Engineers are hard at work on a rail tunnel which will eventually take traffic off Switzerland's roads and onto trans Alpine shuttle trains. Andrew Bolton reports.
Deep beneath the Swiss mountains, an eerie rumble rises above the noise of the excavators and tunnel boring machinery in one of the country's most technically challenging tunnels.
Workers on the Herrenknecht tunnel boring machine (TBM) in the eastern bore of the 30km Lotschberg base tunnel carry on without batting an eyelid. They have got used to the shock waves sent twice daily along the tunnel from explosions triggered by colleagues 2km behind them in the neighbouring bore. While the TBM leads the way, the other tunnel is being constructed using drill and blast techniques.
When complete in 2007 the £1.2bn, 27km Lotschberg base tunnel will speed up rail services between northern and southern Switzerland, bypassing the slower, high level Lotschberg line which snakes along a parallel route through the steep mountains. In the long term the tunnel will be used to ferry shuttle trains carrying long distance cars and lorries through the Alps (see box).
The 30km tunnel is split into four packages. Three are being constructed using drill and blast techniques. Only the southernmost section is being bored, during construction by the MaTrans consortium of Balfour Beatty, Marti Tunnel of Switzerland, Walter Gruppe of Germany and Porr of Austria.
Ground here is mainly hard granodiorite with veins of quartz, making use of an open face hard rock TBM possible.
The other three sections run through relatively unstable areas of slate and water bearing rock, making drill and blast the preferred option.
The MaTrans section of the project was originally awarded as two separate packages: a £100M contract for 10km of running tunnels from the southern portal at Raron, and an £80M, 3.2km spur tunnel and 5.2km running tunnel section from a second portal east of Raron at Steg (see diagram).
MaTrans won both contracts and decided to combine them, using TBMs on the Steg section and the eastern running tunnel from Raron, leaving the southern 4.9km of the western tunnel to be built using drill and blast.
Planning delays held up start of work on the Raron portal, so the Steg TBM started first in October 2000, a year earlier than its Raron sister. In September it completed the Steg drive. Unlike soft ground machines the hard rock TBMs at Lotschberg do not place precast lining segments.
Instead they cut through the ground before a 70mm sprayed concrete lining is applied to the exposed rock. As the machine progresses, Swellex rock anchors are inserted into the tunnel crown for greater stability.
'It is faster with a tunnelling machine, ' says MaTrans project director Francois Bertholet. But he adds that the relatively short length of east tunnel between spur and portal meant the expense of a third TBM could not be justified.
Progress has been impressive.
The eastern bored section was completed last month, just over two years after work began. The machine recorded progress rates of up to 747m a month.
The eastern TBM started work in late 2001 and is making similar headway. It has completed almost 5km and is chewing its way through a section of highly abrasive granodiorite permeated with fingers of granite.
Parallel to the east tunnel, the sprayed concrete lined drill and blast section of the west tunnel is also taking shape.
MaTrans normally carries out two explosions a day after extensive advance drilling.
Work is now almost 50% complete with tunnelling rates running at around 10m a day.
The advantage of combining the two southern tunnelling contracts has meant that the west tunnel TBM team is using the experience gained in the Steg bored section to speed work and anticipate difficulties.
Both tunnels pass through similar ground so lessons learned in the Steg drive could easily be applied to the Raron bored tunnel.
'Using the experience of the Steg bore, we modifed the Raron machine, ' says Bertholet. He says that, for example, the Steg machine cutters needed extensive replacement after sustaining worse than expected damage passing through abrasive ground.
Modifications have included installing triangular protectors on the tunnelling shield which would strike hard sections of rock before the cutters reached them. This has helped, although the abrasive ground is slowing progress to around 20m a day It was also possible to use the Steg experience to help drive its sister machine. 'The geological data from Steg meant we had an exact knowledge of the conditions facing the Raron machine, ' says Bertholet.
At 9.5m diameter, bores for the Lotschberg tunnel are huge as rail tunnels go. Engineers working for client BLS Alptransit have had to balance the need to push trains through the tunnel at up to 250km/h against the power needed to overcome air resistance created by the piston effect.
As a result tunnel diameters are around 9.5m, much larger than for most Alpine tunnels, which normally measure between 8.5m and 9m across.
The Lotschberg base tunnel is being developed in parallel with the much longer rail tunnel at St Gotthard. Both are being built with the long term aim of providing road traffic with a fast rail route through the Alps.
When it opens to traffic in 2007 the Lotschberg base tunnel will provide a faster link across the Alps between Berg in central Switzerland and Brig to the south, relieving capacity on the winding - and slower - Lotschberg high level line.
Initially the tunnel will carry passenger and freight trains. But the Swiss government has long term plans to fit out the second bore and start running car and lorry shuttle trains through it. It is hoped this will take traffic off the roads and reduce traffic pollution in the Alps.