“Normally, buildings up to 15 storeys high could be supported on cast in situ piles in the London Clay. The high tower blocks are too heavy for ordinary piled foundations and will have to be supported on cylindrical caissons with enlarged bases or on deep ‘floating’ rafts sunk into the London Clay, or on combined raft and caissan foundations.”
Chamberlin, Powell & Bon, Architects “Barbican Redevelopment” April 1959
The ground under the site is London clay. It was believed that London clay was incapable of supporting high-rise towers of the sort intended for the site. But AW Skempton proved in the 1950s that bored piles cast in situ would do the job.
Most of the buildings were constructed on “under-reamed bored piles” from 915 to 1220 mm in diameter. They were bored to a depth of anything up to 18 meters. A layer of ballast three meters deep was put on top. As each pile was bored clay samples were taken out and tested on site so that the geometry of the under-reamed bell could be adjusted to accommodate any local variations in ground conditions.
The columns visible at podium level were based on large diameter bored pile foundations. (The columns carry a load of nearly 1,000 tonnes.)