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Undetected sulphates blamed for A10 heave

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HIGH SULPHATE and sulphide concentrations in the capping layer and sub-base of the A10 Wadesmill bypass caused major heave damage, the Highways Agency said this week.

The discovery by independent investigators has forced the Highways Agency to review its guidance on lime stabilisation of soil.

Investigators found that high levels of sulphates and sulphides in the recycled site-won glacial tills, used in the capping layer and sub base, were missed by the first site investigation.

In a written statement the Agency said: 'Pre-stabilisation tests (on the glacial till) showed very few results above 1.2% SO4, but post-stabilisation tests indicated up to 4.3% in the capping (stabilised layer) and 4.1% in the subgrade.'

The Agency confirmed that it would be reviewing advice note HA74/00, to include frequency and extent of site testing, acceptable sulphate limits, sampling and testing methods, and the effects of surface water drainage.

An Agency spokeswoman added: 'The indications are that the high sulphate levels measured after the problem developed were present on the site before work began, and were not brought in by groundwater movements.'

These sulphates and sulphides reacted with the lime to cause an expansive reaction, which formed humps and ripples up to 80mm high. Four large areas along the 7.5km dual carriageway road were affected, and main design and build JV contractor Fitzpatrick Lafarge eventually had to remove and replace 35% of the entire carriageway (NCE 19 August).

A similar problem dogged the M40 in the 1990s. Ettringite and thaumasite, which are tell tale by-products of the heave reaction, were found there and on the A10.

In both cases the solution was complete removal of the damaged areas and other areas where sulphate and sulphide levels were dangerously high.

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