Etanchéité du réservoir supérieur de la chute hydroélectrique de La Coche

Sealing arrangements for the high-level reservoir in the La Coche water power scheme

Chef du Service Etudes Générales Sites 2 E.D.F. Région d'Equipement Alpes Lyon Services de Chambéry

Abstract

Special design and implementation problems for the reservoir are discussed, together with its behaviour during initial filling stages and subsequent additional sealing operations. Reasons for sealing the whole reservoir : Though the site topography would have been ideal for a pump station (L/H = 3), its foundation material, however, consisting of landslide rubble and very permeable moraines overlying Trias formations sandwiched-in between sheets of "Tarentaise" calcareous breccia made it necessary to line the whole reservoir fully watertight, plus providing sealing arrangements underneath the drainage system to allow close monitoring of leakage rates. Considering the difficulty of providing this type of lining and the risk of serious trouble in the event of major settling of the foundations, the use of bituminous concrete and pitch-vinyl compounds was ruled out. Instead, it was decided to rely on reinforced-concrete slabs with water stop joints (Fig. 2) for the main sealing arrangements. Suitable deep sealing arrangements were determined by tests with two polyester or polyamide felts hot-rolled in an asphalt bath, and with a 1 mm PVC membrane reinforced with corrugated nylon fabric. The latter was finally adopted because of its lower cost. The reservoir was sealed on completion of excavation work in 1974 and 1975. An indication of sealing efficiency was obtained by measurement of the following during initial reservoir-filling tests : a) Variation of the overall leakage rate (see Fig. 6) b) Settling of drainage and instrumentation tunnels (see Fig. 8). Reservoir basin instrumentation is illustrated in Figure 9. Since the reservoir has not yet been tested under its full head, no final conclusions can be drawn, but the final tests are expect to confirm the efficiency of the sealing arrangements provided for.