ART's success was TBM design

1 July 2003

Breakthrough was successful last month on the second bore of the US$218M Airside Road Tunnel (ART) project at Heathrow airport, UK.

The event marks the completion of tunnelling on ART, four weeks ahead of schedule. The 1.3km twin bore tunnel was excavated by a dual mode Herrenknecht TBM, capable of operating in EPB mode and compressed air mode. It is also the first completed tunnel on Heathrow's Terminal 5 (T5) project, which got underway earlier this year (see T&TI, May 2003, p6).

Morgan/Vinci JV constructed the Mott MacDonald-designed 8.1m i.d. tunnels, which will provide a private road access from the central terminal area to the remote airport stands to the west of the airport, just south of the new T5 complex. The first bore was completed in December 2002, after starting in June 2002 (see T&TI, July 2002, p28-31). The second bore started in February 2003.

The design of the TBM was crucial to the success of the project. The tunnel had to run at a shallow depth, because of gradient restrictions (5.3%) and the location of other tunnels, which had to be negotiated, including the Heathrow Express tunnel, and fuel tunnels. This meant that the machine would be within 0.5m of the Terrace Gravels at the portals. The rest of the drive was through London Clay.

The overriding concern for the client (BAA) was the issue of settlement. ART passes under the world's busiest airport, and the tunnel's construction could not affect the taxiways that pass above it. The Heathrow Express tunnel, which was crossed during construction, also had to remain operational throughout construction.

Therefore, a machine was designed that would be able to support the face with compressed air throughout construction, and quickly change to EPB mode if the machine faced free-flowing ground.

Other features of the TBM include: the ability of the cutterhead to move forward 750mm to provide face support and excavate ahead of the skin with copy cutters if required; and the annulus around the TBM was filled with bentonite to provide ground support – retained by a cutting head bead.

The design was successful. Seventy-five points were measured along the centre of the tunnel, all with trigger limits of +20mm to -20mm. However, the actual settlement fell within the range of +7 to -18mm.

The preferred method of operation was compressed air, so that the spoil would remain uncontaminated and be safely used elsewhere on the T5 site, rather than be exposed of off-site. However, during both drives, only a small amount of foam was added to the face to aid lubrication.