Lake Mooselookmeguntic is a jewel in the Rangeley Lakes region, a natural glacial lake in western Maine. Its Upper Dam is one of a number of facilities on the Androscoggin River watershed used to regulate flows for hydroelectric power generation downstream, flood control, and industrial utilization. Schnabel provided design-build services for the complete renewal of the over 100-year-old dam’s spillway structure.

Constructed in the mid-1850s, Upper Dam raised the lake’s elevation and increased its storage for controlling flows for log driving. Because of the facility’s existing dependencies, maintaining pool levels and regular flows during construction was critical. Construction of an auxiliary spillway was essential to providing necessary flow capacity during removal of the main spillway. Throughout construction, we provided resident engineering services to respond to differing site conditions and contractor needs.

We designed multiple cofferdam and support of excavation systems that allowed the staged deconstruction of the existing spillway. The initial phase involved the installation of a sheet pile cofferdam to hold back 20 feet of water to allow the construction of an auxiliary spillway. The cofferdam used driven H-piles to provide toe resistance in dense glacial till to supplement the limited penetration of the sheet piles. The sheet pile cofferdam transitioned to a drilled-in soldier pile support of excavation system through the dam embankment and finally to a cantilevered sheet pile cofferdam at the downstream side. The drilled-in soldier piles were integrated into the new auxiliary spillway walls to provide uplift resistance. We also provided the dewatering system design and installation oversight.

The phase two cofferdam consisted of a double row of sheet piles, with two levels of tie rods and granular infill. The 30-foot wide cofferdam also served as a roadway to allow access to the other side of the dam during construction and as a construction platform. The double-row cofferdam retained 25 feet of water for more than three years while the existing spillway was removed and a new service spillway was constructed.

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