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  Lessons learned from design and construction of Nant-de-Drance Powerhouse

  The Nant-de-Drance hydropower scheme (900 MW), awarded "Major tunneling project of the year" prize in 2014, connects two existing artificial reservoirs. With the exception of the upper dam heightening, all works are undertaken underground. They include 16 km of tunnels, two vertical shafts of height 440m and 6 caverns. The exceptional size of the powerhouse cavern (194m x 32m x 52m), located at 600m depth in a metamorphic rock formation of sedimentary origin, required the definition of an appropriate excavation, support and lining concept. An advanced rock behaviour model, based on an important investigation campaign, 3D finite element modeling as well as analysis of unstable blocks allowed the optimization of the support and the lining. Monitoring during construction and back-analysis allowed the successive reduction of support measures and the streamlining of the excavation phasing. The resulting savings largely exceeded the incurred expenses for additional investigation and calculations. Schedule of the cavern excavation were accelerated by reorganizing the phasing of the different stages, creating a margin to make up delays elsewhere. Structural design was also particularly challenging due to the unusual load cases to which the structure is subjected during construction and operation, as well as the large volumes of concrete being poured.

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