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'Tractor and plough' companies lack stabilisation skills


BOOMING DEMAND for soil stabilisation has left specialist contractors short of the skills needed to cope with extreme ground and weather conditions, geotechnical experts said this week.

Fears have been raised over the expertise in the industry after problems surfaced with the lime stabilised capping layer below the new A10 Wadesmill Bypass in Hertfordshire.

Last week the Highways Agency said the cause of the carriageway surfacing problems was probably sulphate attack on the lime stabilised capping layer (news last week). Problems may also be linked to the prolonged dry spell last summer.

'Some companies have been in the business for more than 10 years, and they know what they're doing, ' said soil stabilisation consultant Chris Holt. 'But three or four years ago some 'tractor and plough' companies invested in the new generation rotary mixers, thinking it was an easy way to make money. It isn't, especially if the ground is very dry or very wet.'

Highways Agency senior geotechnical advisor Alex Kidd said: 'Lime stabilisation only works over a fairly narrow spectrum of moisture content, and it's easy to get it wrong at either end of the spectrum.

Good site control is vital.'

The last major problem with sulphate attack on lime stabilised capping layers occurred on the M40 in 1990. Experts agree that the subsequent and continuing upgrading of the Agency's specification for lime stabilisation had produced very effective guidelines, and most were shocked by the apparent problems on the A10.

'More than 90% of our work is on commercial projects, only 5% is on roads, ' said one major lime stabilisation contractor who did not wish to be named. 'The Agency is an intelligent client with a good specification and a testing regime that picks up most problems.

'But there are plenty of commercial developers who just want a firm level platform to build off, and depend completely on the expertise of the specialist contractor.'

If the ground to be stabilised is very dry, normal practice is to add water to the lime/soil mix before compaction. Failure to add enough water results in a poorly compacted layer with a high air void content. These voids encourage the oxidation of any sulfides that might be present and create ideal conditions for the later formation of expansive ettringite crystals. (See box).

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