PLANS TO bury the A303 at Stonehenge in a tunnel will die unless the heritage lobby throws its weight behind the scheme, say sources close to the project.
A £186M cost increase because of diffi cult ground conditions has seriously undermined the project's economic viability as a road scheme alone.
But the proposal could survive if the positive impact of reduced traffi c and pollution on the world heritage site was taken into account.
This would involve organisations like the National Trust and Unesco withdrawing their objections to the scheme.
Roads minister Stephen Ladyman ordered a review of the project at the end of July after revealing that expected costs of the 2.1km tunnel had risen from £284M to £470M. The move came as it was announced that the public inquiry inspector had recommended the scheme should go ahead.
Ground conditions are the main reason for the cost escalation.
About £100M of the increase was down to the discovery of quantities of soft, weak phosphatic chalk, and an unexpectedly high water table.
These factors significantly complicated the planned tunnelling process, which was to have used the open face, spray concrete method.
Use of the Freedom of Information Act has brought to light several other previously unaccounted for factors, including:
a £60M allowance for the true cost of construction inflation
a risk allowance of £13.7M to handle new legislation for working in confined spaces
£5M for project insurance
£5.5M set against the possible introduction of a fire suppression system.
Preliminary ground investigations had not thrown up any unusual problems. On the appointment of Balfour Beatty/Costain under an early contractor involvement arrangement in March 2002, other tunnel options were evaluated. The decision to switch to a 2.1km long bored tunnel was made in December 2002.
The contractors ordered a supplementary ground investigation to be carried out in the winter of 2002/03. But they did not get detailed results before the public inquiry started in February 2004.
The effect of the waterlogged phosphatic chalk on the construction method is signifi cant. It means increased dewatering, greater temporary support and a slower rate of construction. This was expected to extend the programme from three years six months to four years four months.
The result is a £50M increase in the cost of the works and a £23M increase in supervision costs.
Allowances for risk and inflation takes the cost increase related to bad ground to £100M.