Cased secant piling is proving to be an effective solution for a three level basement at Berkely Homes Ebury Square development. NCE reports
What was once exceptional eventually becomes the norm - and so it seems that pretty much every substantial new development in London today will include a deep basement excavation. Usually it is some kind of variation on a secant piled perimeter retaining wall, with plunge columns installed in the load bearing piles to enable top-down construction.
As any technique matures, so the construction priorities shift from proving the design assumptions and the overall buildability to more commercial considerations around improving efficiency and competitiveness. In this respect Cementation Skanska’s foundations for Berkeley Homes’ residential project in Ebury Square, Pimlico, central London, is a significant breakthrough in piled retaining wall construction.
The majority of piles have been formed using a cased secant piling (CSP) technique. This is essentially a cased continuous flight auger (CFA) process and offers the high productivity of CFA, but removes the associated risk of over-flighting, and achieves the tight tolerances required for secant piled walls.
While CSP is reasonably well established in the UK market - Cementation introduced the technique in 1998 - at Ebury Square Cementation Skanska is using Soilmec’s latest version of the technique.
Critically, Soilmec’s system can go slightly deeper than rival manufacturers’ equipment with casings potentially extending to 20m and the auger to 26.5m, just making a three-level basement within its capabilities.
In use, the CSP attachment uses two independently controlled, counter-rotating rotary heads. The lower head drives the casing, fixed to a cutting shoe which cuts concrete away from female piles, and at the same time the upper head drives the auger. After the hole is driven, concrete is pumped into the hollow auger, while the casing is withdrawn.
The Ebury Square wall comprises 358 alternating male and female piles. These are 750mm in diameter and the females vary in depth between 11.5 and 13.5m. The interlocking male piles, which determine the wall’s structural performance, extend to 45m, although the majority, some 134 in total, are 26m in depth.
At Ebury Square, the steel-reinforced male piles use C35 concrete while the female piles have a target strength of 10-15N/mm2. As Cementation technical manager Chris Robinson points out, this makes it a hard/firm wall. The often-described “hard/ soft” specification for secant walls, Robinson explains, has been made obsolete as piling plant has become more powerful and can now comfortably cut through higher strength female piles, and these have become the norm.
The 45 longer piles, which are required where loads are higher, are rotary bored, but all the interest is with the 26m long piles. This is right at the limit of Soilmec’s current CSP technology, and the project is the first in which a three level basement in London has been constructed using the technique.
Although Cementation has had to go through a learning curve with the new process, project manager Gareth Hales says that daily production of up to 10 CSP piles from each rig is achievable.
Access restrictions mean the site is effectively divided into two areas, with the basement extending only below the larger of the two areas. For the smaller area Cementation has installed conventional rotary bored bearing piles using a Soilmec R132 rig.
The three level basement is restricted to the main part of the site, and bearing piles here have been constructed at three diameters, with six piles at 750mm, 37 at 1,200mm, and 19 at 1,500mm. All but the six smallest include embedded plunge columns.
Soilmec’s system can go slightly deeper than rivals’ equipment with casings potentially extending to 20m
The subsequent semi-top-down basement excavation will be to 10.5m below the piling platform level, extending to 12m locally. With the need to set the plunge columns relatively deep, Cementation Skanska’s Bentley Works team developed a novel socket to enable it to disconnect the up to 7.1m long followers from the permanent plunge columns.
Cementation Skanska started on site in May, which meant that for the first two months of the project it was operating through the wettest summer on record. This, says Hales, added to the usual logistics challenges of running a central London project, such as tight space, and planning the supply and removal of materials.
It was always expected that local residents would be sensitive to disruption and noise, and keeping the roads around the site clean was given a high priority.
And beyond this, the near continuous deluge meant extra effort was needed to ensure site conditions were safe under foot and that staff moral remained high.
Another interesting aspect of the project is that the design is being carried out to Eurocode 7, and Robinson says the approach necessitated close collaboration and interaction with consultant Waterman Structures, but intriguingly resulted in shorter piles than the conventional design to British Standards.