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Geotechnical: Streatham Hub

With leisure facilities, supermarket, apartments and transport interchange, the Streatham Hub is certainly cramming a lot into one area and is calling on some complex geotechnics. Claire Symes reports.

The time it takes to complete the piling work for a new development is usually a good indication of the complexity of the scheme or the scale of the site.

For the Streatham Hub in south London it is a combination of the two factors that means it will take over six months to complete the ground engineering called for by the new mixed used development.

Challenging urban development

Drilling rigs from Bachy Soletanche are currently just over half way through work to install more than 1,000 piles on this challenging urban redevelopment for main contractor Vinci. While the underlying London Clay may present an

ideal situation in terms of foundation construction, the complex loadings imposed by the planned development called for a lot of focus when it came to the pile designs.

When completed the Streatham Hub will boast a leisure centre - complete with an ice rink - a 5,800m2 Tesco Extra supermarket, underground car parking, residential development and a bus interchange.

Redevelopment of the site is being funded by Tesco’s urban regeneration business Spenhill. Construction of the new leisure and transport facilities was part of Tesco’s planning agreement with the London Borough of Lambeth.

“It is the loading of the buildings that presents the main challenge to the pile design”

Vijay Pookat, Bachy Soletanche

“The building at the southern end was used as a bus garage from the 1980s but was later converted to a private garage and also used as a go-karting centre with a Transport for London (TfL) hub fronting onto the main road.

“In the centre of the site is a church, which is home to a nursery school, which will remain on the site and in use during the redevelopment, although an extension to the building has been demolished,” says Vinci project director Steve Diggines.

“At the northern end of the site, close to the station, was an ice rink and swimming pool dating back to the 1920s.

Although the swimming pool has been closed for eight years, the ice rink was still in regular use and a temporary one has been installed in Brixton until the redevelopment is complete.”

The redevelopment will, from south to north, include a new five storey leisure centre with swimming pool and ice rink; a link building running along the western boundary of the site behind the church, with access to the basement car parks provided by a ramp along the northern boundary of the church that uses the natural east to west slope of the site. Beyond the ramp will be the Tesco store itself with 250 residential properties over the top, plus a new TfL hub next to the station.

CHP plant

To the rear of the TfL hub and Tesco service yard, a combined heat and power plant will provide heating for the whole development.

Vinci has drawn on the strengths of its subsidiary companies to value engineer this scheme. “The Tesco store was originally planned to be built on a Cobiax slab but we convinced Tesco to change to a post tensioned design to save on headroom in the car park and reduce spoil from the basement excavation,” explains Diggines.

Freyssinet is going to carry out this element of the work for the Tesco store, as well as the leisure centre and bus depot. Another subsidiary Sol Data is carrying out noise and vibration monitoring, Eurovia is carrying out the road improvements and Bachy Soletanche is delivering the geotechnical solutions.

Diggines believes that these skills, along with the company’s experience of working close to railway lines, was a key reason for Vinci securing the contract.

Consultants for the scheme have been novated to Vinci, with Peter Brett Associates undertaking design work for the Tesco end of the development and Halcrow Yolles responsible for the leisure centre.

Two way split

Vinci has split the scheme into two areas - the leisure centre and link building forming the first and the supermarket and TfL hub the other - a project manager for each reports into Diggines. But for Bachy ­Soletanche project manager ­Vijay Pookat, the project is divided in terms of piling technique - continuous flight auger or rotary bored.

“The foundations work is split into two parts with CFA bearing piles being used for the TfL hub, CHP and Tesco service area, while rotary piles are being used for the Tesco store and leisure centre,” says Pookat. “A ­contiguous piled wall is also being installed around the southern boundary of the leisure centre and adjacent to the church to enable the basement levels to be excavated safely and without impacting on the church and other buildings on the south eastern corner of the site.”

In total there are 1,050 piles to be installed around the site.

“The geological conditions are as good as they get in London - brown weather London Clay from 2m to 9m below ground and the blue London Clay beyond,” says Pookat. “It is the loading of the buildings that presents the main challenge to the pile design.”

Piling work for the leisure centre started with the contiguous piled wall formed from 700mm and 900mm diameter piles installed to depths of between 14m and 17m. There are two sections of contiguous piled wall - the larger is an L-shaped section 22m long, formed from 24 piles, behind buildings remaining on the A23 frontage. Eighty six piles have been installed 80m along the edge of Natal Road to the south of the site. “The other section is a 12m long wall to protect the church which has been tied back using four piles placed behind the wall and a capping beam,” says Pookat.

The leisure centre building will be supported by 750mm ­diameter piles bored to 44m below ground level with casings used to 6m. The piles are being installed on a 6m by 8m grid ­pattern.

Tight tolerances

“Halcrow wanted a very tight tolerance on settlement because of the ice rink, which will be located on one of the upper floors, with the swimming pool in the basement,” says Pookat. “The tolerance was originally specified as 2mm, but with the shrinkage in the concrete likely to exceed that it was impossible to design for, so we are now working towards 10mm.”

Piling work on the contiguous piled wall was completed in March and the rest of the piling work at this end is expected to be finished in late April. Once complete, the ground works contractor will construct the pile caps and load transfer beams to support the complex basement arrangement, with column loads reaching up to 6,000kN.

The Tesco store will be supported on 750mm diameter rotary piles bored to 44m with some pile loads expected to reach 4,3000kN due to the wide spans required within the store. “Column spacings are 16m by 7.5m and the average pile load will be 3,650kN,” says Pookat.

Just over 50 of the 247 piles required in this area have been installed so far.

Piling work on the leisure centre and supermarket is scheduled to be completed by the end of June but, if work continues at its present rate, Pookat expects to finish four weeks early. Designs for the link have still to be finished, but work is expected to start in early August.

Work on the whole scheme is scheduled to be finished in November 2013 but the planning conditions mean that the Tesco store cannot open until all the other developments are complete.

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