Pile size and depth are fundamental to its behaviour. A pile
works in two ways – through adhesion to the ground around it, in
which the greater the surface area of the pile the more friction
there is with the surrounding soil; and through end-bearing, in
which load is transmitted through the pile shaft to the ground
directly beneath it. Pile design presents a quandary – the bigger
the pile, the greater the load it can carry but the more
susceptible it is to swelling.
We had to look carefully at the size and corresponding capacity of
piles to make sure they could take the loads that would be imposed,
but ensure
that they would not be susceptible to excessive movement from
swelling.
A balance between opposite forces had to be considered and
struck.
It was crucial that excessive ground movement was not translated to
the terminal building superstructure. Terminal 5 has a grid of
columns at 18m centres supporting concourse floors. The columns
rise from a base slab founded on principal and intermediate piles
on an 18m by 9m grid. Differential movement between piles would be
translated to the columns;
as one moved relative to its neighbours, stress would be introduced
to
the beams supporting the terminal building’s floors. Maximum
allowable movement over the length of beams was 1:500, or 36mm over
18m, meaning that displacement of piles had to be rigorously
controlled.
