AUSTRIAN CONTRACTOR Porr Tunnelbau was this week continuing to battle difficult ground conditions at the Tauern road tunnel in the Austrian Alps.
A second parallel bore is being excavated next to the original 1972 tunnel south of Salzburg, which caught fire after a traffic accident in 1999 claiming 12 lives and injuring 50 (NCE 10 June 1999).
ngineers have recorded 100mm settlement in weak ground that comprises sand and gravel landslide material at the north end of the 6.4km drive.
The contractor said that a mixture of excavation and drill and blast was chosen as a tunnel boring machine could not easily be used through the mountain's variable materials. A combination of support methods are being used to protect workers and stabilise the 11m wide excavation.
'Usually we would use rock anchors to stabilise the tunnel but the material here is not good enough, ' said Porr project manager Hans Gaulhofer. 'So we are using shotcrete with steel lattice arches and wire mesh.' Porr is placing wire mesh panels against the walls. It is then installing the steel lattice arches at 1.7m intervals and adding another layer of mesh on top before applying a 300mm thick layer of jet grout.
However, in some areas this has not been enough to make safe the working area.
Where ground is particularly prone to movement, site workers have been securing the profi le using 2.5m long, 220mm wide sheet piles driven around the top edge of the excavation's face, 7° upwards from the horizontal.
These help prevent uncontrolled ground loss.
Excavator work on the north drive is progressing at about 2m per day.
Throughout the tunnel the contractor is grouting in minimum 4m long self-drilling steel bars perpendicular to its circumference.
The number used depends on the rock quality encountered, but it has proved hard to predict what lies ahead.
Although Porr was partly chosen on the strength that it built the original Tauern tunnel and would have experience of the ground, it says it is difficult to predict when sands and gravels will end.
Far better for tunnelling is the 'schiefer' (slate) at the site's south end where drill and blast is progressing almost four times as fast.
In the firmer rock at the south side, there has been only 8mm movement.
Here the tunnel walls are secured using a single mesh layer, a lattice arch and 150mm of shotcrete.Every metre, engineers reclassify the rock to decide if the self-drilling anchors are needed and in what arrangements.
Excavation work on the @110M (£70M) contract started last September and is due to finish in autumn 2009 with the tunnel opening for traffic in 2011.
The new tunnel is about 30m from the original and will be connected every 50m along its length by cross tunnels.
A full length feature on this project will appear in the spring issue of European Foundations.