FIRST OF 1,500 concrete rings was this week put in place for one of two 1.3km bores for a new airside road tunnel at London's Heathrow airport.
Tunnel boring will start later this month using a 90m tunnel boring machine (TBM) and is expected to take a year.
The new tunnel will give vehicles access to remote aircraft stands from the airport's central terminal area.
Close monitoring for heave and settlement will be required as the tunnel passes under a key taxiway and across the tunnel for the Heathrow Express rail link. Average settlement along the whole tunnel is expected to be between 30mm and 40mm.
The tunnel is being built to help operator BAA use aircraft stands at times when terminal docking points are overloaded.
The tunnel comprises two parallel bores, each housing a 6m wide single traffic carriageway.
The two tunnels will be connected with 3m diameter cross passages every 130m.
The first bore will start from the air side western portal where the TBM has been assembled. It will drive 1.3km to a reception chamber being constructed as part of the eastern portal in the central terminal area.
The TBM will then be removed and transferred back to the western portal to start the second bore.
The German made Herrenknecht TBM will operate under compressed air through the London clay, but can be switched to an earth pressure balance mode when passing through gravels to minimise settlement.
BAA is carrying out the project using framework contracts with an alliance comprising Morgan Vinci JV, Laing Civil Engineering, Amec and Mott MacDonald.
Tunnelling will account for £59M of the £130M contract, the rest being the civils work required for the portals and cut and cover operations to connect each portal. The tunnel will open in early 2005.