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Pile test makes savings on London supermarket

PRELIMINARY PILE testing on a new supermarket site in west London has allowed a reduced factor of safety to be used for foundations and retaining wall piles. This has resulted in shorter piles, which are cheaper and quicker to construct. Testing was undertaken on sacrificial piles, loading them to three times their working load, and on six working piles at one-and-a-half times their working load.

UK Foundation contractor Stent Foundations completed groundworks for the commercial and retail development in Kensington last month.

The brownfield site, sandwiched between the busy A4 Cromwell Road and the West Coast Main Line railway, is being developed by contract manager Bovis Construction and consultant White Young Green for client Tesco Stores. The £800,000 foundation works for the store, car park and office block was finished in just 11 weeks.

In all, some 709 auger bored piles were installed in three areas of the site, together with three retaining walls, for the car park, the road and an adjacent office building. A sheet pile retaining wall was also built to support the railway during construction.

Piles were installed through made ground, Terrace Gravel, weathered London Clay and London Clay and are between 15m and 25m long. Generally 900mm diameter piles were used for the office block and 600mm and 750mm diameter for the store and car park, depending on load and ground conditions across the site. Groundwater was encountered in the gravel and so boreholes were cased off 1m into the clay, around 4m below ground level.

Reinforcement cages for the piles varied according to the design load across the site, ranging from light reinforcement in the store area up to heavy for the office block and car park, both of which have single storey basements. Heavier reinforce- ment was needed to counter lateral loading on the piles from the new basement walls and also clay heave, a direct result of excavating the basements. Reinforcement cages were fabricated on site to allow a flexible piling programme.

The car park retaining wall lies parallel to and just 4m from the busy live railway line along one side of the site. Stent used a fully hydraulic piling rig to ensure that there was no risk of the jib falling on to the railway.

Retaining walls were designed as contiguous cantilevers for retained heights of 3m. Where greater heights - 9m - had to be retained, and there were tighter deflection criteria (next to the road and railway), the retaining walls were propped. This allowed smaller diameter piles to be used. The Cromwell Road retaining wall also had to be temporarily propped as did the sheet piling next to the railway as both had tight deflection criteria.

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