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Pushing the boundaries

PILING & FOUNDATIONS

Jet grouting is increasingly enabling developers to maximise available space within basements constructed in tight city centres. Paul Wheeler reports.

Development and use of jet grouting in the UK could be described as a solution looking for a problem. Over the last decade or so projects have been sporadic and applications have varied widely.

This is in sharp contrast with continental Europe, especially Italy and Germany, where the technique is routinely used in retaining wall construction or when underpinning, particularly for historic structures.

Now it sees that jet grouting is coming of age in the UK. Keller Ground Engineering, one of a few companies with UK experience in the technique, has seen strong growth in the past year.

Most of the jobs have been in London and have involved building relatively small but deep basements, below or adjacent to structures. The big advantage is that the overlapping jet grouted columns can be formed right below an original wall - maximising the plan of the new basement.

Space savings are considerable compared with the conventional approach to this problem - a piled retaining wall running along the inside of the party wall.

This is because a piling rig cannot form piles right up against a wall. Add to this the space occupied by the piles and finally some kind of cavity wall construction up the inside, all of which eats into the basement area.

Keller business development director Martyn Singleton says that with a conventional retaining structure up to 1.5m can be lost at each boundary. On a typical London plot only 8m across, a basement built with a secant pile wall could be little more than 5m wide, reducing the potential basement floor space by a third.

Keller used jet grouting in 1999 on Norman Foster's Great Court project at the British Museum. It jet grouted a block of stabilised soil below the circular Reading Room, which was then cut back in line with the structure as the new 5m deep basement was excavated.

The technique has also proved popular with wealthy homeowners in Belgravia and Kensington , helping them to maximise space in their houses.

In one case Keller formed an 8m deep basement in water-bearing sands and gravels - a job that was complicated, Singleton adds, by the concerns of the 'great and good' neighbours of Belgravia.

Jet grouting was also used in a hotel development on the site of the old Magistrates Court at Marlborough Street, adjacent to the London Palladium.

Work required basement construction to a depth below the foundations of adjacent structures. Keller used jet grouting to deepen adjacent footings. Again the technique was chosen because it maximised the limited space and enabled excavation and construction to proceed without temporary propping.

Another recent jet grouting project was in the City of London, where Keller used it to support facades and adjacent structures during construction of the new Merrill Lynch headquarters.

Jet grouting is not cheap, but London's high land prices mean the space benefits it offers are cost effective. It has also been used in Birmingham, where earlier this year Keller underpinned existing structures to allow construction of a basement car park for a new residential block.

Another benefit is that jet grouted soil behaves more like a gravity block than a wall, which removes the need for pile embedment (the often considerable section of pile below basement level which is essential for stability).

The advantages of this were apparent last autumn on a job at Torrington Place for University College London, which lies right above the proposed Crossrail route underground railway route between Liverpool Street and Paddington stations.

A conventional piled solution would have involved piles extending unacceptably close to Crossrail, but jet grouting enabled Keller to underpin walls and create the new basement walls without the need for deep embedded piles.

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