UNIQUE GROUND conditions are the main reason for costs of the Stonehenge tunnel escalating from £284M to £470M.
Around £100M of the increase was down to the discovery of quantities of soft, weak phosphatic chalk - 'unique' in the UK - and an unexpectedly high water table.
These factors significantly complicated the planned tunnelling process which was to have used the open face, sprayed concrete method.
Information released to NCE last week under the Freedom of Information Act also reveals several other previously unaccounted for factors.
The increases include:
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
and £5.5M set against the possible introduction of a fire suppression system.
The bad ground conditions were discovered by contracting joint venture Balfour Beatty/ Costain shortly after draft orders were published in June 2003 ahead of the public inquiry in spring last year.
Preliminary round investigations had not thrown up any unusual problems.
But these had been carried out assuming a 2km long cut and cover tunnel would be constructed in accordance with the June 1999 preferred route announcement.
On appointment of Balfour Beatty/Costain under an Early Contractor Involvement arrangement in March 2002, alternative 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 unstable, waterlogged phosphatic chalk on the construction method is signifi cant.
It means increased dewatering and the need for greater temporary support during open face New Austrian Tunnelling Method tunnel construction.
Even by attacking the tunnel from both ends the slower rate of construction 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 in inflation takes the cost increase related to bad ground to £100M.