IT MAY be the giant arch that grabs the headlines but even Wembley's roof engineers admit that some of the most difficult engineering is in the stadium's foundations. Over 3,700 piles are needed to support the new stadium. And piling contractor Stent Foundations has the job of installing them.
Of the 3,700, the massive pile base for the eastern side of the stadium's arch is the most substantial piece of work yet completed. A total of 900 cu m of concrete forms a 33 m foundation made up of 1,500 mm diameter piles and took three weeks to construct. The western side's arch base, including 1,200 cu m of concrete, is set to be poured later this month.
'Each one is on the scale of a basement on a City development, ' says David Hendries, head of Multiplex's Wembley team.
Work on the piles for the stadium is expected to begin in mid-March. Having been on site since September, working alongside demolition crews and earthmovers, Stent is in the thick of things.
'We have another eight months to go, ' says works manager Kenny Morrison. 'It is a conventional straight drive technique into London Clay. It is just the sheer volume of them. The hardest part is the logistics, getting enough muck out and enough concrete in.'
This is echoed by project manager Anthony Reynolds. 'The amounts are not that big, it's coordinating all the retaining walls and the arch bases and the different cage types. There has been a lot of standardisation to minimise risk, ' he says.
The piles vary in length from 16 m to 35 m - the height of the old twin towers - with the majority 600 mm or 750 mm diameter. Designed in clutches of three and four, they become more condensed at the tallest part of the stadium to compensate for increased loads. A total of 45 working test piles dealing with loads of between 300 and 200 tonnes are on site.
Rebar cages ranging in diameter from 600 cm to 900 cm diameter are pre-fabricated off-site or made on site according to size. A just-in-time delivery approach aims to have 100 cages on site at a time, with two steel riggers on site and two others working off site. Four rigs are kept permanently on site.
'Any more and we wouldn't be able to keep them all supplied with concrete or steel, ' says Mr Morrison.
A temporary 52.2 m piling wall has also been constructed to allow earth works contractor Griffiths McGee to excavate down to the level of the new stadium. Temporary works are also being established for the jacking systems that Cleveland Bridge will use to manoeuvre the arch into place.
In total, Stent estimates it will use 'several thousand' tonnes of steel and around 70,000 cu m of concrete on the job.
'We're getting through around 50 loads of concrete - pouring 350 cu m a day - and four or five loads of steel a day, ' says Mr Reynolds.
Later, material for the 20 m, 2.75 m thick pile cap to support the perimeter of the stadium will include recycled concrete removed from the site by Griffiths McGee.