The excavation of the two elliptical retaining shafts is taking place on the right-hand side slope of the Paudèze river. The two underground structures have already reached a depth of over 15 m. The geological conditions are challenging, characterized by the presence of weak marls with poor mechanical properties.

The excavation is performed by steps of 1.5 m maximum depth. The primary lining is installed right after the completion of each excavation phase. It is made by steel sets installed on the excavation perimeter and by shotcrete reinforced by wire-mesh.

Radial drains of 30 m length are also installed at different depths inside the shafts. They allow the rock mass to be drained thus improving the slope stability conditions.