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Subterranean saga

T5 Tunnels

Least obvious of all the major works on the T5 project are the road and rail tunnels. Alan Sparks reports on progress below ground.

With a total price tag well in excess of £180M, the 13.5km of tunnels linking T5 to the outside world presented a whole range of challenges for the T5 team and lead contractor Morgan Vinci.

One tunnel dominates the rest, both in terms of size and the technical complexities involved.

The £140M Airside Road Tunnel (ART), just1.3km long but with twin bores of 8.1m internal diameter, was driven just 8m beneath a live taxiway.

The ART connects two defined portals; one at the eastern edge of the new T5 campus and one at Terminal 3 within the Heathrow central terminal area. It will provide an all-weather road link between the remote stands and the central terminal area.

Despite its massive bore, driving the ART should have been straightforward in London Clay below 6m of water bearing gravels and 2m of made ground.

Unfortunately, the site is crisscrossed by the existing Piccadilly Line and Heathrow Express tunnels.

Both the £44M Heathrow Extension tunnels and one of the £30M Piccadilly Line Extension tunnels are able to get a clear run beneath the existing Piccadilly Line.

But blocking the ideal route for the ART was the Heathrow Express, forcing the project designers to consider some, initially unattractive, options.

Tunnelling far enough below the Heathrow Express to be sure of not affecting its stability would have created unacceptably steep gradients at each end.

Finally, it was agreed that the ART would pass above the Heathrow Express, in a very tight thickness of clay beneath only 3m of the water bearing gravels. Either side of the Heathrow Express, the tunnel would burrow down through favoured deeper clays, forming a 'W' shaped path in elevation.

Finding the right kit to do the job was essential. 'Nobody had ever used a TBM to drive through London Clay which was able to switch from earth pressure balance face support to mechanical support with compressed air - until now, ' says BAA head of rails and tunnels Ian Fugeman.

'Developing a machine with this variability meant that we had engineered protection should we encounter the gravel and have to deal with groundwater at the face.'

Together with manufacturer Herrenknecht, the tunnelling team developed the innovative technology and undertook trials in Germany to prove the new technique. To provide the necessary pressure at the inclined face, a 750mm diameter piston with a capacity of 400t per hour was used; unique for this type of operation.

Forming the tunnel lining are 1.7m long and 350mm thick reinforced concrete segmental rings. Wedgeblock lining panels are used on the other T5 tunnels, but here bolted segments with waterproofing gaskets were used to provide added strength and cope with any water. The segments were tapered to enable a high level of control of the tunnel alignment.

A bentonite slurry injected around the TBM provided temporary ground support. Then cementitious grout was injected through the tailskin in the annulus behind the segment to provide immediate ground support. Minimal settlement and zero impact on the airport's operations proved that the strict ground movement controls paid dividends.

As with other parts of the project, the use of prefabricated reinforcement has cut handling and on-site working drastically for the ART portal construction. Post-drilled fixings in the linings were also avoided, thanks to prior planning.

Sleeved pile reinforcement enabled the complete pile head to be cracked off and removed as a single section, says Fugeman.

Again, this is an example of early design improving safety, eliminating the need for hand-held pneumatic tools.

Morgan Vinci's tunnels at T5 include the Airside Road Tunnel, Piccadilly Line Extensions, the Heathrow Express Extensions, the Storm Water Outfall Tunnel and the A3044 services tunnel. Mott MacDonald is consultant for the civils work, Laing O'Rourke has led the construction of the portals and Amec is charged with production design and mechanical and electrical work for the ART.

Managing so many teams with overlapping works has demanded high levels of scrutiny and excellent communication to ensure efficient teams are developed with no duplication of roles. The norm on tunnelling work is for mechanical and electrical works and cross passages to wait until main tunnelling work is complete. But at T5, integrated teams have allowed these operations to run in parallel, saving time on the tight programme.

Delays have threatened the tunnels programme, but as yet, the burrowing brigade has met its tight schedule. 'Bad weather meant the launch chambers for the Piccadilly Extension were at one stage 12 weeks behind programme, ' says ART production leader Selby Thacker.

'But by working together with the civils contractor, we were able to access part of the site on programme and prepare for the first launch. As a result, we were able to resolve the potential delay and start the drive on time.'

With an eye on the future, the location of piles for the main terminal and satellite buildings was selected with thought for the present and possible future tunnels. This involved use of 3D CAD technology to ensure that BAA's expansion aspirations for tomorrow are not curtailed by today's construction.

The underground rail station has six platforms; two for the Piccadilly Line Extension and two for the Heathrow Express Extension. The third pair is to provide capacity for potential future rail links to high-speed lines bursting out to the west or a CrossRail connection into London.

A host of rail, fire and safety organisations has to approve the plans; smoke control being the main safety design consideration, says Fugeman. 'The back of house spaces for forced ventilation are quite enormous and are required to re-vent smoke entering the passenger concourse and emergency evacuation routes in the event of a train fire, ' he adds.

Tunnel update Piccadilly Line Extension Two tunnels 1.7km long, each 4.5m internal diameter: One drive complete, one 70% complete.

Heathrow Express Extension Two tunnels 1.6km long, each 5.7m internal diameter. First drive planned to start end of February and the second in June. Shafts are built.

Airside Road Tunnel (ART) Two bores, both 8.1m diameter and 1.3km long. Completed in June 2003. Fit-out and cross passages ongoing.

Storm Water Outfall Tunnel (SWOT) Single bore 4.1km long and 2.9m diameter. Handed over in January 2004.

A3044 service tunnel Twin bore 140m long and 3m diameter pipe jack. Completed in August 2002.

Track Transit System Cut and cover 600m long.

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