________________ The power plant technology for the Swiss pumped-storage power plant Nant de Drance is being installed underground. A Herrenknecht Gripper TBM drives the 5.6-kilometer-long access tunnel (5). Following successful previous projects, the hard rock TBM was especially modified for this jobsite. The back-up system, which serves to supply the actual tunnelling unit, is adapted to the tunnel incline, for example. The tunnel boring machine has to deal with a gradient of up to 12 percent and curve radii of 500 meters. Disturbance zones are explored by systematic pre-drillings. In areas with very high water pressures, the rock to be bored through is stabilized with concrete injections. Two vertical pressure shafts, the penstocks (2), connect the Vieux Emosson reservoir (1) with the power house cavern (3) inside the Alps. The Herrenknecht Raise Boring Rig (RBR) drills vertically downwards, and in the next working step, the pilot borehole is reamed to a diameter of 2.44 meters. Subsequently the borehole is enlarged to 8.40 meters using a Shaft Drilling Jumbo.
The Herrenknecht Gripper TBM, which was especially adapted to the project, proved its worth, dealing with a gradient of 12 % and curve radii of 500 m.
For vertical pressure shafts, the Herrenknecht Raise Boring Rig produced a pilot hole with a diameter of 2.44 m in two working steps – with top daily performances of 62 m when pre-drilling and 33 m when reaming backwards.
With a Herrenknecht Shaft Drilling Jumbo, the penstocks are being enlarged to a diameter of 8.40 m.
Tunnel lengths: 5,600m, 2× 424m
Geology: gneiss, graywacke, granite
TBM: Marti Tunnelbau AG
RBR: Marti Contractors Ltd.
Shaft Drilling Jumbo: Östu-Stettin Hoch- und Tiefbau GmbH
Gripper TBM, S-522
Cutterhead power: 3,500kW
Raise Boring Rig (RBR), MI-005
Installed power: 600kW
Diameter drilled: 2,440mm
Shaft Drilling Jumbo, MI-009
Configuration: 3 booms
Diameter drilled: 8,400mm
The Nant de Drance plant uses the difference in elevation between the Vieux Emosson (1) and Emosson (4) reservoirs. The closed circuit between the two reservoirs does not have any influence on natural water courses. Since the power house cavern (3) is underground, impact on the environment is low. The plant is designed to render 900 MW turbine and pump output. The power can be made available within 2 minutes. This is especially important for Swiss Railways (SBB): In the morning, when locomotives start almost simultaneously and according to timetable in all parts of the country, the power needed amounts to several 100 MW.
Herrenknecht technology for vertical shafts
Penstocks, the pressure tunnels for hydropower plants, run vertically or on a downward slope. The appropriate machine system to build such galleries can be chosen from the Herrenknecht range according to the project conditions. In Nant de Drance, two connecting shafts between two caverns located one above the other are being built. In this operation, the modularly designed Raise Boring Rig that can be quickly installed and easily removed is being used. The rig makes a vertical downward pilot drill with a diameter of around 300 millimeters down from the upper cavern. In the lower cavern, the drill bit is replaced with a reamer which enlarges the borehole as it is pulled back up. Then the shaft diameter is enlarged to 8.40 meters using a Shaft Drilling Jumbo.