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Stent scores with the Gunnners

PILING

Stent has been sinking piles for the new Arsenal stadium faster than you can say Freddie Ljungberg. Joanna Booth finds out how the project has sprung back into motion after a nine-month delay due to lack of funding

ARSENAL fans have had a double treat this season, as well as watching their team reign unbeaten at the top of the Premiership, eagle-eyed fans have been able to watch over the progress of their new stadium a spitting distance from the old ground at Highbury.

The Gunners are expected to move into their new home in time for the 2006-07 football season.

The £357 million, 60,000 capacity stadium will stand 46 m-high on a 6.9ha site. But standing on the future pitch area, all this seems very far away at the moment.

The ground is currently occupied by a working waste transfer depot, processing trucks of rubbish.Around this is a hive of activity, as Stent piling works proceed side by side with Keltbray demolition and formwork contractor Byrne Bros.

Principal contractor Sir Robert MacAlpine was forced to mothball the project in April 2003 when the club's deal to finance the stadium ran into difficulties. Stent had been piling for a month when the call came to halt work and de-rig.After Arsenal secured a further loan, work began again nine months later.

'Very little of the design had altered when we came back, ' explains Max Gwynn, project manager.'They've added a few temporary works, changed a few loadings, but we could basically pick up where the previous team left off.'

All 1,637 piles for the stadium are large-diameter bored piles.Under the podium structure there are 1,360, 600 mm-diameter piles.

In addition, 277 larger piles are situated outside the stadium footprint to the north and south, where a suspended slab will allow service trucks to enter the stadium at a lower level.These piles are 750 mm diameter and 20 m deep, with a load-bearing capacity of 2,200 kN.

The 600 mm piles come in three lengths - 20 m, 18 m and 16 m.The longest will support eight shear cores, which house stairwells and carry the loads from the stands.The shorter piles will carry the lower tiers.

'We can use one design of cage for the whole lot, which is a dream as far as we're concerned, ' adds Mr Gwynn.'We really hit the sweet spot on the design.There are 24 m of London clay before we hit Thanet sand, so we can keep the piles in the stable ground.

We use a short piece of casing then just dry bore through the clay.'

Stent's cage supplier Romtech developed a stillage system for the pile cages specifically for this job.

Space is tight, so it was important that cages weren't littering the ground.

'Each stillage contains 25 cages, ' says Mr Gwynn.'The slings are already in place, so we can just crane them around after the rigs. It makes the whole process a lot more efficient, and we have fewer health and safety worries.'

Despite its acreage, the works on the site are tightly packed and look somewhat chaotic.'We have weekly interface meetings with the other subbies, ' says Mr Gwynn.

'We use barriers to control the piling zones.Our two rigs are Casagrande B125s.They're hydraulic and fast enough for this kind of ground, but the bodies are also quite small. It's a real advantage on a site like this, ' he adds.

Stent will not be able to finish all the piling works until the waste transfer depot can be removed.'They're building a replacement facility on the other side of the Holloway Road, ' says Mr Gwynn.'When that's working, this one can be demolished.'

The team will return in the autumn to complete 457 of the piles when access is available.The £3 million Stent project will drive 25.5 km of piles, using about 7,500 cu m of concrete.Up to 30 wagons of concrete arrive at the site each day. Each batch is logged, and using the Sherpa IT system (see above), the records show which delivery of concrete goes into each pile.

Each rig completes an average of 14 piles a day.'It's all about getting into a rhythm, 'maintains Mr Gwynn.'We put in the casings in the late afternoon, so the next morning the rig can drill along the row of them, with the concrete pours following that.'

Checks take place to determine that pile positionings are correct. If a casing is more than 75 mm out of position, it will create an eccentric loading on the pile cap.When the casing is pulled, further checks take place on the steel and finally the concrete.Once piling is complete, static load tests are done on 10 per cent of the piles.

Sandwiched between residential property, noise can be an issue on site.

Working hours are strictly between 8 am and 6 pm, but there has been the occasional complaint from disgruntled locals.

'Mind you, I don't think we make much more noise than the crowd at Highbury, 'Mr Gwynn reveals.

Sherpa takes up the challenge

THE ARSENAL site is among the first tranche of projects to pioneer Stent's IT system, Stent Handling Electronic Roaming Piling Assistant.

Using Tablet PCs, Stent employees can record pile data out on site, rather than storing paper records or laboriously re-keying information later on. Sherpa creates what amounts to a birth certificate for every pile, registering details at each stage of the construction process. It records designed levels and then as-built levels to automatically track problems if the figures differ too widely.

Users have different profiles.A banksman will record the casing and drilling details, the concrete ganger fills in sections about pours and cages and the concrete technician registers concrete deliveries.

The tablets link into the system by a wireless network.As well as keeping the data, the software processes it to provide summary snapshot views for a week, and generates a picture of the stadium footprint, indicating where piles are completed.This data can be viewed from anywhere through the Stent intranet.

Sherpa manager Azlan Cuttilan co-ordinated the roll-out programme on site, and found that workers coped with the learning curve.

'They can pick it up very easily, but they forget, ' he says.'We've found the best way is to talk it through, then go and stand with them while they work, and then throw them in at the deep end.

'They need to adapt it to their jobs.A banksman needs to keep an eye on the rig at all times, so he needs to plan when it is safe for him to use.

After a while it becomes second nature.'

Mr Gwynn finds that the system can reduce error.'It compares the asbuilt data with the original figures, and if, for example, the pile is too short or the cage is too low, there'll be an on-screen alert.'We have a non-conformance report level of 1 per cent, but this could reduce it further.'

Progress of piling at time of going to press

First phase nearly complete ROLF Christiansen is Arsenal Stadium project director for Sir Robert MacAlpine.

'We're very pleased with Stent, ' he says.'The first phase of the piling is basically complete. Part of the contract is to build a new recycling plant for London Waste.

'When this is finished we can demolish the old one and get access to the last quarter of the site.The next stage is to erect two steel footbridges over the railway around the site boundary.Demolition is more or less complete, and concrete works are progressing.

By the end of the year, we hope to have three stands substantially complete and have started the second phase of piling. Early next year, we will begin work on the roof.

'The funding issue did cause a problem - it was impossible not to, as we were delayed by the best part of a year. It did give the design team time to look over things cold.

'Picking things up wasn't too hard as the same core team remained, and it's not hard to remotivate people working on a project with such a high profile.'