Tailrace wall remedial case study for a hydro-electric facility in Medway, Maine

Abstract

Across the nation, dams constructed several decades to centuries ago with fatiguing components are in need of remedial measures to extend the facility design life and safely maintain functionality for future generations. Early settlers discovered the State of Maine’s remarkable river network and the ability to generate significant hydropower which gave rise to the construction of several dams in the late 1800s to mid-1900s.

The Medway Hydroelectric Project is a run-of-river facility located on the West Branch of the Penobscot River in Medway, Maine. The project was constructed in 1922 with commercial operations commencing in 1923. The facility consists of a 343-foot-long concrete gravity dam with a total height of 35.3 feet. The powerhouse contains five generating units with a total installed capacity of 3.44 MW and a maximum hydraulic capacity of 3,450 cubic feet per second. On September 9, 2021, a portion of the left downstream concrete training wall at the earthen abutment rotated into the powerhouse tailrace. It was anticipated that a portion of the existing timber crib beneath the wall foundation degraded along with inadequate structural design of the wall, causing the failure. The ability to install a new wall system during the winter without geotechnical investigations and foregoing the incorporation a cofferdam system was critical to avoid further embankment erosion during spring runoff. This paper presents the design considerations and construction implementation of a soldier pile wall remediation design intended to expedite construction, minimize impact to the dam, reduce further embankment erosion, and allow for powerhouse operation to continue during construction. The paper also discusses construction challenges, such as undermining of an upper retaining wall, construction of a working pad, the presence of boulders, and the sealing of the wall to prevent soil migration.