Controlled sinking of an open end caisson in weak rock

A concret open-end caisson (depth 60 m, diameter 30 m) was sunk for the shaft of a reversible hydro-electric power plant, first through 10 m of clay and through weak rock (very hard marly clay) for the remaining 50 m. The sinking of such a heavy structure to the required depth calls for a very strict control of centre line inclination and penetration depth at every stage of undercutting the edge. Different failure modes of soil under the cutting edge have been studied of which the most adequate was implemented. Shaft behaviour during excavation and the experience acquired during the construction of this outstanding structure are described.