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Vibro-columns - Building houses on a backlled quarry has called for a rethink of foundations after piling proved too pricy.

On a sodden December day, with mountains of earth being mauled by excavators and tipper trucks churning the site into a mire, it is hard to see Linden Homes' vision for the smart new 'Watercolour' housing estate at Merstham, Surrey.

The developer has already erected a row of show homes on one corner of the site, but most of it is still very much in the hands of contractor Keller, which is in the final stages of landscaping and ground improvement works.

'If you think things look messy now you should have seen the site when we started work in November last year, ' says Keller regional manager Derek Taylor.

'It looked like the moon.' The Watercolour site was a sand and gravel quarry.

Extraction from parts of the quarry was under way until Keller started work. Simultaneously, the worked out areas of the quarry were being landlled.

'Backfilling was under way from the 1960s until mid-last year - mainly demolition waste from Redhill town centre and from construction of the M25, ' says Keller commercial director Trevor Snell.

At its deepest the ll is 17m deep. 'There's quite a mix of materials. There was no control over placement and no compaction. It's very nonhomogenous, ' Snell adds.

'The material is too deep to compact with surface rolling techniques, ' continues Taylor.

'Keller was approached by Linden's engineers in 2004 to suggest ways of making the site suitable for development. They were looking for an economical way of making it buildable.' Because the ll is so deep and also because it has been placed so recently there were concerns about settlement. Houses, built on raft foundations, would settle faster and further than adjacent roads and areas of hard standing, it was feared. Keller was asked to limit settlement to between 35mm and 50mm, and to keep differential settlement to within 1:500.

'It was originally proposed to pile the site, but there was a big question over whether it would be possible to get adequate cover at a sensible price, ' says Snell. 'It was also uncertain what length of piles would be required.

'We concluded that while it would be economical to pile the houses, it wouldn't work for the paved areas.' Keller instead proposed putting in vibro-stone columns to achieve the required 50kNm 2 to 75kN/m 2 bearing strength under the houses, and surcharging the areas of hard standing and roads to accelerate natural settlement into one to two months.

Keller has done reproling and site improvement work under a £5.5M contract, giving it unusual freedom in organising operations.

'Often you'd come in after a landscape contractor and have to work with what they'd left you. Here, we've been able to use cut material for surcharging before moving it on to its final ll location, for example, ' says Taylor.

Materials have been mixed with 3% to 6% cement to control moisture content and improve homogeneity, and laid to 95% maximum density.

The vibro stone columns have been installed to depths of 6m to 7m across most of the site, though there are areas where they are much shorter, at 2.5m.

For the house foundations, columns have been installed at 1.5m to 1.8m centres.

Keller has just installed the last of the 17,000 columns and is rapidly handing areas of the site over to Linden Homes as surcharging operations come to a close.

AM

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