UNEXPECTED GROUND conditions have slowed progress on the last 10% of the AlpTransit project's 34.6km long Lötschberg tunnel in Switzerland.
Carboniferous and sedimentary rock has slowed progress from the 10m a day achievable in the harder self-supporting granite to between 2m and 4m a day.
'The rock was quite unexpected, ' said BLS AlpTransit chief geologist Hans Jacob Tiegler.
'We had expected this 2km long central section to be just granite.'
The Lötschberg tunnel forms the western half of the huge AlpTransit project. A straightforward drill and blast method was being used to drive the tunnel.
But three months ago, only 800m into the bore, contractor Satco hit Triassic sandstone which proved more abrasive and difficult to control than the predicted granite.
Tiegler explained that softer rock meant the tunnel required considerably more support during construction. Fortunately, he added, there was less methane in the carboniferous rock than feared, enabling the ventilation system to cope well.
BLS AlpTransit is managing the smaller of Switzerland's two deep-level high-speed railway projects - the other runs beneath the Gotthard pass. BLS insisted the project was 93.8% excavated and on target for a 2007 opening.