Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Heathrow's hidden gems

Terminal 5 tunnels - Trains will start running through new tunnel extensions to Heathrow's Terminal 5 in a couple of months. Andrew Mylius looks at their construction.

Heathrow's gleaming Terminal 5 so dominates the airport's landscape that it is easy to neglect the major construction that has taken place underground.

'Concorde used to be British Airways' icon. T5 will be BA's icon for the 21st century, ' says British Airports Authority head of rail and tunnels Ian Fugeman.

But Fugeman is quick to point out that beneath T5, the extension of London Underground's Piccadilly Line and BAA's Heathrow Express have been impressive feats of engineering.

Some £650M of tunnelling has taken place as part of the total £4.3bn T5 construction works, namely the large diameter bored airside road and storm water outfall tunnels, the cut and cover tracked transit system connecting the main terminal with its satellites T5B and C, and the two rail tunnels. The road and drainage tunnels were completed in 2003.

With civils work complete, track laid, and power, signalling and communications systems in place, the rail tunnels are due to be commissioned in two months time. The first passengers are scheduled to arrive by train at T5 in exactly a year's time.

The risks associated with tunnelling have been reduced, beyond site investigation and design, by handing construction for all of the tunnels to the same firms. Mott MacDonald has carried out design, supervision and interpretation of data from settlement monitors; a joint venture of Morgan Est and Vinci Grand Projects was tunnelling contractor; Balfour Beatty Rail Projects installed rail and traction power in the two rail tunnel extensions; and Balfour Kilpatrick carried out mechanical and electrical fit-out.

Signalling is the only activity carried out by different contractors - a reection of the different technologies used on the two systems and the critical sensitivity of signalling to their operation. London Underground PPP contractor Tube Lines has carried out signalling on the Piccadilly Line extension and Westinghouse has installed signalling on the Heathrow Express extension.

As with all other T5 construction work, tunnelling risk has been insured by BAA, on condition that contractors open their account books to scrutiny and, if problems do occur, that they muck in to resolve them quickly.

While the tunnelling approach to the two rail extensions was essentially the same, there were important differences. When the Heathrow Express was built the construction of T5 was already on the horizon, and stub tunnels were constructed, enabling tunnel boring machines (TBMs) to drive straight off without disrupting services to Heathrow Terminals 1, 2, 3 and 4.

To construct the new twin Piccadilly Line bores, however, Morgan Vinci rst had to construct a box from which to launch its TBMs.

To connect the new tunnels with the existing Piccadilly Line at Terminals 1, 2 and 3 a new junction box had to be constructed. This has a larger cross-section than the adjoining tunnels, extending close to water-bearing gravels overlying the London Clay through which the tunnels themselves were driven. Top down construction was selected over the alternative of driving bored tunnels all the way and then enlarging them to make space for the connection.

This was to eliminate the risk of hitting the water-bearing gravels and enabled Morgan Vinci to use an open-face TBM rather than a more expensive earth pressure balance machine, which would have been essential to deal with ground water.

Bachy Soletanche installed diaphragm walls for the junction box through the existing Piccadilly Line tunnels at the start of a 20-month line possession, allowing the box to be excavated. The bored tunnels stopped just shy of the box - the TBMs retreated the way they had come so that the drives would not risk destabilising the existing tunnels. Short lengths of hand dug, sprayed concrete-lined tunnel were created to make the connection.

A form of New Austrian Tunnelling Method (NATM) developed by Mott MacDonald and Morgan Vinci, known as Laser Shell, was used for the approximately 500m of non-bored tunnel on the two rail extension projects - cross passages and connections.

'It's a single pass excavation method, ' says Morgan Vinci project director Selby Thacker.

'The tunnel face is inclined to reduce the risk of face collapse.

There's no lattice reinforcement system to install, which makes the method faster than conventional NATM, and we apply the lining as a single layer of sprayed concrete. It gets the full structure in place very rapidly.' Laser sighting is used to monitor and control tunnel geometry during excavation and lining.

Both lines terminate in a shared cut and cover station box, nestling under the main T5 building. This is equipped with six platforms in all, in anticipation of high-speed rail links from west of Heathrow and/or the arrival of Crossrail.

Rail for both railways is a track slab system: rail and sleepers were made up into 100m lengths.

Piccadilly PPP Contractually, construction of the Piccadilly Line extension has been unusual. 'BAA is building the extension for London Underground, which will own it. We're effectively a PPP contractor for the project. We're funding the project as well as building it and will be paid through a share of farebox takings for journeys to and from Heathrow, ' says British Airports Authority head of rail and tunnels Ian Fugeman.

London Underground set technical and performance briefs for the project. It demanded a capacity of 19 trains per hour and that the tunnel diameter be larger than normal on the Tube to allow for easier emergency access and egress alongside trains.

London Underground service director for the Jubilee, Northern and Piccadilly Lines George McInulty notes that 'the history of rail projects is one of delivery problems. We have had a policy of intruding [over the course of the project] to make sure things go properly. Paradoxically, the fact we have done so means that things are going well, and that means we actually have to interfere very little.' One of the key technical challenges was meshing the new line's new signalling with the 1960s' system currently in operation. 'It shares the same technology, but it's the latest version of that technology, with associated improvements in performance and robustness. And it looks and feels the same in the control room so there's no new operator training needed.' London Underground's PPP contractor Tube Lines installed the signalling under contract from London Underground. 'It was odd to be paying for work over which we had no control, ' Fugeman says.

London Underground will carry out testing and commissioning of the tunnel. Signalling, traction power, communications systems, forced ventilation and smoke extraction systems have to be put through their paces before trains start running on 14 May, enabling the operator to start training its 600 drivers.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.