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Dumpers for goal posts

Foundations Wembley stadium

Teams of contractors are swarming over the pitch at London's Wembley Stadium. Gareth Beazant reports.

Football fans adorned in their club colours used to strut down 'Wembley Way' towards Wembley Stadium's twin towers expecting to see their teams triumph. But with the home of English football now undergoing a much debated rebuilding, the teams have changed.

The ú433M (US$671M) stadium has its own 'overseas' manager, Australian firm Multiplex, which is responsible for project design and construction.

The firm supervised construction of Stadium Australia for the 2000 Olympics in Sydney. Multiplex appointed Griffiths-McGee to demolish the old Wembley Stadium.

Demolition started on the north west London site last September, opening the way for piling specialist Stent to begin the foundations. Supporting one of the world's biggest stadiums is a challenge in itself, but the centrepiece of the design, a 315m long and 133m high arch spanning the pitch, also has to be tackled.

Work began with seven, large diameter, bored, cast in place test piles, installed across the footprint of the new site.

'Seven provided a representative sample of the site, ' explains Stent project manager Anthony Reynolds. 'This confirmed our value engineering studies carried out before the work began.'

After the test piles were done, the main contract works could begin beyond the perimeter of the old stadium while demolition work continued.

The new stadium requires 4,000 piles of 600mm, 750mm and 1.5m diameters, plus 1,500mm giants for the arch bases. The arch will be built on site and lifted into place, so temporary piles are needed for five jacking points, four turning bases and five restraining bases during its installation.

About 5,000m 3of concrete is needed for pile caps. Each connects about 60 piles which go to depths of 35m - as deep as Wembley's landmark twin towers were high.

Stent has four rigs averaging 10 piles a day. These guzzle about 460m 3of concrete, and 8m 3concrete trucks are being used instead of the standard 6m Organising the delivery of concrete, pile cages and other materials is a key part of the project.

'We have calculated that we need a delivery through the gates every six minutes of every working day, ' Reynolds says. 'We've spent a lot of time standardising the cages and working on 'just in time' delivery.'

Prefabricated pile cages are used for the smaller piles (transport is not feasible for those bigger than 1,050mm). These are stacked in a rack that can be moved to where they are needed. This alleviates site storage issues and aims to reduce waste levels associated with storing materials on site.

'Typically the prefabricated cages arrive on Monday and are in place by Wednesday, ' Reynolds says. 'If they were fixed on site, the raw materials would arrive on Monday and they'd end up spread across site, welding would take place - which raises quality and safety issues - and they would probably be installed by the Friday at the earliest.'

The spacers come fixed with handles so they can be lifted and used almost immediately. 'We've aimed for safety, quality and efficiency and if those are right the programme will follow on time, ' Reynolds says.

Stent's contract for Multiplex also involved a short stretch of sheet piling where the stadium site bounds Wembley's exhibition centre. The sheet piles were 15.5m long and driven between 13m and 14m. Piles for 14 tower cranes will also be needed.

The site slopes by 13m and cut and fill operations to form sufficient level space for the new stadium will see 500,000m 3ofmaterial excavated and redistributed. Stent is installing a contiguous pile wall to retain this material under a separate subcontract.

So far this has proved much quicker than a reinforced concrete wall, which would require formwork and more space.

Stent is due to finish piling this summer and the stadium will open in 2006.

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