SULPHATE ATTACK on the lime-stabilised capping layer below the dual carriageways of the 7.5km Wadesmill Bypass in Hertfordshire was this week threatening to delay its opening.
The phenomenon has caused a surface heave problem that has slowed progress on the £25M project.
In 1990 lime-stabilised clay capping below the M40 heaved under sulphate attack, causing major damage and delay.
'The only effective way to deal with the problem is to remove the entire affected carriageway and all sulphate-bearing material below it, said landfill specialist Viridis technical manager Murray Reid.
JV design and build contractor Fitzpatrick Lafarge had originally hoped to open the bypass in January, eight months ahead of the official August target (NCE 13 February 2003).
Settlement problems with fill behind bridge abutments, and the discovery of surface irregularities, had already put back the opening.
But now that an independent consultant has identified sulphate attack under the main carriageway, even the August deadline could be under threat.
In a statement this week, the Highways Agency confirmed the findings and said: 'Additional testing to determine the extent of remedial works is being undertaken as quickly as possible so that these works can begin early in April.'
It also reported that the suspect backfill behind the bridges was being excavated and removed from site.
Much of this is believed to be recycled site-won granular material from a major cutting on the bypass.
Elsewhere on the bypass route there are glacial tills.
These are typical materials which could require lime stabilisation.
The technique has been widely promoted as an environmentally superior alternative to removing otherwise unsuitable cohesive fill material from site and replacing it with high quality fill from distant quarries.
'But after the M40 problems the Agency issued lime stabilisation guidance notes, updating them as recently as 2000, and there seem to have been no similar problems since, ' said Reid.
He confirmed that Agency road specifications and guidance notes say that any material chosen for lime stabilisation has to be tested for sulphate and sulphide content.
Adding lime to cohesive soils dries them out and stiffens them, but if sulphate ions are present, an expansive chemical reaction occurs.
Last week the Agency was still insisting the road would open by August. In its latest statement it said: 'The contractor will endeavour to complete all remedial work by the contract completion date.'