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Tight access and an explosion-proof basement are just some of the challenges on a piling contract in central London. Max Soudain reports.

Mason's Yard, a small square behind Jermyn Street near Piccadilly Circus, packs an interesting history into a confined space.

In the early 1770s it was a stable yard owned by Hugh Mason, co-founder of a little grocery shop called Fortnum and Mason. By the 1960s the shop was world-famous and Mason's Yard was at the heart of swinging London: it was here that John Lennon met Yoko Ono, at her exhibition at the Indica Gallery in 1966.

Art galleries are still thick on the ground in the area and by summer next year another will dominate Mason's Yard. The White Cube gallery will be Brit Art champion Jay Joplin's third in London.

The gallery will replace a threestorey structure built in 1889 to house the St James & Pall Mall Electric Lighting Company power station. This was decommissioned in 1928 and it became a substation.

Main contractor Cosmur Special Projects demolished the building last December and broke out the ground floor slab to give access to the basement that once housed the power station's steam turbines.

Foundation contractor Van Elle arrived on site in mid-January to install piles for the gallery and to build a secant pile wall around the perimeter of the basement. This will allow the basement to be deepened by 2.5m.

'It's basically an underpinning job, ' explains Van Elle director Matthew Large. 'We are installing the wall in sections.' Cosmur Special Projects is freeing up stretches of wall by excavating through the basement floor slab, which is generally 2m thick. Site manager Richard Cushley explains that the floor had been upgraded with mass concrete to protect the surrounding area in the event of turbines exploding.

'The basement walls are also thick - 700mm to 800mm of reinforced concrete with a brick structure behind, ' he says.

Piles are installed in the opened up sections, which quickly become filled with water. 'There are water bearing gravels under the slab, with water level about 1m to 1.2m below it. This fl uctuates with the River Thames, ' Cushley says. London Clay lies below and the piles found here.

Large says: 'The original plan was to install a contiguous pile wall with grouting between the piles.' The rigs this would have required would have been too tall for the basement, and removing the roof to make more headroom was not an option.

Design changed to a secant wall which could be installed by two smaller machines - a Klemm 702 and a Hutte 302 - from inside the basement. Even so, there is only 3.5m of headroom, and the Klemm rig is 3.2m high.

The pile wall is comprised of 200 'hard' and 200 'soft' CFA piles up to 18m deep. Soft piles are mainly 300mm diameter at 450mm centres.

These are installed first and are left to achieve 3-4N strength in seven days, Large says. Hard 400mm diameter piles are then drilled in between.

These are reinforced for their full length with a single 273mm diameter steel tube inserted in 3m sections (because of the headroom) into the concreted pile.

Because the original basement wall has some convex sections, Van Elle is also installing smaller, 273mm diameter piles along these stretches to maximise space and to achieve a straight wall.

As well as providing lateral support and preventing water ingress during excavation, the wall will support the new building, helped by bearing piles across the basement. Van Elle is installing a total of 60, 450mm diameter piles, some of which will be tension piles to prevent uplift of the new, deeper basement slab due to the groundwater beneath.

The job involved 'a lot of brain work beforehand', Cushley says.

Van Elle bought a lorry with a special low headroom cab to allow it to pass under the arched entrance to Mason's Yard. It also needed a small crane to move equipment around in the basement. The firm eventually found a suitable piece of plant on the internet.

Van Elle is due to finish in early summer. Once its rigs are gone Cosmur can start breaking out the rest of the basement fl oor and deepening the underground space. This will be followed by the gallery's steel frame.

The building is due for completion in July 2006.

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