ABSTRACT
The compensation reservoir Châtelard is an asphalt lined basin owned and operated by the Swiss Federal Railways. It is located in a narrow valley that has been partly filled to the downstream by a spoil area of tunnel muck derived from the construction of Nant de Drance pump storage scheme. The area between the asphalt lined embankment dam and the spoil area provided good opportunity to increase the storage volume of the basin. The Swiss Federal Railways took a brave decision for the alternative with the greatest increase of storage volume but also maximum impact on the existing scheme. With the chosen alternative, the active storage volume was doubled from 200’000 m3 to 394’000 m3.
The spoil area was not originally planned to serve as a water retaining structure and efforts needed to be made to investigate its properties and analyze its stability under static and dynamic loads as well as post-earthquake state with ruptured surface sealing. It was concluded that the original grade of the spoil area slopes of 1:1.5 (v:h) is too steep to meet the required factors of safety for a water retaining dam and an additional embankment to flatten the slope to 1:2.5 (v:h) was planned and implemented. In agreement with the supervising authorities, a fiber-optic deformation measurement system was included within the asphalt sealing of the new dam in order to monitor closely the embankment for settlements and the asphalt sealing for unacceptable strains during first impounding. The monitoring system was enhanced with geodetic measurements, piezometers within the embankment and a drainage system beneath the asphalt sealing.
The project implementation started in April 2021 and first impounding took place in February and March 2022. The first full impounding resulted in strains of 250 microstrains (0.025% or 0.25 mm on 1 m) in different areas of the new dam. Only along the plinth between the asphalt surface sealing and the rock cliff strains of up to 1’500 microstrains (1.5 mm on 1 m) were observed. All in all, the deformations observed with the fiber optic cables remained small and well distributed (no abrupt changes), indicating no deformation critical to watertightness. Three weeks after first full impounding, the geodetic measurements showed a maximum horizontal displacement of the five measuring points located on the new dam of 3.5 mm and a maximum settlement of 6.8 mm. After 9 months of operation, the horizontal deformation is still of the same order of magnitude and the settlements only increased slightly to 8.8 mm. These values confirm an appropriate compaction of the spoil deposit and the new earthworks embankment with flattened slope. Also, the piezometric pressures and drainage water quantities confirmed a satisfactory behavior during first impounding. After 9 months of normal operation the behavior is still fully up to expectation.
With this project, the Federal Railways facilitated a considerable increase in the efficiency of the existing scheme and improved its capability for flexible energy storage and just in time generation according to the ever-changing demands.
