Underground Pumped-Storage Station

The Nant de Drance project entails constructing an underground pumped-storage power station located between the Emosson and Vieux Emosson dams at an altitude of approximately 1,700 meters. The facility is accessible via a five-kilometer-long tunnel from below and a two-kilometer tunnel system from the surface. Vertical wells transport water to the turbines to generate power.

Complex Tunnel Excavation Methods

Construction tasks include hard-rock boring for the main tunnel using a boring machine with grippers, and excavation of access tunnels and caverns through blasting. Shafts are created using raise drilling and additional blasting, while the lined tunnel is excavated via blasting as well. Management of 1.5 million cubic meters of excavated material and the heightening of the Vieux Emosson dam are also part of the contract.

High Altitude Construction Challenges

The project required handling complex material preparation and navigating challenges associated with working at high altitudes. Notably, the tunnel boring machine had to maintain an 11% uphill gradient. The entire construction involved 390,000 cubic meters of concrete and 20,000 tonnes of steel.

10-Year Project, 742m CHF Cost

Commencing in January 2008, the project was completed by January 2018. Nant de Drance SA, Finhaut VS served as both the building owner and contracting entity, with AF-Colenco AG, Baden-Dättwil handling project design and construction management. The total construction cost amounted to 742 million CHF, which equates to various amounts in other currencies including 761 million USD.