Ayres Associates provided final design and construction observation for the addition of 80% more spillway capacity to the Byllesby Dam. This high hazard Federal Energy Regulatory Commission (FERC) dam in southeastern Minnesota needed a new spillway with two 65-by-14-foot spillway gates, a new post-tensioned anchor wall, 330 feet of new foundation cutoff walls into bedrock, excavation of 40,000 cubic yards of earth and 9,000 cubic yards of bedrock, removal of 2,500 cubic yards of existing structures, and the raising of 1,500 feet of perimeter dike around Lake Byllesby.
To reduce construction dewatering costs for the Dakota County Water Resources Department, the new spillway gates were constructed in the dry downstream channel of the existing fuse plug embankment, and the fuse plug was subsequently removed behind a low hazard cofferdam. About 14,000 cubic yards of quality material from the new spillway excavation was recycled and hauled five miles away for reuse at the perimeter dike raise project.
The Ayres team reviewed earlier design concepts, coordinated geotechnical borings and laboratory rock analyses, designed the new spillway structure, analyzed the probable maximum flood hydraulics, and assisted the County with a FERC “mini” potential failure mode analysis. The project’s challenges included wetland mitigation and the presence of thinly bedded rock, for which FERC required bedrock erodibility analyses and appropriate scour countermeasures. The team also contended with high groundwater levels and challenging construction operations during a near-record cold winter.
Construction of the $7.2 million project began in May 2013 and ended in July 2014. Construction required contractor coordination with FERC, the Minnesota Department of Natural Resources, the Minnesota Department of Transportation, and the U.S. Army Corps of Engineers for permits, as well as coordination with a new state trail and pedestrian bridge, relocation of overhead high-voltage transmission lines, and providing access for a concurrent sluiceway abandonment project. To accommodate the coordination communication, separate gate and concrete prime contractors, two independent testing agencies, and numerous subcontractors’ inquiries, this project relied on an online construction observation and administration database hosted by the County and maintained by Ayres Associates.