For basement construction swelling ground can be
dealt with in two ways. A massive slab can be cast directly on the
ground and anchored with very long piles to resist uplift. Or the
base slab can be formed above the ground, propped on piles, leaving
a void beneath into which the ground is allowed to swell.
Principal disadvantages of the first option are that it is a heavy
solution, with larger material requirements and correspondingly
higher construction costs. The second option is far more economical
and BAA selected it.
But in so doing, BAA also selected the more complex design option.
Foundations need to support the base slab and terminal
superstructure, but also to resist swelling, which wants to push
the piles out of the ground.
: there are many basement levels resulting in excavation to
different depths across the building’s footprint. And loads imposed
by the superstructure differ, depending on where baggage-handling
plant, check-in desks, retail or waiting lounges are located, for
example. This means that the amount of force exerted on piles
differs, introducing significant variability to the size of piles
needed, and the behaviour of those piles in contact with swelling
clay.
