The main Terminal 5 building has a footprint equivalent to 60 football pitches, measuring 396m by 176m. And at 22m, its basement would be deep enough to swallow an eight storey building.

Excavating a hole of this size in London clay was unprecedented. Basements of similar depth had been created for high rise buildings, but their footprints were a fraction of the size. The scale of the Terminal 5 excavation posed a significant challenge.

London clay is described as an over-consolidated soil. Aeons ago the Thames valley was thickly covered in deposits that compressed the underlying clay. Over time these deposits have been eroded in the valley but London clay is still highly consolidated. When it is relieved of load, for example by excavation or tunnelling, it loses equilibrium and starts to swell – it effectively wants to fill the hole.

For the scale of excavations at Terminal 5 in the region of 50mm of swelling is instantaneous. But London clay can be expected to continue swelling for several decades. At Terminal 5 it was anticipated there would be up to 250mm of movement over the coming 50 or so years. Read about how we countered the swelling clay.

Savings

Our testing and detailed analyses enabled the number of underreamed piles to be literally decimated. In all only 43 underreamed piles were eventually needed for Terminal 5A.

Overall, a huge 30% reduction in the volume of concrete was achieved, and major savings in construction time, significantly helping the Terminal 5 project’s bottom line. By reducing the scale for underreaming, project safety was also improved.

We applied the lessons learnt from the design of Terminal 5A to foundation design for Terminal 5B and C, involving installation of 326 piles apiece.