RAILTRACK HAS warned that costs could double in the multi-million pound stabilisation of the Georgian Strood Tunnel in Kent.
The network operator has already spent £4M and a series of collapses means it has had to close the 2.5km long tunnel, which cuts through the north Kent Chalk escarpment, four times since December 1999.
Railtrack southern zone director Michael Holden said the final repair bill could be much higher.'We could easily spend the same again, ' he warned.
After the first collapse, which caused a derailment, shotcrete and steel supports were used to shore up the roof near one of seven backfilled construction shafts. Soon after, another collapse occurred next to the works.
Repairs were carried out and the tunnel reopened but further collapses occurred earlier this year, with a 16m deep crater opening up in an orchard above a shaft. Less dramatic subsidence has also occurred above some of the other shafts, forcing Railtrack to shut the tunnel again and at one stage to evacuate residents.
The 19th century tunnel is the oldest tunnelled structure on the UK railway network.It was built to allow British soldiers to reach the north Kent coast unobserved during the Napoleonic Wars and converted to a railway tunnel in the 1840s. It was excavated in the chalk and is only partially brick lined.
The shafts were backfilled from ground level but not always to full depth, with brick arches built to support the fill.
Railtrack published Scott Wi lson's init ia l report into the collapses last month.
The report raised the prospect of a long battle between Railtrack and Southern Water after it emerged that a burst water main could be to blame for the collapses.
It concluded that collapse of the tunnel lining in December had not caused the shaft collapses in April and that the root cause was the same - excessive groundwater movement.
'The problem is something to do with changed water patterns. But at the moment we don't understand what has happened, ' said Holden.
Sources close to the project confirmed that the most likely cause was a leaking water main, although tests on the water had proved inconclusive.
Southern Water admitted there had been three incidents involving leaking water pipes in the area since October 1999 but claimed all were minor and denied they could have contributed to the ground failures.
Foundation contractor Fondedile is lining and filling two of the collapsed shafts with foamed concrete. Site investigations are now Collapse of construction shafts above the Strood Tunnel is thought to have been caused by excessive groundwater movement.
being carried out to see if the other five shafts need repairing.
Railtrack has set up an expert panel of geologists and engineers to look at the long-term water management needs for Frindsbury Hill, through which the tunnel passes.