Completion of an area of base-ment at T5A alongside the very deep early bag store area was critical to the programme for assembling the building’s roof.

To enable simultaneous construction of both the basement and bag store areas, we installed a 9m tall contiguous pile in place of the battered earth slopes common across the rest of the Terminal 5 site.

Open cut excavation, in which the sides of the excavation are sloped to eliminate risk of collapse, was elected for the vast majority of construction because it is more economical – no deep piled retaining walls are required. Backfilling is carried out after the substructure has been built, allowing construction of upper levels. But open cut is space hungry and prohibits construction in parallel of adjacent deep level and shallow or surface level structures.

The piled wall we installed to allow simultaneous construction of the deep level baggage store and shallower basement had to withstand high long term pressures from swelling clay. Also, as the wall was to a shallower depth than the main building piles and consequently had different long term movement characteristics, it had to be kept separate from the main basement structure. We achieved this by making the wall act as a cantilever and providing a sliding joint at the wall/ base slab junction, allowing the wall to move vertically at a different rate to the building.

Basement construction was divided into three bays, which we tackled in sequence. We started excavation at the south end of T5A, with erection of the structural frame to ground level taking place while excavation of the second bay got under way. Roof construction commenced while excavation of the third bay was in progress.