A £50 million car park is being constructed while the Heathrow Express terminal directly underneath is kept open. Damian Arnold visits the site at Heathrow Airport to see how Expanded Piling dealt with a precision job
MORE than a decade after the Heathrow Express tunnel collapse, the ground that gave way so dramatically is set to support a £50.6 million car park.
Work on the main structure has just begun after four months of complicated ground works that explain the £50 m illion cost of an otherwise convent ional structure that would typically cost £18 million.
The site is presently buzzing as steel girders are lifted in to place but client BAA can take comfort from the fact that the difficult part of the job below ground is now complete.
Th is included more than 14 months of geological surveying and modelling by Buro Happold to study ground conditions made very uncertain by the migrating soils full of tunnel debr is f rom the collapse in October 1994.
The team of principal contractor Laing O'Rourke, engineering consultant Buro Happold and piling contractor Expanded Piling has devised a solution that sees massive 1,800 mm diameter piles d r iven precisely between the Heathrow Express tunnels, passenger walkway and passenger terminal keeping the Heathrow Express running. The piles also have to avoid ground alongside the HEX tunnels that has been reserved for the Crossrail tunnels expected to bored in the next few years.
After almost a year on site the piling operation ? comprising more than 360 piles of 60 mm diameter capped in groups of mainly six to eight, plus nine 1,800 mm piles ? is drawing to a close.
BAA project leader David Rebello hails an operation that had to be pinpoint accurate to ensure that the HEX concrete tunnel walls 34 m underground did not crack or the tunnel itself move sideways.
'Perfect verticality was needed, ' says Rebello. 'If the alignment is off by even two or three degrees (about 20-30 mm) then you've got serious problems.
The tunnel lining could be damaged on both sides.
'There are a lot of unknowns in the first 25 m of that ground. There are pockets of concrete and remnants of sheet piling.' Ensuring the accurate positioning of the piles and monitoring of the shifting soils after the Heathrow collapse called for an intricate ground monitoring operation using Total Station software that can be linked up to real-time internet viewing. Under a colour coded system controlled by tunnel consultant Mott MacDonald, blue means the operation is proceeding exactly to BAA design parameters, green calls for a reassessment of the operation, amber calls a halt to the piling operation and red stops the trains as well.
The 1,800 mm diameter piles are positioned between the two Heathrow Express rail tunnels.
These piles, up to 60 m deep, are driven in a steel casing for the first 40 m to reduce the friction that might crack the tunnel linings. They need to be driven more than 6 m below the tunnels so that their load bearing does not exert pressure on the tunnel linings.
The operation is eased as the uncertain ground conditions give way to virgin London clay at around 25 m down.
'The intensity of the London clay at 35-40 m is perfect for anchoring piles, ' says Rebello.
Starting construction above ground brought a new set of challenges posed by building a steel and concrete seven-storey structure on top of the Heathrow Express buildings that break the surface.
Originally the cofferdam was built to accommodate a hotel development on top of it using the same circular footprint of the site. A circular configuration of loadbearing columns was built into the top of the HEX development to act as a foundation for more building. Faced with the problem of building a rectangular car park on top of circular foundations, a rectangular 'transfer deck' of structural steel girders up to 24 tonnes was placed between the columns to transfer the loads under a solution that Rebello says was unique for a car park.
The thickness of the steel pieces of the transfer deck has the added advantage of fire rating the above-ground roof structure of the Heathrow Express buildings. The Heathrow Express control room and plant room that break the surface of the cofferdam are not fire rated and a barrier is needed between them and the underside of the car park.
The steel member forming the underside of the car park would remain in place for 90 minutes, giving enough time for both facilities to be evacuated.
While massive steel girders were being lifted into place by a 500-tonne crane when Construction News visited the site, an estimated 56,000 vehicles a day were circumnavigating the site at the world's busiest international airport. Those that use the existing car park have a bird's eye view of construction, making the site feel like a goldfish bowl, says Laing O'Rourke project manager Tony Backler.
Already constrained by busy roads around the perimeter, the site also has restrictions on height because the 30 m cranes needed to lift the steel sections interfere with the air traffic control towers. A height restriction of 26 m is placed on buildings so as not to interfere with radar.
Under a modelling exercise with National Air Traffic Services a solution was found to reprogramme the nearby control tower monitoring air traffic from the south-east to tell it the crane was not there.
Ideally there would have been two 30 m cranes on the site but the team has had to make do with one, so the car park will be constructed in two halves.
Beneath ground the main construction challenge was to ensure the busy Heathrow Express terminal operated as normal while piles were being driven all around by the massive Casagrande 800 machine under a 22 hour construction schedule of 7 am-6 pm and 8 pm-5 am.
HEX passengers are able to use the service as normal with the only issue being the need to ensure that the sensitive monitoring equipment rigged up around the station is not disrupted by passers by. A misplaced mop from a member of the HEX cleaning staff has already caused the whole operation to stop.
So far the intensive 14 months of planning has paid off and the project is on schedule but the construction challenges will not have ended after the project is finished.
When the Crossrail tunnels are bored, as seems likely within the next 10 years, the whole car park will have to be jacked up by 25 mm to counteract the settlement that is likely to occur. But there lies a construction challenge for another day.