CDA 2005 Annual Conference Congrès annuel 2005 de l’ACB Calgary, Alberta, Canada CANADIAN DAM ASSOCIATION October 3 – October 6, 2005 ASSOCIATION CANADIENNE DES BARRAGES 3 Octobre – 6 Octobre, 2005 TREATMENT OF DEEP PERVIOUS FOUNDATIONS AT THE PERIBONKA DAM SITE Annick Bigras, Geotechnical Engineer, Hydro-Québec Équipement, Montréal. Bernard Gagné, Head geotechnical engineer Péribonka Projet , SNC-Lavalin inc, Montréal. Anthony Rattue, Head of geotechnical department, Energy division, SNC-Lavalin inc., Montréal. Pascal Garand, Senior Geotechnical Engineer, SNC-Lavalin inc. Energy Division, Montréal. Yousef Hammamji, Head of geotechnical eng. div., Hydro-Québec Équipement, Montréal. Pierre Vannobel, Geotechnical Engineer, Hydro-Québec Équipement, Montréal. ABSTRACT: The Peribonka hydroelectric project, located in Quebec, is presently under construction. It includes a 80 m high dam and two dikes consisting of zoned earth fills with a till core, an underground power house with an installed power of 385 MW, a diversion tunnel of 2005 m 3 /s capacity and a spillway of 5300 m 3 /s capacity. The dam site presents difficulties related to the presence of a deep canyon under the river bed filled with alluvial sand and gravel. The possibility of dewatering and of executing an open excavation to construct the dam on rock is precluded due to the high permeability and the depth of the alluvial foundation. The design includes the construction of a roughly 10 m high working platform in the river and the construction of a plastic concrete cut-off wall down to bedrock. The project relies on the application of several construction techniques that are at or beyond the limit of precedents in terms of present knowledge and equipment capacity. These construction techniques consist of vibrodensification of alluvial soils to a depth of nearly 50 m, the consolidation of alluvial soils using the “Tube à Manchette” technique on either sides of the cut-off wall, both driven down to a depth exceeding 125 m. A 46 m deep plastic concrete diaphragm wall will also be constructed under dyke A. An extensive testing program has been conducted to assess the mechanical behaviour and to assess the sensitivity to erosion of different plastic concrete mixes. This paper covers essentially the dam and dykes structures. It presents a description of foundation conditions, typical cross sections of retaining works, of the cut-offs, and of foundation treatment methods. It also presents the monitoring instruments planned and a resume of the research program on the plastic concrete. Dam site location The Peribonka hydroelectric project is located in the province of Quebec to the north of Lac Saint-Jean (see figure 1). The Peribonka reservoir will be contained by the dam located around kilometre 152 on the Peribonka River, and by two dikes located less than 2 km north of the dam. The dam site is located immediately upstream of the confluence of the Manouane river with the Peribonka river. Alcan currently operates three powerhouses on the Peribonka river. 1