COSTS OF building a £145M road tunnel in Surrey are set to rise by up to £30M if the National Trust insists on a 350m extension, contractors warned this week.
Balfour Beatty is working on alternative plans to extend the tunnel under an early contractor involvement arrangement with the Highways Agency.
The project is intended to eliminate a notorious bottleneck on the A3 London to Portsmouth road at Devil's Punchbowl.
Balfour Beatty is examining the proposal to extend the twin bore tunnel in case the National Trust insists on the extension to protect land through the Devil's Punchbowl Site of Special Scientific Interest.
Consultant Mott MacDonald is helping to work up the alternative design, which includes a 330m extension to the 1.7km long sprayed concrete tunnel that forms the key central section of the scheme.
In total the scheme includes 6.7km of dual carriageway and will already cost £145M.
The National Trust supports the scheme in principle but has detailed objections about the impact of the southern portals on an area called Tyndall's Wood.
This is a serious obstacle as much of the area around Devil's Punchbowl - including Tyndall's Wood - is owned inalienably by the National Trust for the benefit of the nation.
This means that the National Trust cannot be forced to sell the land for development, even if the public inquiry inspector and transport secretary Alistair Darling give the scheme the goahead.
If the Trust refuses to lift its objections, special parliamentary procedures would be required to advance the scheme.
The Highways Agency claims its published scheme would have an overall beneficial environmental impact.
But it admits that there would be considerable adverse impacts from the site clearance, construction and operation of the scheme, particularly through the Tyndall's Wood area.
At present the road will emerge from tunnel halfway across Tyndall's Wood. It would then cross the remainder of the wood on embankment 11m to 12m high.
It then swings west through woodland in cutting up to 18m deep, emerging to cross the Hazel Grove grade-separated junction which will cater for all traffic exiting and entering the A3.
The location of the junction is heavily constrained, with schools and housing close by.
The option being drawn up by Balfour Beatty would see a 200m cut and cover tunnel formed on top of the embankment.
This then gives way to a short 130m long bored section in place of the cutting.
This bored section would be just 10m below ground in places and would demand careful tunnelling.
'This would be a relatively delicate bit of tunnel as we can't use standard techniques, ' said Balfour Beatty contract manager Paul Hoyland.
The contractor has come up with a preliminary proposal and estimates that it will cost between £25M and £30M.
The option will be formally published on 18 June, ahead of the public inquiry which begins in September.
This is expected to run for two months, with a decision in spring 2005. Darling will then make his decision.
The National Trust council will then vote on whether to remove its objection and allow the project to proceed.