WHILE tunnels drive forward, work is also advancing on the T5 station, which will service platforms for the Piccadilly Line, the Heathrow Express and a potential future Crossrail or Airtrack line.
'There are six platforms in all, ' explains Keith Heard, T5 station delivery manager for BAA.'And they will occupy a very spacious area some 250 m long and 90 m wide.'
That is the size of Paddington, he adds, but with the additional fact of being entirely underground.The station sits beneath the main T5 building and the associated car park/arrivals deck in front of it, though at right angles to the building above.
Platforms will each be separated by internal walls and will also have platform door screens for safety; the trains will also be separated by internal partitions between tracks creating 'tunnel boxes' for each.
To counter the closing-in effect of the separators they will be heavily glazed, says Heard, to allow a line of sight through the station and give an early awe-inspiring effect on arriving passengers before they are confronted with the impressive architecture that BAA has designed into the main terminal above.
To keep the 'wow factor' going most of the passenger movement will be in glass-sided lifts up through the central canyon between the main T5 building and the car park, to the top (departures) deck.There will be five lifts per platform, each taking 55 people.There will also be some escalators through the main building.
Mr Heard says construction of the 14 m-deep excavation is relatively straightforward.The station sits within a concrete diaphragm wall box where it is built from a mainly concrete and steel frame structure.
Complications are primarily logistical, since contractor Laing O'Rourke must fit its work around the jobs going on above in the main building '. . . and we are always last in the chain, ' adds Mr Heard. Steel structures in the station are built by Rowen Steel.
The design team of architect HOK and engineer Mott MacDonald has gone for largely precast or prefabricated elements with an architectural standard finish already on them before they leave the factory, which calls for careful handling on site.
'Everyone has put in a huge effort to achieve these high finishes, ' he says.'And it took a lot of effort to determine what could be done and what we wanted.'
Full-scale mock-ups of the steel columns were carried out in Bolton for example.
One final element will be an unusual roof structure of inflatable ETFE panels, which are lightweight and translucent.
These were used to clad the domes of Cornwall's Eden Project but this will be the first time they are used in the construction of a station.