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A sticky problem solved

CFA piles Piling for a Leeds project has been less than straightforward.

Among the challenges facing piling contractor May Gurney at East Street, Leeds, is mudstone. The material is hard enough to create major friction as continuous flight auger (CFA) piles are bored. Heat generated bakes the mudstone to the auger flights, obliging workers to break it off with jack hammers.

The contractor is installing foundations for main contractor Shepherd Construction and developer Gateway on what will be a £135M mixed-use scheme, consisting of 640 apartments, a 215-bed hotel, office, commercial and retail space. There is a basement car park for 500 cars.

Buildings are up to 13 storeys high, applying high compression, tension and horizontal loads.

Because the water table is only 4m below surface, with rock of variable strength and depth underlying wet, weak and gravely clay, CFA piles were selected for the foundations. These are being combined with rock sockets installed in the mudstone. Driven piles were ruled out as too noisy, says May Gurney northern area manager Nick Sharp.

Shepherd onstruction installed sheet piling around the perimeter of the site, enabling excavation of the basement areas before May Gurney started work. The firm completed the £642,000 first phase of work in February last year and is now getting under way with a second phase. For the development's highly loaded structural cores 79 piles of 900mm diameter are being installed in groups of four.

Elsewhere 700mm and 750mm piles are being used.

Because depth of the mudstone varies across the site, probe boring is being used in advance of pile installation to ensure that they bear onto capable strata.

Once the founding strata had been identified, a series of preliminary piles was installed and tested to confirm the load carrying capacity of the rock socket within that type of mudstone strata.

From the test pile results, the CFA pile diameters and lengths were determined and checked off with supervising engineer Connell Mott MacDonald.

'In boring to form the rock sockets for the test piles a lot of friction and consequently heat was generated, ' Sharp says.

'The clay and mudstone baked on to the augers. It had to be regularly removed using jackhammers and a jet wash.' He says that through experimentation, heat generation has been overcome by modifying the arrangement of auger teeth.

May Gurney's new challenge is co-ordinating the movements of its two large piling rigs on the confirmned site.

Work is due for completion at the end of next month.

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