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THE ROAD TO SUCCESS

TUNNELLING - Tunnelling engineers are battling with difficult ground conditions at the Tauern road tunnel in the Austrian Alps. Damon Schünmann went to see the NATM project in full swing.

The Austrian government decided to build a second tunnel next to the original one, south of Salzburg, after it caught fire in 1999. A traffic accident caused the tragedy, claiming 12 lives and injuring 50.

The new tunnel will double the A10's capacity through the mountain, a major route that carries holiday traffic from Austria and Germany to Italy. The current single tunnel, opened in 1975, causes a major bottleneck, particularly during summer weekends when traffic jams 30km to 40km long are often reported. The second tunnel will ease the congestion and provide an escape route from one tunnel to the other - a distance of 30m - via cross shafts that connect the two every 50m.

Contractor Porr Tunnelbau won the 2110M job and decided to use NATM through the mountain, that lies between Flachau and St Michael.

The contractor says a mixture of excavation and drill and blast was chosen because a tunnel boring machine could not easily be used through the mountain's variable materials. These consist of sands and gravels at the north side of the mountain and mainly phyllite to the south. Porr is using a combination of support methods to protect workers and stabilise the excavation.

'Usually we would use [traditional] rock anchors to stabilise the tunnel but the material here is not good enough, ' says Porr project manager, Hans Gaulhofer. So site workers are installing self drilling bolts that prevent the borehole from closing in unstable ground. 'We are also using shotcrete without steel fibres, steel lattice arches and wire mesh, ' he says.

For the problematic north end of the over 11m wide bore, Porr first lays wire mesh panels against the walls. It then places the steel lattice arches at generally less than 2m intervals and adds another layer of mesh on top and applies a 300mm thick layer of jet grout.

But in some areas this is not enough to make safe the working area. Where ground is particularly prone to movement, site workers have been securing the profile using up to 70 'liner plates' driven around the top edge of the excavation's face.

The 2.5m long, 220mm wide plates go in 7 upwards from the horizontal and help prevent loose material from owing down from the crown.

A two boom Atlas Copco Rocket Boomer L2 C is installing these using a modi d COP1838 rock drill. This rig is also installing self drilling 4m long, 32mm diameter bolts - perpendicular to its circumference - that are grouted in place for additional support of the tunnel prole.

Site workers tension the bolts 12 hours later. But because rock movement creates tension, only some of the bolts need additional tightening.

Although six different companies worked on the original tunnel, Porr was partly chosen as the single contractor because it built the original Tauern Tunnel, and would have experience of the ground.

But it admits it is difficult to be certain when the sands and gravels will end at the north end of the drive. 'We have now passed through over 400m of gravel and sand and we expect another 20m remains before encountering rock, ' says Gaulhofer.

Excavator work on the north portal is progressing at about 2m per day.

Engineers have recorded 100mm settlement in the weak ground at this end of the 6.5km drive. This is the maximum amount allowable.

But in the former rock at the south end, being worked at the same time, there has been only 8mm movement.

Here, Porr is also cementing in the self drilling steel bars and the amount used depends on the rock quality encountered. This phyllite allows far quicker tunnelling and the drill and blast is progressing almost four times as fast as the north end excavator advance. These tunnel walls are secured using a single mesh layer, a lattice arch and 150mm of shotcrete. Every metre, at both ends of the work, engineers reclassify the rock to decide if the self drilling bolts are needed and in what arrangements.

Another Atlas Copco machine is boring for the drill and blast and bolt installation here. The new Rocket Boomer E2 is the first of its kind to be used in the Alps.

Excavation work on the project, started last September, is due to finish with a breakthrough in autumn 2009, with it opening to traffic in 2011.

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