The owner of the scheme (FMHL) wanted to increase the capacity of the existing hydropower plant by building a second 240-MW underground powerhouse with two 120-MW pump-turbines. The project comprises the following underground works: access tunnels, headrace tunnel, powerhouse cavern, tailrace tunnel, shafts and chambers and a new surge chamber.

The project required explosive blasting for excavation of the powerhouse cavern. This produced around 160,000 m³ of spoil (power house: 94,000 m³; tunnel: 66,000 m³; surge chamber: 15,000 m³). Given the quality of the rock and the low contaminants content, the material was recycled for concrete aggregate, filter drains, and high-quality hardcore. Around 20% of the spoil was used for concrete on the Veytaux powerhouse project, the rest being stockpiled and processed in a neighbouring quarry. Since the project is close to Lake Geneva, and part of the works is below the lake level, controlling seepage was an important issue. A grout curtain was therefore built around the areas below the lake level. A large water treatment plant capable of handling an average flowrate of 20 l/s was also set up.