October 1, 2006
By Canadian Consulting Engineer
The Toulnustouc hydroelectric project involved constructing a concrete face rockfill dam and a dyke to enlarge the existing Lac-Sainte-Anne reservoir. Located north of Baie-Comeau on the Toulnustouc River, the Lac Saint-Anne reservoir covers 213 square kilometres and forms part of the Manicouagan Complex serving Hydro Quebec’s Manic 1 and 2 and a plant owned by the Manicouagan Hydroelectric Company.
The new dam and dyke are located about 14 kilometres downstream of the existing dam. The new structures enlarge the existing reservoir by 22 square kilometres to accommodate a 526 MW power plant. The south dyke, which encloses the reservoir, is located in a secondary valley approximately 500 metres southwest of the main dam.
In June 2000, Hydro-Qubec hired RSW to provide civil engineering services for the dam, south dyke, intake, spillway, headrace tunnel, power plant and tailrace canal. RSW’s mandate was initially to participate in a value engineering review and prepare a synthesis report of feasibility studies done by Hydro-Qubec. RSW went on to prepare the scope of work studies, detailed engineering and tender document preparation. The company also provided technical assistance during construction.
Adapting the dam to a Nordic environment
The primary choice for dams in Quebec is zoned rockfill because of the availability of glacial till that can be used in the impervious core. At Toulnustouc, the till encountered in nearby borrow areas was considered too pervious for this type of dam.
During the pre-feasibility studies carried out by Hydro-Qubec, a concrete face rockfill dam (CFRD) was identified as a potential alternative and cost-effective type of dam construction. A concrete face rockfill dam uses rockfill and crush rock transitions, materials available nearby from the spillway excavations. The total volume of fill is reduced because steeper fill slopes are used. The concrete face slab ensures impermeability.
The concrete faced rockfill dam had only been used once before in Quebec, in 1978 for the Outardes-2 project. Since then many improvements had been made to the construction techniques and the use of new materials in projects around the world. Hydro-Qubec sought examples in South America, where many dams of this type are being built.
It was necessary to adapt the concept to a Nordic environment, where there is ice loading and limited time available for construction. For this task, RSW developed and integrated techniques that are not usually combined.
Water tightness is provided by a reinforced concrete face slab constructed on the upstream face of the rockfill and with a concrete plinth on bedrock. The parapet on the crest provides additional freeboard during exceptional flood conditions and it permits a reduction in the rockfill volume.
Coping with varying hydrostatic loads and rockfill settlement
The design of a dam with a concrete face slab has many challenges. These are associated with the functions of the main structural elements (the upstream slab, the plinth, the parapet), as well as the loads that are applied to these elements. The slab must accommodate the varying hydrostatic loads from the reservoir, the large variation in temperature and the ice loads in winter. As well, the slab has to accommodate the rock fill deformation on which the slab rests. The parapet must be able to resist the hydrostatic loads and adapt to the rockfill settlement, all while maintaining water tightness. The plinth anchored to the bedrock is also used as a cap for the grout curtain to make the dam completely watertight.
A complex three-dimensional geometry was used to design the reinforced concrete plinth at the closure of the face slab with the bedrock. A finite element model was also developed to understand properly the deformation of the fill and its impact on the upstream face slab. The results were used to design the reinforcing steel and to select the design criteria to ensure watertightness and durability, as well as to select the right type of joints between the slab panels.
The sequence of construction of this type of dam requires that the plinth and the rockfill are built in parallel; the face slab is placed on the rockfill afterwards. For this dam that is 500 metres long at the crest and more than 70 metres high, the slab was poured in panels 15 metres wide to control cracking due to shrinkage as well as to facilitate construction. Between each panel vertical joints are designed to ensure water tightness for the varying deformations. Nine different joints were developed and used at different locations.
Construction and benefits
Work on the dam started in 2002 with diversion works that consisted of a tunnel excavated into rock on the left bank and a portal with closure gate. The dam was completed with the construction of the crest parapet in early 2005, six months early.
The project was developed with the strong support of the local population, who were involved in the construction. The development of the project with only a small increase to an existing reservoir area minimized the environmental impact.
The reservoir filling started on February 10, 2005 and continued until April 29 to the maximum operating level. Since then, continuous instrumentation readings and surveys indicate that deformations are less than what were originally planned and there is very little dam leakage: around 10-15 L/sec.
Name of project: Toulnustouc Dam
Award-winning firm: RSW, Montreal – prime consultants, civil engineering (Claude Chartrand, P.Eng., Michel Claisse M.Sc.Eng., Normand Beausjour, M.Sc.Eng., Marie-Hlen Briand, Ph.D.Eng., Hafid Bouzaiene, Ph.D.Eng., Claude Boisjoly, M.Sc.Eng., Gustavo Gonzaga, Ph.D.Eng., Robert Quenneville, Tech., Andr Bergeron, B.Eng.)