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Simplex answer to a loading question

PILING - Many developers have tried in vain to revitalise the centre of Hounslow over the years.Alasdair Reisner talks to Simplex Piling to find out how it is helping parent company Vinci beat the Hounslow hoodoo

IT IS HARD to see what the problem is with Hounslow. Everything seems to be in favour of the town on the western fringes of London. Its transport links make it ideal to commute from into the city, while its proximity to the M25 and Heathrow offer an excellent location for businesses. It is also far enough away from the hubbub of central London to offer a relaxing place to live.

So why is it that every time someone tries to redevelop the town centre the scheme falls flat on its face?

This was the question on the mind of Ray Daniel of Blenheim Norwest, the developer building the Blenheim Centre, a 110,000 sq m mixed use development at the heart of the town. Before Mr Daniel, three other developers had tried and failed to transform the town centre.How was this going to be any different?

Mr Daniel says the only way the scheme was going to work was by bringing in expertise from the construction sector as early as possible, to refine the plans as much as possible to make the project affordable.

'By bringing all of the specialists in you get coal-face technology being applied right at the design stage.We then know what we are doing and can build confidence. If we had developed it in the traditional way we would have had elements of over-engineering rather than bringing savings through innovation. In the long run that is what has made the project viable, ' he says.

But rather than find subcontractors for every discipline to help him, Mr Daniel approached Vinci, which could offer a 'one stop shop' for the skills needed.This included Norwest Holst as main contractor, Norwest Holst Soil Engineering for site investigation and Freyssinet for the post-tensioned concrete frame.

The piling work fell to another Vinci group company, Simplex Foundations.Over the past three years the firm has been working with Mr Daniel to ensure that the Blenheim Centre is set on a firm footing, according to Simplex director David Donovan.

'We started out advising on various cost options.We looked at all manner of solutions to reduce the length of the piles required because the deeper you go the more material you have to carry off-site, ' he says.

The first thing Simplex needed to know was precisely what it would be dealing with.

'We worked very closely with consulting engineer SKM.We wanted to know exactly what the loading was going to be on each node and on the core.There are some big loads both horizontally and vertically and we had to investigate the cheapest and most effective way to carry them, ' he says.

With this knowledge the team set about putting in three non-working test piles to be tested to destruction.This gave a good idea of how the ground would work with the piles on the completed project.An area of poorer ground in a corner of the site was chosen to bore the three piles (26 m x 600 mm, 20 m x 600 mm and20 m x 500 mm); if the right results were achieved there, there should no problems with the rest of the site.

'To have such a testing regime is unusual even for a job of this size but with those three we could be confident, both for ourselves and the client, that there were not going to be any hiccups, ' says Mr Donovan.

Along with the non-permanent test piles the team also installed some working test piles.

'We have done four of them.We jack the working piles against four extendible anchor piles.Typically a 600 mm pile on site needs to be loaded to around 200 tonnes so with these working test piles we loaded them up to 300 tonnes.They have been excellent, really proving that the design we have is efficient.'

This, says Mr Donovan, is the key to Simplex's work on the Blenheim Centre; not piles that are too weak, not piles that are too strong and overengineered - piles that are just right for the job.

'I've been in this business quite a few years now and this is the most efficient piled foundation I have ever come across, ' he says.'On any traditional contract you will find the piles are over-designed.There is a degree of design built into the piles that is lost forever.On this occasion we have had the opportunity to do preliminary test piles that have allowed us to optimise the results.'

But is there not a danger that by working so close to the margins of what is acceptable you may end up accidentally overstepping the capabilities of the piles, compromising the overall structure?

Mr Donovan replies: 'There is no risk.The testing is used to confirm that the piles conform to traditional piling standards.Within those design regulations there is a statutory safety requirement that everyone designs to.We make sure we adhere to those safety standards.Typically all the piles on site have a minimum factor of safety of two.This means that if their design load is 1,500 kN/m, it should be capable of carrying at least 3,000 kN.'What we don't want is to have a design safety factor of 2.5 or 3 because then you are just spending money unnecessarily.'

In fact the only area where the piles have an extra level of redundancy built into them is underneath the tower cranes that will service the site.

This was a requirement of London Underground to safeguard the cranes as their arc of collapse would have taken them across the Piccadilly Line along the northern edge of the site.

The method of piling used has been rotary bored piles.While the underlying ground across the site is London clay, there is layer of around 3 m of gravel above it, necessitating the use of temporary steel casings to hold the bore open during pile installation.

Around 700 piles have been installed under an £800,000 contract.Of those, 130 are 750 mm diameter.There are also 160 500 mm piles.The remainder were 600 mm piles with each pile diameter chosen to meet the loading demands of that part of the site.

'There were any number of combinations open to us, ' says Mr Donovan.'We could go down to 40 m with very thin sections or stop at 20 m with a much fatter section. It was a case of building each into the design and checking the costs.'