The hydroelectric generating plant facilities at the Soo consist of two powerhouses with a total installed capacity of 18.4 megawatts (MW), rated at a flow of 12,700 cfs. The Main Powerhouse located at the foot of the rapids has three generating units each of 4.8 MW capacity installed in 1951-52 and one generating unit of 2.0 MW capacity installed in 1954. Its adjoining features include the north and center dikes, which form the headrace, and the footbridge, which provides access to the powerhouse across the Unit 10 tailrace.
The Unit 10 powerhouse with a single unit of 2.0 MW capacity installed in 1932 is located at the head of the rapids. There is a dam to the north of the powerhouse that was formed when abandoned sluiceways were closed off by reinforced concrete bulkheads. To the south, gate bays, which are no longer in use, were filled with rock fill material to form the South Dam.
The power generated at the Government plants in excess of the Government’s needs, is sold by contract to Cloverland.
The Main and Unit No. 10 Powerhouses are situated just north of the four navigation locks, near the St. Mary’s River Falls at Sault Ste. Marie, Michigan.
Main Powerhouse and Related Features
The Main Powerhouse is a conventional indoor type structure. The main generating room is approximately 45 feet wide by 216 feet long. The substructure is reinforced concrete with the super structure walls being masonry construction. Foundation for the Main Powerhouse is Jacobsville sandstone. Provisions were made in the original structure for expansion of the plant.
The North Dike extends westward from the Main Powerhouse to the Compensating Works along the north side of the headrace. At the east end of the north dike, where it adjoins the Main Powerhouse, are the sluice gates. For approximately 1,350 feet upstream of the Main Powerhouse, the headrace has concrete-lined side slopes that are keyed into bedrock. Upstream from the concrete-lined section of the north dike, the headrace widens and includes the headrace for Unit No. 10. Upstream from Unit No. 10 there is no concrete lining. The headrace is excavated into bedrock.
The center dike lies along the south side of the headrace extending from the Main Powerhouse to the Unit No. 10 building. It includes a crib dam, which connects the eastern end of the dike to the Main Powerhouse. The north side slope of the center dike is lined with concrete. The south side slope of the center dike faces the tailrace of the Unit No. 10 Powerhouse.
Public Law 14, dated 2 March 1945, authorized the construction of the first step of a two-phase development plan for the hydroelectric powerhouses at Sault Ste. Marie, Michigan. The first phase was the construction of the present main powerhouse. To avoid interruption of service from the initial plant during construction of the second phase, the crib dam and footbridge were constructed so that the second phase powerhouse excavation could be accomplished with the crib dam and footbridge in place, and then removed for headrace excavation. This second phase was never authorized, and the crib dam remained in place until the work was performed in 2010.
The crib dam was constructed of 12-inch by 12-inch wooden timbers and is filled with stone. The upstream face is constructed of two perpendicular layers of 3-inch by 8-inch tongue-and-groove wood sheeting. The crib dam is founded on bedrock (Jacobsville sandstone) and is approximately 302.5’ long by 28’ wide by 36’ high. Top elevation is 606.4 ft (reference IGLD 1985) and bottom elevation is 570.0 ft. The downstream end of the crib dam connects to the Main Powerhouse, while the upstream end ties into the concrete-paved center dike adjacent to the headrace for the Main Powerhouse. The downstream end is buttressed with fill stone on the south side.
A contract to repair the crib dam was awarded in FY10 and was completed by the end of FY11. Work involved the addition of a sheet steel wall in front of the crib dam, placing tremie concrete between the sheet steel and crib structure, removal of the top 12 feet of the crib dam and replacing it with a concrete double L wall.
Unit No. 10 Powerhouse and Related Features
The Unit No. 10 Powerhouse is located just north of the Sabin Lock, near the headrace of the St. Marys Rapids. It is situated at the center of a concrete and masonry dam, which originally housed other generators. When the generators were removed, the raceways were blocked with stoplogs and backfilled with concrete. The powerhouse was constructed in 1934 and became operational the same year. The plant’s superstructure is constructed of brick masonry on a reinforced concrete substructure. The foundation is supported by Jacobsville sandstone. The Unit 10 generator has a fixed blade turbine, and generates 2.0 MW.
The North Dam of Unit No. 10 extends from Unit 10, north to its junction with the center dike of the headrace for the Main Powerhouse. This dam consists of sluiceways closed off by reinforced concrete bulkheads. In 1995-96 a SSP closure dike was constructed on the upstream side of the dam to stop leakage.
The South Dam of Unit No. 10 extends south from the plant to the backside of the north wall of the Sabin Lock. The gate bays were filled with rock fill when the turbines and other equipment were removed, forming the south dam. The piers adjacent to the gate bays are constructed of brick over concrete. Part of the south dam is the precast concrete plank walkway supported by steel framework, which is currently rusting, corroding, and deteriorated. In 1995-96 a SSP closure dike was constructed on the upstream side of the dam to stop leakage.