Grand-Mre Powerhouse

The Rocher de Grand Mre Hydroelectric Powerhouse was built in 2004 adjacent to the historic Grand-Mere plant that dates from 1916. The plants are located on the St. Maurice River, in the municipality of Shawinigan, in the region of Trois-Rivires. The area is entirely urban, with an Abitibi-Consolidated paper mill, a CN railroad and high voltage pylons in the immediate vicinity.

The new plant is a run-of-river type and has an installed capacity of 220 MW. The plant flow capacity is 1,040 m3/s and its three five-blade Kaplan turbines produce 1,214 GWh annually.

Its components include a power station with built-in water intake, main and secondary spillways, a remote controlled regulating structure, two gravity dams and an electrical switchyard. The combined length of the new and existing water containment structures is 785 metres.

Constructing a project of this size in an urban environment, coupled with the necessity of keeping the existing powerhouse in operation, was a challenge.

The limited space required ingenious solutions to permit the excavation of overburden and rock in dry conditions. Three phases of cofferdamming were required. In phase 1, to enable the construction of the powerhouse and main spillway, a steel sheet pile cofferdam founded on bedrock was constructed to the right of the regulating structure. In phase 2, the cofferdam in phase 1 was removed and a second sheet pile cofferdam was constructed between the wall of the main spillway and the sixth pier of the existing spillway. In phase 3, the cofferdam built in phase 2 was removed during a period of low flows before the spring flood.

Piers of exceptional slenderness for the main spillway and regulating structure were designed because of the limited space. While normal practice in stability studies is to consider a pier-and-a-half width of each adjoining rollway as a unit, in this case the unit was taken as a monolith comprising the extreme pier, the next intermediate pier, the rollway between these piers and the half rollway adjacent to the intermediate pier. In addition, these two hydraulic structures have radial gates operated by hydraulic jacks.

During winter there are daily fluctuations in the river’s water level of up to 1.8 metres at the location of an ice boom downstream. This project was the first time an ice boom has been used to accommodate daily level fluctuations of this magnitude. Tecsult developed a new design. Innovations included:

* ballasting the anchor cable with concrete weights at cable mid-points to ensure that the boom always remains in the same unique vertical plane, and

* groups of piles deeply embedded in the river bottom to resist thrust from the cables of 3,000 N per cable.

The new power station was architecturally blended with the existing plant, which has been designated as a historic site. The project won a Leonard Prize from the Association of Consulting Engineers of Quebec (AICQ) in 2005.

Client: Hydro Qubec

Prime consultant: Tecsult, Montreal (Gaetan Thibault, ing., Michel Cloutier, ing., Andr Guala, ing., Slavianka Saad, geol., Jean Lemire, ing., Lo Farnesi, ing.)

Subconsultants: Dessau-Soprin (control & automation), Breton-Banville & Assoc. (detail engineering, auxiliary services)

Contractors: Alstom, Kiewit