Breakthrough was achieved last month on the first bore of Heathrow’s US$88.5M Airside Road Tunnel (ART) project. The 9.2m diameter Herrenknecht dual-mode TBM will now be transported back above ground to the western portal where it will begin the second tube this month.
Capable of operating in both EPB and compressed air mode, the TBM started boring the 1.3km long twin-bore tunnel on 18 June 2002, placing 1.7m long x 350mm wide rings as it went. Each ring is made up of seven segments and a key, resulting in an 8.1m i.d. Once complete, 12,000 segments will have been placed along the twin-tunnel route (July 2002, p28).
A dual mode TBM was selected because of possible mixed ground conditions. Although the tunnels run through London Clay, requiring compressed air support, the crown comes close to a layer of Terrace Gravels. The alternative EPB mode will control the ground and minimise any settlement.
Analysis of surface settlement on the airport infrastructure has been based on achieving 1% face loss – about 30mm-40mm surface settlement – as opposed to 1.2% face loss on the Heathrow Express with an open-faced shield.
The project also consists of 120m of cut and cover tunnels and eight cross passages linking the twin-bored tunnels.
The Morgan/Vinci JV is carrying out all tunnelling construction. Mott MacDonald is undertaking all civils, tunnels and M&E definition design, and AMEC is carrying out the M&E detailed design and installation. Laing Civil Engineers is responsible for the heavy civil work at each portal.
The tunnels will link Heathrow Airport’s Central Terminal Area with aircraft stands located on the western edge of the airport, and should be open to passengers in February 2005.
