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Once more with feeling

FOUNDATIONS: Extensive probing was needed to find buried foundations during piling for redevelopment of a power station site in Portishead. Dan Simpson reports.

Portishead in north Somerset was once home to the west of England's largest power station. Opened in the 1920s, the coal-fired station closed in 1980 and in recent years Portishead has expanded to become an important commuter town for the nearby city of Bristol.

After extensive development elsewhere in the town, it is now the turn of the power station site and dock area. The 17ha Port Marine scheme includes 900 homes plus retail, leisure and entertainment properties on a site overlooking the mouth of the River Severn estuary and a new 500berth marina.

As with any brownfield site, its history has thrown up a number of engineering problems.

Even after remediation, the problem of the power station's foundations had to be overcome.

This particular industrial legacy became apparent during a earlier phase of the scheme in July 2001, when foundation contractor Aarsleff Piling hit obstructions while driving precast piles. Many of the piles were broken and some positions had to be aborted.

'The previous phase piled by Aarsleff was probed, but on a grid pattern. We were unlucky because the probing missed nearly all the obstructions, ' says Crest Nicholson commercial manager Julian Howarth. Crest Nicholson Residential (SW) is client and developer of the Port Marine project with Nicholson Jones Partnership acting as consultant.

Howarth explains that to minimise these problems on an adjacent phase of the project which began at the end of last year, Aarsleff and Crest Nicholson decided the best solution would be to probe every pile position.

This meant any shallow obstructions could be located and removed or any changes in position of the piles could be made before the piling started, cutting costs and reducing delays.

Site investigation, carried out by Integralé Geotechnique, revealed that most of the site was underlain by up to 8m of fill followed by stiff clay, although next to the harbour wall fill was up to 14m thick. The power station foundations were installed through the fill and on to the clay and those encountered during piling were mainly concrete slabs.

Probing for this phase, which started in early December 2001, was carried out using a 278mm diameter tube driven to a depth of 8m using a Banut 700 machine.

'The Banut rig was chosen as it has more power to pull the mandrill probe out of the ground, ' explains Aarsleff piling manager Pat Deighan. 'We achieved a maximum of 114 probes in a 10-hour day.'

Probing was finished before Christmas, and the first few piles were installed before the holiday period.'The probing went so well we were able to pull back the piling by two weeks, ' says Deighan.

With all the pile locations cleared, Aarsleff finished installing its 250mm precast Centrum piles at the end of January. The piles were driven to between 10m and 17m and designed using dynamic formulae during driving.

Piling continued while the obstructions were being excavated or the piles were redesigned if they needed to be relocated. Joints had to be incorporated in the piles next to the dock wall because of the increased thickness of the fill.

The piles, installed singly or in groups of up to four, support the column loads of seven- and eightstorey steel framed apartments. 'Up to 300m of piles were installed a day, ' says Deighan.More than 530 piles were placed on Aarsleff 's two most recent contracts with each pile cap carrying an average load of about 600kN.

Development is being carried out in a series of phases and is expected to be complete by 2006.

Aarsleff has now installed the piles for four phases of the housing development and is a strong candidate to win a further phase, known as phase 15.

This part of the scheme straddles an outfall pipeline from the old power station and part of the development is over a deep, 7m thick basement slab, also part of the old facility. It is planned to install rotary percussive minipiles to key into the basement slab, transferring the load from the structure above.

Deighan says phase 15 will probably be probed only in areas where high SPTs were recorded in the site investigation, which should save the client unnecessary costs.

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