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GROUND IMPROVEMENT - Lime cement stabilisation is just part of the recycling philosophy for a new apron at the expanding London Ashford Airport.

It could reasonably be expected that construction of a major new airport runway would generate a good deal of road traffic to and from site. But at London Ashford (Lydd) Airport in Kent principal contractor Foster Yeoman Contracting (FYC) made it a priority to adopt a sustainable approach throughout.

FYC's £3M contract for client London Ashford Airport (LAA) includes construction of 24,000m 2of new apron as well as work to upgarde the main runway to handle international Boeing 737 flights.

The overall surfacing operation will consume 1,400t of bitumen and involve laying 30,000t of asphalt.

But 35,000t of deliveries have been kept off local roads through a determined effort to recycle on site material. This involved breaking up material from a redundant second runway for re-use as aggregate, and insitu ground stabilisation by contractor O'Keefe Soil Remediation.

After discussions with FYC and consultant Parsons Brinckerhoff, O'Keefe proposed replacing the client-detailed 500mm of imported Type 1 material with two 250mm layers of stabilised sub-base material with a CBR strength of 30%.

With LAA's agreement, site investigation by O'Keefe's soil testing laboratory started in June last year.

O'Keefe operations manager Mark Jones says: 'A series of trial pits were excavated across the proposed new apron area and a range of suitability and acceptability tests were performed on the samples.' Testing included soil classification (Atterberg limits and sieve analysis), acid soluble sulphate and total sulphur tests, compaction, moisture content value and moisture relationship tests and assessment of CBR strength with differing amounts of lime and cement powders. Results were assessed to develop the final mix of lime and cement to achieve the required 30% CBR strength for the sub-base.

First stage of stabilisation was to remove the top 250mm of the fine sandy clay with a little gravel that underlies the apron area.

'The material was initially treated to 250mm depth with lime powder to 'modify' the clay fraction of the soil before being stabilised with Portland cement to enhance the bearing capacity, ' Jones explains.

'Wetter areas of the formation layer were dried out using additional lime powder before cement was added, ' he says. Excavated material was then replaced and stabilised in the same way.

Compaction and grading using O'Keefe's digital 3D ground modelling system catered for fluctuations in the apron surface without the need for profiling and intensive engineering support, while maintaining finished tolerances of better than ¦15mm.

O'Keefe monitored stabilisation with a series of quality control tests, including stabiliser addition and moisture condition value. Independent CBR validation testing at a rate of one test per 1,000m 2of treated soil was also carried out to confirm the stabilised material had achieved the required 30% value.

Recycled aggregate from the redundant second runway provided raw material for a 200mm cement-bound base on which the asphalt top layers are laid. Run-off will be cleaned in reed beds before discharge.

Although in poor shape, the main runway did not need full depth reconstruction. Cracks are being repaired and the runway resurfaced. A phased approach has enabled FYC to close off outer 500m sections while keeping the airport's central 1km of runway open.This is being tackled during nighttime possessions.

Initial work, which was due to finish at the end of the year, is part of a programme of improvements to advance the airport to two million passenger capability by 2010.

Recycling can bring substantial cost savings but there are other benefits, says FYC project manager Colin Timmins. 'The wonderful thing about recycling is that you are your own master.' With material readily available on site, there is no risk of delay from delivery hold up, he says.

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