Tunnel workers had the unusual luxury of working in the hot sunshine in August to assemble the first of the TBMs for the Steg drive. It will shortly slide into an initial 60m heading formed by drill and blast and shotcreted, while manufacturer Herrenknecht tests performance on a 100m drive.
The 140m-long machine is an unusual design, with two Tamrock rock support drills mounted immediately behind the 180-tonne solid cutterhead and its 10 water-cooled 400kW motors. The drills are usable at any angle from a circular mount and complement shotcrete spray equipment also on board and a wire mesh erector produced by Herrenknecht. Arches can also go in at this early location.
"Though we are expecting relatively good rock for much of the drive," says Jens Classen, who is supervising the TBM for the consortium, "perhaps 6km out of the 8km." About 64 cutters will produce up to 300mm chips with another six over-cutters.
Two big grippers hold the machine, which also has a hydraulic "leg" mechanism for moving forwards on to the precast invert segments that will be laid as it goes. These are 1.5m long and 4m wide. In the trailer section there are two more drills, the segment crane used to move in arches as well, ancillary equipment, and heat exchanges for the coolers. Segments and arches will be brought up by diesel trains.
"We will use 2.5m ventilation all the way, but heat is likely to be a problem because the rock temperature could get quite high and the machine produces a lot of heat too," says Classen. Swiss regulations stipulate that the maximum permissible working temperature is 28°C. Cooling water will come from the Rhone which, although relatively small at this point, is at only 7°C, since it comes off its glacier source only 25km upstream.
Rock burst is the other possible problem to face, with a cover for the tunnel rising to 1,800m.
Meanwhile preparation continues. "Once we are in we can set up the conveyors that will transport the muck out," says Malcolm Lorimer, Balfour Beatty manager. Marti Mossedorf is making those. Material will be stockpiled in what is a fairly spacious site in flat land on one side of the Rhone river and then removed by the client by railway.
Space is rather tighter at the double portal at Raron, where the site is now under preparation. The mountain rises from the edge of the Rhone, leaving a platform less than 100m long which will just accept a full length of machine, if it is set obliquely. A steep bluff is being anchored back, and bridges will be built for spoil to pass over a conveyor bridge to a railhead, which has been set up on an old Swiss military airstrip. Once again, the client is disposing of material, which will be variously re-used for concrete linings or embankments where possible.
The first part of the drive here involves negotiating about 100m of transitional material "through which we will have to drag the machine", says Peter Hanslip, Balfour Beatty site production engineer.
A bridge downstream gives access via a tight underpass, but Mr Hanslip says the team is not looking forward to getting in some of the bigger machine parts when they arrive at the end of the year.
