Running the length of the terminal building two levels below ground is a 1.7km concrete box girder. 36m wide and 8.5m deep, it contains six tunnels. At the centre are two 9m wide corridors for the airport's rapid internal transit system, the advanced people mover; two 3m service tunnels flank these; and on the outside come two 9m tunnels dedicated to high speed baggage transfer.
APM stations are sited at the terminal's main concourse, at its Y junction, and at the control tower. Expansion will take the APM to a new terminal building to the west of the control tower, beyond Chek Lap Kok's granite platform into reclaimed ground.
Embankments were heaped up above the tunnel to pre-compress the ground and limit subsidence prior to construction. However, the 'beam', with its 1.2m floor slab and 1.8m ceiling, is designed to flex. It also contains six closely spaced joints to accommodate rotational movement as the landfill settles.
The joins are sealed in the same way as a submerged tube. The tunnel is designed as a water retaining structure, with cracking in the concrete calculated to less that 0.2mm. The concrete itself carries out 'autogenous healing' - cracks will self repair if invaded by water.