophysical surveys to assess the potential for seepage and piping through the embankment, abutments and spillway.
The earth and rock fill Nolin River Dam is eight miles north of the confluence of the Nolin and Green Rivers, with 703 square miles of drainage area above the dam. The lake is 5,795 acres and the crest of the east-west oriented dam is almost 1,000 feet long, 166 feet high and constructed against abutments comprised of sand and limestone formations.
An emergency spillway was constructed by blasting through sandstone northwest of the dam as the result of a previously encountered void on the upstream slope of the embankment. Beneath the sandstone formation near the floor of the spillway, there was an unconformable clayey contact between the sandstone and underlying limestone. Electrical resistivity surveys using the dipole-dipole method were deployed along the downstream abutments, reservoir rim and spillway measuring ground voltage between electrodes. Subsurface features exhibited various resistivity values, and this survey allowed the team to determine characteristics of the subsurface formations and develop a profile of conditions.
The team then performed ground-penetrating radar (GPR) surveys along the upstream crest of the embankment near the location of the void. This technique transmitted a broadband electromagnetic signal into the ground, which travels through underlying materials and is reflected by subsurface features to uncover subsurface conditions such as voids. The data was used to assist the client in evaluating possible dam repair sites.