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Concrete poses a challenge to Colindale piling

Piling through London Clay ought to be a simple task but a new housing development in north London presented some real challenges for Bachy Soletanche

Demand for housing in London means that the skyline on Edgware Road in Colindale, north London, is changing and will soon feature a new 17-storey luxury apartment building. While work on taking the structure from the ground up has started on time, constructing the foundations for the new tower and other parts of the development was not straightforward.

“The Cemex mix was reaching its 28-day strength within four days.”

Rob Cunningham, Bachy Soletanche

Concrete that reaches its 28-day strength in just four may be great for pile sequencing but the lack of workability presented a real challenge for foundations contractor Bachy Soletanche. The mix design was not the only issue that had to be overcome – there were also more than 50 different reinforcing cage designs planned for the site.

Capitol Way is set to transform the skyline of Colindale

Capitol Way is set to transform the skyline of Colindale

The new development will also include several four storey residential blocks to the rear – with basement parking for 450 vehicles – with shops and restaurants on the ground floor. The new 18,000m2 development, which is being developed by a joint venture of Royal London Asset Management and Kitewood Development, replaces warehouses that have occupied the site since the 1970s.

“The Capitol Way site used to be occupied by a Wickes warehouse, but the area was once used for munitions manufacturing so there was a risk of UXO at the site,” says Bachy Soletanche contracts manager Rob Cunningham.

Bachy has recently completed the foundations for the second phase of Capitol Way. The first phase was completed this summer and involved the installation of 541 piles, while the second phase involves the installation of 468 piles and was carried out alongside main contractor Shepherd Construction’s building work on the £64M contract.

The ground conditions are near perfect London Clay, but the site investigation suggested there were elevated levels of sulphates and perched groundwater, which resulted in a challenging concrete mix design.

“High sulphate resistant concretes are not very workable,” explains Cunningham. “We used DC4 28/35 mixes from London Concrete and Cemex to meet the site demands but that only gave us one hour of workability – less in the heat of the summer – and we are normally used to working with four hours on CFA piles. The concrete from London Concrete contains ground granular blast furnace slag which actually slowed the curing but the Cemex mix was reaching its 28-day strength within four days.

“On the positive side, it has meant that we have been able to install piles within the same group on the same day.”

The CFA piles vary in diameter between 450mm, 600mm and 700mm and in length from 8.5m to 26m depending on the loadings. The piles are grouped in pairs, threes, fours or even fives to help meet the 2,000N loading expected from the building. “It is generally cheaper to construct small diameter long piles and group them to meet the demands of the structure,” says Cunningham.

Bachy used three rigs for the work – one for each diameter pile to help speed up the construction and keep the work on track.

The basement level parking added challenges for the installation as there were five different piling platform levels that have influenced the design and resulted in 51 different cage types. The cages were delivered to site preformed by NGR but the site team had to be careful to select the right ones for each pile location.

“The basements are being built from the ground up and this called for a pile projection of up to 5.5m,” says Cunningham.

A central reinforcing bar has also been installed to alleviate the effects of heave from the pressure relief resulting from the basement excavation work being undertaken by Shepherd.

The work also included construction of a contiguous piled wall around part of the eastern boundary of the site. The 40m long section was formed by 120 piles to a depth of 8.5m and will provide support to the neighbouring site when excavations to 3m are carried out to create the entrance to the underground car parking areas below the new development.

With the foundations element
of the project now safely delivered, the whole site has been handed
over to Shepherd which is tasked with constructing the 460 apartments ready for the marketing operation to start in 2014.

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