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Groundworks in the centre of the world's busiest international airport have to be threaded through a tangle of services. Damon Sch³nmann reports from Heathrow.

There are tricky sites, really tricky sites, and then there are nightmares. And it's definitely the latter facing engineers replacing a multi-storey car park at Heathrow.

Reminiscent of large scale brain surgery, mistakes on this project would have equally disastrous consequences.

The new structure, in the heart of the central terminal area, will sit over the area of the infamous Heathrow Express tunnel collapse in 1994, which is now protected by a circular cofferdam and ground improvement piles. The ground is densely packed with services and there are about 56,000 vehicle movements around the site every 24 hours.

Airport operator BAA had little choice but to open this Pandora's Box: the car park serving Terminal 3, built in 1966, had passed its life expectancy and was beyond economic repair. It will be replaced by a 1,550 space car park with a 15,000m site.

Logistical problems have dogged the construction team at every turn.

BAA project leader David Rebello says: 'We had a bit of a problem with our tower crane strategy as it would give a signature and block out a section of radar.' The 26m height restriction imposed by the National Air Traffic Service (NATS) meant the top level of the car park could not be completed. After negotiations, a 30m height was agreed.

Rebello says: 'We had 837 services in the ground, such as high voltage services to Terminal 3, NATS fi bre optic cables, others for airline communications and airfield ground lighting cables.

'The infrastructure has been going in for over 60 years and it's every man for himself in the ducting systems. It's an absolute maze to relocate these and we need backup procedures to backup procedures.' BAA's own IT cables run through the site and severing one of these could have cut off up to 76,000 computers. Prevention was down to the strict site management of principal contractor Laing O'Rourke. The service relocations began in September 2004 and were completed in June.

Laing O'Rourke won the £22M contract for the civils work, substructure and superstructure and was involved from the early stages of the scheme.

Senior project manager Adrian Brown says: 'There's a live rail service underneath that's got to stay open and so we had to plan for this. The planning was led by the contractor from the concept stage rather than just the construction design. It allows us to lead and design in buildability.' Rebello adds: 'You walk out with a drawing that works.' The next constraint in a list of many arose from the need to build a car park over the tunnels for the Heathrow Express link to London's Paddington Station. These are up to 40m deep, and over the circular cofferdam built to secure the ground after the Heathrow Express tunnel collapse. Also below ground here is a concourse link between Terminals 1 and 3. If this were not enough of an encumbrance, areas below the site have been earmarked for future rail expansion.

Geology is gravels from about 4m to 7m overlying London Clay. Before work began, structural engineer Buro Happold did an extensive site investigation to see how the ground disturbed by the 1994 collapse is shifting. Rebello explains the collapse means the soil to the north and south east of the site is pretty poor.

On the north east side of the cofferdam, a block of hundreds of heavily reinforced, 750mm diameter ground improvement piles further complicate work.

The ground is so potentially sensitive, con rained and packed with infrastructure that BAA insisted directors at foundation subcontractor Expanded Piling and parent company Laing O'Rourke sign up to a higher level of accountability for the key parts of the project.

Expanded began work at the end of January with proof digging to 2m to check for services. Overlapping this work, in February it started installing the fi st of 342 piles to support the six-storey car park.

The piles fall into two main groups. Most are 600mm diameter and go 20m to 25m deep. These are area bearing, says Rebello, with generally eight to a pile cap. When GE visited in July there were about 40 of these to be installed with completion scheduled for the end of the month.

These 'multipiles' were the preferred option as single, deeper piles would have interfered with future rail routes earmarked for the site.

Another complication, this time on the west of the cofferdam, is caused by a former fuel farm. The decommissioned 51mm plate steel tanks that remain sit on a 2.5m reinforced concrete foundation and are filled with foam and cement.

Rebello says: 'The 600mm piles solved 90% of our problems.

We had to core through the 2.5m concrete base with a Soilmec rig and the pile caps were formed in the 2m excavation of the proof digs.' But the multipiles could not be used for every part of the site. To support the northern perimeter of the car park and the down ramps, eight, 1,800mm diameter piles have been built at depths of 55m to 65m.

Brown says: 'The Heathrow Express tunnels would have been in the zone of influence of the multipiles and so there would have been a load on them.' But there was not much room to squeeze in the deeper piles. 'The clearances were extremely tight, such as 980mm from a pile to a TBM recovery shaft and 1.9m between another and the concourse between Terminal 1 and Terminal 3, ' says Rebello.

The challenge here was to get these to depth with only minimal defl ction. Laing O'Rourke and Expanded worked on a system that could get them down with a maximum deviation of 100mm. With the directors of the two companies signed up to the piling document there was plenty of incentive.

Rebello says: 'The 1,800mm piles first fi lled me with horror, but when I looked at it again after the initial shock I could see the brilliance in it. There were questions of whether we would hit a tunnel but it never worried me because I had total faith.

'The complexity of what we're doing here is exceptionally risky so we're applying the motto 'if it can't be done safely, don't do it'.' Signs adorning the diffi cult site emphasise this philosophy.

A big Casagrande C800 rig fitted with a 35m mast installed the deeper piles. The machine was modified to produce 380kNm maximum torque and the power was needed to oscillate four, 10m steel sleeves into the ground as straight as possible.

Four total stations continually recorded verticality and precision welding ensured the casings were as near plumb as possible. Although the maximum defl ection had been set at 100mm, the rig crews achieved 75mm.

But one pile became notorious.

Designated E/11a, it set the team one more puzzle. The only location for it in the cluttered ground was next to a Heathrow Express line, and it needed to go through the 750mm diameter ground improvement piles.

A cutting edge on the piling sleeve did the job, but needed to get below 30m - the bottom of the concrete - without wearing out to avoid having to withdraw it. This had to be done while catering for the additional deflection risk from the concrete.

Laing O'Rourke site manager Tony Backler says: 'There were a lot of services in the ground; just about everything that you wouldn't want was there. The big piles were seriously worrying but the ground behaved as predicted and it went well.' The top 40m of the piles do not take a load, so there is no threat to the surrounding infrastructure such as the Heathrow Express tunnels and the underground concourse. About 20m of friction and end bearing pile extends beyond this depth to actually do the work. The eight, 1,800mm piles went in between April and June with new service routes going around them.

Laing O'Rourke is employing the fluid advanced separation technique, which uses water at about 345N/ mm 2, to cut the piles to length.

A subway connection in the southern corner of the site will get people from the car park to Terminal 3. Consisting of two lifts, a lift lobby and a staircase, this is being built inside an excavation secured by a 22m deep secant pile wall.

The £50.6M project includes demolition of the old car park. The new building is due to open at the end of June 2006.

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