When detailed design got under way in late 2001 the desire was to construct piles as open bores without any support to the pile walls prior to concreting, as this would be the most economic and programme efficient option. But the length of piles was limited by the local geology. At a depth of around 50m the London clay becomes granular and water-bearing. Water entering the bottom of a pile bore would make it susceptible to collapse. Open bore piles would therefore have to terminate above this level.

Prevented from installing long piles by ground conditions, it became necessary for us to utilise a combination of straight-shafted and underreamed piles (piles with an enlarged base) to support the heavier column loads from the building superstructure. Straight-shafted piles are formed by boring a hole with an auger – a tool like a vast drill bit. The pile is cast within this borehole by installing steel reinforcement and then pouring concrete.

Construction of an underreamed pile is essentially the same, but involves enlarging the base of the pile to increase its bearing surface. Straight-shafted piles for Terminal 5 would be up to 2.1m diameter, and underreams would be up to 6.2m diameter. Mott MacDonald anticipated that underreams would be needed on 450 of the main building’s 1,100 piles.

However, underreaming is time-consuming and carries higher safety risks than construction of conventional straight-shafted piles, as there is higher potential for collapse of the borehole sides. In order to understand to what extent underreaming really was necessary, our specialists devised unusual and rigorous tests to determine how piles would behave. We designed the tests to reveal the effects of swelling on the strength of the London clay and pile shaft capacity over time.