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Lime stabilisation layer suspected in case of buckling bypass

INVESTIGATIONS INTO heave that badly damaged a new 7.5km bypass in Hertfordshire were continuing as GE went to press.

Up to a quarter of the new carriageway on the A10 Wadesmill bypass was found to be buckled, cracked and ridged.

Some surface defects occurred close to the bridges after heavy winter rain. The most likely cause is seen as sulphate attack on the lime stabilised capping layer below the full depth flexible road construction.

Sand and gravel from a cutting on the project was processed to be reused on site. But it was found to be too susceptible to frost heave to be used below the main carriageways, so main design and build contractor Fitzpatrick Lafarge and its consultant URS opted for full depth flexible construction on a lime-stabilised capping layer.

Elsewhere on the bypass route there are glacial tills, materials which could require lime stabilisation.

The Fitzpatrick Lafarge joint venture will now have to explain how the battery of chemical tests demanded by the Highways Agency failed to detect the presence of dangerous levels of sulphates or sulphides in the stabilised soil.

Late last year the Agency improved test methods for sulphate and sulphide by including a direct test for sulphide content in its specification. Earlier forms of sulphate and sulphide tests were known to be labour intensive and more prone to human error.

In 1990 lime stabilised clay capping below the M40 heaved under sulphate attack, causing major damage and delay.

'The only effective way of dealing with the problem is to remove the entire affected carriageway and all sulphate-bearing material below it, ' said Murray Reid, technical manager at landfill tax specialist Viridis.

He confirmed that Agency roads specifications and guidance notes say 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.

Determining exactly how much of the road will have to be repaired is the most important next step.

The Highways Agency said it was carrying out additional testing with a view to start remedial works this month. It insisted the road would still open on schedule in August.

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