CS7B - Concurrent Session 07B: DMAD Dam Rehabilitation Phase II

DMAD Dam is located along the Sevier River near the town of Delta, Utah. Although original construction records are not available, it was raised to its current geometry in 1959. The embankment is about 1,200 ft in length with a structural height of 37 feet and has a storage volume of about 11,000 acre-feet. The reservoir provides storage for irrigation diversions and cooling water for the Intermountain Power Plant. In June of 1983, the east end of the embankment and original spillway failed during a large flood event. The spillway was replaced, and embankment repaired in 1984.

In 2006, main portions of the dam were rehabilitated to address embankment seepage. During rehabilitation efforts loose liquefiable materials combined with high seepage pressures were found under the existing spillway suggesting the potential for the existing spillway to be unusable following a major earthquake. Geotechnical modeling demonstrated that densification of these loose materials could potentially stabilize the spillway structure to meet minimum standards. Various ground improvement and mitigation methods were considered, and compaction grouting was found to be the most reasonable, minimally invasive, and relatively low-cost mitigation solution. Given concerns with the level of mitigation needed to improve the site soils and access logistics for the equipment, a full-scale compaction grouting program was developed and implemented with pre- and post-improvement assessments using cone penetration soundings. Results from the compaction grouting program combined with other hydraulic and operational concerns led the owner, regulators and design team to move forward with replacing the spillway.

The project commenced in the summer of 2021 and was completed in the spring of 2022. Some key features included: a) lowering the crest of the spillway apron 10 feet and flattening the spillway chute to 3.3H:1V to increase capacity and stability of the spillway, b) removing approximately ten feet of liquefiable soils below portions of the spillway and basin, and c) buttressing the remaining liquefiable soils by filling the plunge pool and scour hole with structural fill and riprap. Implementation of these mitigation measures demonstrated minimum regulatory standards were met and increased seismic resiliency of the spillway.