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Underground progress moves T5 a step closer to take-off

UNDERGROUND - RAIL Over 13.5 km of tunnels are being built as part of the huge Terminal 5 project at Heathrow, including an extension to the Underground.Adrian Greeman reports

THEY wrap you up well when you visit the tunnel boring at Heathrow.Not only do you get hard hat and steel-capped boots, high-viz jacket and safety goggles, but there is even a neat pair of white gloves to wear.

And ear plugs, of course.

Even if you can no longer hear or feel much else, there is no missing the message the T5 team wants you to take away: safety and organisation on site are critical features of this giant project, and nowhere more so than in the inherently dangerous tunnels and underground works.

And there are plenty of those.Terminal 5 will handle roughly 30 million passengers every year - equivalent to the whole of Gatwick Airport.Apart from transit passengers, most of these have to get to the new part of the airport and leave it again. Cars buses and taxis will carry some of them via a dedicated new link to the M25 motorway but a large proportion will use trains and tube lines. It is part of the overall philosophy of the terminal to encourage public transport.The target is to achieve 50 per cent of passenger movement using rail.

'These links must come in underneath the airport, which means tunnels, ' says Ian Fugeman, head of rail and tunnels for T5.'And there are other tunnels too, such as a 4 km-long surface water drainage outfall and baggage tunnels.'

Much of the major tunnelling has been carried out, including the 2.9 mdiameter water runoff tunnel, a smaller service tunnel for power lines and communications and, biggest of all, the 5.68 m-diameter twin bores for the Heathrow Express rail link to Paddington.These tunnels, passing under taxiways and aircraft parking stands, extend the line some 1.6 km from the existing Central Terminal Complex to the T5 'campus' at the western end of Heathrow.

Remaining attention focuses on the Piccadilly Line, two slightly smaller 4.5 m bores running for 1.7 km.These will tie the terminal's underground station into the Piccadilly loop line, which currently carries tube passengers around the airport to a station at Terminal 4 and then to one at the three central terminals, before heading back into London.

The first part of the work is straightforward tunnelling. Like most of the underground works on the project this passes through London clay, often described as near to ideal for tunnelling work. Both bores start from a pit shaft close to the main T5 site - also used to start the Heathrow Express tunnels - and then terminate 100 m short of the existing loop line.

At this point, near the central complex of terminals 1,2 and 3, an open excavation is needed for a cut and cover operation to link the tunnels into the loop.

'That will require closing the Piccadilly Line within the airport for some 20 months, ' says Mr Fugeman.

'A bus shuttle system and free use of the Heathrow Express between T3 and T4 will take the passengers.

'We - that is BAA and London Underground Limited - considered all sorts of methods to make the junction including the construction of a step plate junction underground, which could be done while the existing track remains live, as they did for instance to link in the Jubilee Line Extension.'

But while the sound London Clay can be used for the tunnels, the junction lies in an area where gravel layers above dip downwards slightly.

Mr Fugeman says the risk of hitting the less stable ground and from working with limited overall cover anyway, was too great.

'At the cost of disruption to passengers - and also to aircraft movements because we occupy the space of three busy aircraft parking stands - we have to use a cut and cover system, ' he says.

The rail junction will be built within a circular diaphragm wall cofferdam, demolishing the existing tunnel and rebuilding. Finally, short tunnel links will be driven outwards to the blind ends of the new tunnels to connect them.

'That work begins in January, ' says Mr Fugeman.'And it is crucial, of course, that it holds to the programme.'

Contractor for the junction will be the Morgan Vinci joint venture, which is also constructing the tunnels.

The current work bodes well for the schedule, having made good progress.The first bore, begun in November last year, has been completed and the tunnel machine disassembled and rebuilt in the start shaft for the second drive.

'We have been making about 50 m a day, ' says Ian Williamson, BAA's delivery manager for the tunnels, who works alongside the contractor, as favoured by BAA's integrated team approach for construction projects.

Tunnelling in the London Clay is straightforward with the Dosco roadheader, at the heart of the tunnel machine, slicing off the clay before an automatic segment erector positions the 1 m-wide concrete segment rings.The rings use a wedgeblock key for final tightening, a system particular to London Clay work.

Spoil is transported the length of the tunnel on an extendible conveyor from Continental Conveyors.The same firm also supplied a high-angle conveyor, which lifts the material to surface at the shaft. Further covered conveyors move it to loading points, where Moxy dumptrucks pick it up for backfill elsewhere on T5.

One of the benefits on a large integrated project such as this is saving costs such as spoil disposal.

Project interactions have also helped economies on the tunnels in equipment; the conveyors were already used for construction of the stormwater outfall tunnel and the TBM itself was first used at Heathrow a decade ago for construction of the baggage tunnel between T4 and the central terminal complex. It has been refurbished and modernised.

The tunnel itself is surprisingly empty of people, the result, explains Mr Williamson, of a deliberate effort to mechanise as much of the work as possible.The work team of miners and technicians is just a dozen, working a 12-hour shift five days a week. It has not been possible to eliminate movement altogether and a small Schoma locomotive is used to haul segments on rail to the TBM.This is loaded by a mechanical handler at the shaft bottom after a portal crane lifts the units down.They are made at a precast yard in Ridham in Kent, which has served most of the tunnels on the project.

When Construction News was on site, progress was much slower.This was because segments were being bolted as they were erected.'We are passing a shaft excavation from the surface for ventilation, ' explains Williamson.'And have to bolt for this short length for stability because there is no ground support to compress the rings.'

But just afterwards the machine was back to full speed and looking set for drive completion in October this year.They may not have the 'wow factor' of the above-ground architecture but the tunnels will form a central artery for the life-blood of Heathrow.