The £30M Piccadilly Line Extension will transfer trains from Terminals 1,2 and 3 to T5. This will branch off from the loop that takes trains from Terminal 4 on to Terminal 1, 2 and 3 stations and then to Hatton Cross and through to London. This loop will be closed during the works, meaning there will be no Piccadilly Line access to Terminal 4 for 20 months from January 2005.
Building this connection amid the busiest airport in the world will demand complex construction.
Because the cross section of the tunnel will be larger around the connection of two tunnels, it will extend into the water bearing gravels.
'And as it is outside the main site and its bentonite cut-off wall, we will have to build a contiguous piled cofferdam to keep the work in the dry, ' explains Fugeman.
The alternative to top-down construction would be to continue the tunnel drive and enlarge the connection opening between the two tunnels from below ground.
Ten years ago, this was the initial concept based on LUL's construction methods for the Jubilee Line Extension project.
However, subsequent ground investigation suggested otherwise.
'Because we are now closer to the water-bearing gravel layer than originally assumed, this method was proved to be not viable, ' reports Fugeman. 'By digging down into a secure watertight box we have designed out this risk.'
Bachy Soletanche is undertaking the diaphragm walling works next year. 'This presents a real challenge for us, forming diaphragm wall panels around and down onto existing tunnels, ' says Bachy project manager Rob Jackson.
Joining one tunnel from either side to an existing twin-track tunnel is far from straightforward. A pair of Y-shaped junctions will provide the track connection. Driving into the outside of a tunnel would threaten significant structural failure of the existing tunnel and disturb the ground stability.
'So TBMs will drive shy of the cofferdam box that will envelope the whole junction and retreat back from where they came. To connect to the new bores, four short lengths of new tunnel will reach out from the box, ' explains BAA tunnels delivery manager Ian Williams.
Once complete, the 25m deep, 25m by 40m box will be double capped by a concrete slab. This will remain a box and will not be backfilled. Trains will run through the covered box at the step-plate junction.
'Where the Heathrow Express extension connects to the existing system, the complexity of this £20M junction box has been avoided. When the Heathrow Express was built, a head shunt and stub tunnel were constructed to facilitate the junctioning of the new tunnels, ' says Thacker.