UNEXPECTED GROUND conditions have slowed progress on the last 10% of the AlpTransit project's 34.6km long L§tchberg tunnel in Switzerland.
Discovery of carboniferous and sedimentary rock has slowed progress from the 10m a day achievable in the harder self-supporting granite to just 2m-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, ' he pointed out.
The L§tchberg tunnel forms the western half of the huge AlpTransit project. A straightforward drill and blast method was being used to drive the tunnel.
But two months ago, just 800m into the bore, contractor Satco hit Triassic sandstone which proved more abrasive and difficult to control than the predicted granite.
Tiegler explained that the softer rock meant the tunnel required considerably more support during construction. But he added that 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 Gotthart pass. It insisted that the project was now 93.8% excavated and on target for a 2007 opening.
At present a single 9.5km bore is being created for the central part of the tunnel - future expansion may create a twin bore.