The structure of the Barbican estate basically consists of concrete buildings, on top of massive columns, standing on bored pile foundations. The buildings are constructed of reinforced concrete for the most part, but there are some areas where pre-stressed concrete was needed.

The bored foundations consist of “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.

These foundations were sunk into 18 meters of London clay, and then topped with 3 meters of ballast. The foundations have to support the columns visible at podium level.  As columns are often widely spaced, the piles have to support loads up to 1800 tons. 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.

Calculations on whether the ground would support buildings of the size proposed were based on research carried out by Professor A W Scampton, and recorded in a paper written by him and D J Henkel. This was also based on the geological surveys and well boring logs for the area, also provided by Professor Skempton. Boring logs were also obtained from the research carried out for the construction of the new route 11 (London Wall). Professor Skempton’s work demonstrated that bored piles cast in situ would do the job.

This was how the subsoil under the Barbican is made up.

• Gravel to a depth of between 20 and 30 feet below road level.
• London clay to a thickness of 45 feet to 90 feet. The top 2 feet were usually decomposed, soft and compatible clay.
• Below the London clay, ‘the Woolwich and Reading beds’, of varying sands and clays with a total thickness from 40 feet to 60 feet.
• Below them, ‘the Thanet Sands’, about 25 feet in thickness.
• Below them, chalk at a depth of about 150 feet.

All these beds showed a tendency to dip towards the Thames in a north-south direction.

Ove Arup, structural engineers, concluded that conditions were normal for the London area, that the foundation problems were not unusual for large buildings on London clay, and that the foundations would be entirely in the London clay.

Ove Arup carried out site borings and soil tests for the detailed design of the foundations of each of the major building in the development.

The foundations for buildings up to 15 storeys high were built on piles dug into the London clay and cast in situ. This would be done by creating a shaft and a liner.There were two methods for creating the pile. One method was to refill the shaft with concrete as the liner was withdrawn. The other method was to ram the liner and subsequently the concrete so that a bulb was pressed out at the base of the pile for greater protection. (Special arrangements had to be made for buildings partly sited on the new railway tunnel.)

Where buildings are sited over existing sewers, pile groups were arranged on both sides of the sewer at a safe distance from it and the piles taken so far down that no load was transmitted to the earth immediately surrounding the sewer. Between the pile caps, heavy reinforced concrete beams carry any load from above so that no pressure is exerted on the sewers.

The tower blocks produce loads which are greater than conventional piles can carry. The towers have three storeys below podium level. The basements do not go down deeply enough to be used as a raft foundation in themselves. So a large number of concrete cylinders with enlarged bases were proposed by Ove Arup, with the basement walls resting directly on lines of cylinders with deep beams between them. They were created using ‘cylinder caissons’. These are cylinders which are dug in first and which can be kept dry so that concrete can then be poured without fear of it getting wet and losing its potential strength before it can dry,

They recommended that for Lauderdale Tower and Shakespeare Tower there was sufficient thickness of clay for these cylinders to provide adequate carrying capacity. There was some concern about Cromwell Tower because the underground level known as ‘the Woolwich and Reading Beds’ reduced the clay thickness to 45 feet there, which was much less than in the other parts of the estate. Special measures were taken to prevent water entering the foundations before the concrete could be poured and set, including widening the basement structure and deepening it so that it could act as a raft.

St Paul’s Cathedral may seem a long way away. But there was concern that the Barbican building’s foundations might affect the water table of St Paul’s Cathedral by interfering with underground flows of water. Investigations found no record of any streams underneath the site and therefore no reason to be concerned that the new development might create a different situation than existed before the works. However, it seemed that there might be a risk that water extracted for construction might affect the water table under the cathedral. But Ove Arup reported that, even if this happened, it would tend to increase the strength of support under the cathedral. They said that if it was felt desirable to maintain the groundwater level under St Paul’s, the cost was likely to be so small “that it would not be a crippling expense to replenish the depression or saucer under the cathedral”.