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HOMES FIT FORHEROES

Permanent load-bearing sheet piling is being used on a residential scheme in Cheshire named after a local sporting legend.

A multi-storey apartment block taking shape in Warrington, Cheshire is the latest development to be named after the town's rugby league hero Brian Bevan.

Bevan, considered by some to be the best winger in the history of the game, scored 796 tries for Warrington during his 19-year career, from 1945 to 1964.

The Watkins Jones development is on an infi lled bend of the River Mersey, near Brian Bevan Island on Wilderspool Causeway, a main route into Warrington. The site was formerly a bus depot and then a transport garage.

Architect O'Connell East designed the 44 one- and twobedroom apartments with underground parking and, as is becoming more common for both commercial and residential schemes, specified that the basement should be formed by permanent sheet piles.

The piles will retain the ground during excavation and form the permanent walls of the car park, as well as carrying vertical loads from the development's steel frame superstructure - estimated to vary from a nominal 16kN/m to a maximum 820kN/m.

Dew Piling designed the sheet piles based on loadings provided by structural engineer Robert West & Partners. Ground conditions are fill over silts and sands, sandy gravels and firm to stiff clay.

Dew says the technique is cheaper than concrete and generally quicker to install than bored piles.

Workers installed Marcelo PU and AU profi le sheets between 8.8m and 25m long for the 185m perimeter basement. Pile choice depended on loading and ground conditions with the greatest loads borne by the extra skin friction provided by the longest piles.

The £210,000 contract was in two phases. Dew used a resonance-free 14m ABI leader rig - capable of driving piles up to 15.8m long - for about two-thirds of the 249 piles.

The second phase, to close the rectangular basement, used a more traditional technique. A crane-hung resonance-free ICE18RF vibrating hammer drove the piles longer than 15.8m as far as possible and the final level was achieved using a BPH2400 impact hammer. This phase was completed in late February.

Using resonance-free equipment minimised the effect of piling on neighbours, Dew says. Pile walls are supported by the ground fl oor and basement fl oor slab, except where there are access ramps - here piles act in cantilever.

Foundation work included a number of satellite piles outside the cofferdam, to carry vertical loads for balconies.

After excavation of 3.2m of ground, the sheet pile clutches were sandblasted and welded. As the top of the basement slab is well above the 100-year maximum fl ood level, being watertight was not the most important issue, but welding was a structural requirement. This ensured the vertical point loads imposed by the building were evenly distributed on the sheet piles.

'The piles cut through the ground like a knife through butter, ' says Dew Piling contracts engineer Andrew Pool.

But the company's piling division senior design engineer Dave Thompson explains that some of the longer piles could not be driven to full depth. When these met refusal, the piles either side were driven further to spread the redistributed load.

Pool says although the foundation work was relatively straightforward, the tightness of the site and the closeness of houses made it challenging.

'Before we started the contract Watkin Jones engineers carried out structural surveys on all adjacent properties and installed monitors around the site to ensure that vibration did not exceed agreed levels, ' he says.

'When the conventional piling rig was being used to drive the longer piles, noise levels were monitored and agreed with local environmental officers.'

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