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Unconventional wisdom

PILING - The ground beneath a former engineering works in Wolverhampton, earmarked for a housing development, presented a tricky challenge.Alasdair Reisner discovers how Pennine Vibropiling's stone columns made the earth move for Taylor Woodrow

IN MATTHEW'S gospel it is the wise man who builds his house upon the rock while the foolish man builds his house upon the sand.

But on a former GKN factory site in Wolverhampton Taylor Woodrow intends to build houses not on rock, nor even on sand, but on a mixture of backfilled colliery spoil and ash.Given the poor engineering properties of such material, is this not a major risk?

Fortunately not. Instead of going through the expensive and longwinded process of laying down the pad foundations that such ground would normally require, Taylor Woodrow has brought in the services of geotechnical specialist Pennine Vibropiling.

The firm's geotechnical engineer, Ralph Cook, explains: 'They were looking to us to provide a system where they would be able to make cost savings by changing from a raft foundation to a traditional strip footing with some reinforcement in.

'This was designed in co-ordination with the National House Builders Council.We looked at the ground conditions and suggested a form of treatment that would be suitable to take the settlements that were occurring in the ground.'

Pennine's solution was to limit these settlements by installing around 4,900 vibro stone columns across the site.These columns offer a secure surface on which to start building homes.

So why choose stone columns over a piled solution?

'One of the reasons a piled solution may not have been suitable was that there were deep obstructions such as boulders in the made ground. If you were going to use driven or CFA piles you might be struggling to penetrate that material, ' says Mr Cook.

The process is also much quicker and cheaper than piling.As proof of this, just yards away from Pennine's vibro rig, Taylor Woodrow workers are already at work installing blockwork for its first homes.

'If you had been using a concrete piling system you would have had to put reinforcement in, ' says Mr Cook.'You would then have had to wait a set time for your piles to cure.Then the piles have to be tested. It is a much slower process which means you can't get on with building.'

A final benefit associated with the use of vibro stone columns is the fact that they can be formed without producing any uprisings.Under recently introduced regulations any spoil produced would have to be taken away and treated as industrial waste to be buried in specialist landfill sites.

Mr Cook says these benefits only came about because Taylor Woodrow brought together its team for the job at an early stage, allowing innovation to help deliver the project.

'We had a team together that was able to confront any problems at the design stage, which allows a smooth transition to work on site. Every issue we have seen on site has been anticipated because of the earlier design input from all parties' So in choosing to build their houses on stone columns it appears the team is made up of wise men after all.