Nant de Drance: machine trio from Herrenknecht on a successful course
Gripper TBM. Initially, Marti Tunnelbau AG excavated an access tunnel to the machine cavern located at 1,700 meters above sea level with a Herrenknecht tunnel boring machine (Gripper TBM, Ø 9,450mm). The tunnel is the only safe winter access to the system during the construction phase and the subsequent operations. The Gripper TBM proved its worth under the direction of Marti already in the Lötschberg Base Tunnel project (2000-2003; Switzerland, 10km of railway tunnels) and the San Pedro project (2005-2006, Spain, 3km of railway tunnels).
Raise Boring Rig. In addition to the Gripper TBM, a Raise Boring Rig (RBR) from Herrenknecht is in operation in Nant de Drance (see picture 3). The RBR600VF is drilling two 424 meter deep vertical penstocks. They connect the upper Vieux Émosson reservoir with the machine cavern. Marti Contractors Ltd. managed to complete the pilot borehole (Ø 381mm) for the first shaft from the top down through rock with strengths of up to 150MPa, with best performances of up to 62 meters a day. The reaming (Ø 2,440mm) started on November 14, 2012 and was successfully completed after only two weeks with top daily performances of up to 33 meters. The excavation of the second penstock (pilot borehole and reaming) with the RBR600VF is planned for fall 2013.
Shaft Drilling Jumbo. The third machine that is currently biting through the hard rock in Nant de Drance is the new Herrenknecht Shaft Drilling Jumbo. After its commissioning in April 2013, Östu-Stettin Hoch- und Tiefbau GmbH is now working downwards with the machine in two drilling and blasting rounds of 2.5 meters, each with 120 blast holes per day. The Drilling Jumbo will ream the shafts drilled by the RBR (Ø 2,440mm) to a diameter of eight meters using the conventional method.
The Nant de Drance pumped-storage power plant will be gradually put into operation from the year 2017 onwards. 900MW will then be ready within two minutes to react quickly, especially in times of peak demand. As soon as more power than needed is available in the network, the turbines transport the water from the lower Émosson reservoir at 1,930 meters above sea level into the upper Vieux Émosson reservoir at 2,225 meters above sea level. The Swiss railway company SBB, which is a partner in the project, will acquire a portion of the energy generated in Nant de Drance to cover peak demand in the railway network.