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Working without piles

Spotlight - Geotechnics

Piling is no longer the automatic choice for foundation support and construction. Specialist grouting systems are now playing an increasing role, providing alternative ways to support structures.

Deep basements are now commonplace on inner city developments. The traditional method for forming these structures is by installing bored pile retaining walls. However, the downside of a piled solution is that you lose basement space.

Keller Ground Engineering has chosen to use one of its specialist techniques to build a complex basement project in central London.

The 5m deep basements are needed for a prestigious new hotel being built on a constricted site behind the Grade II listed Palladium theatre in London's West End. Space is at a premium, and the complexity of the project is increased by the need to underpin the neighbouring Marlborough Street Magistrates Courts - also Grade II listed.

To maximise the building area on the relatively confined site, while still controlling settlement of the adjacent buildings, Keller opted to use its Soilcreteregistered jet grouting system to both underpin the existing buildings and form the new basement walls.

Soilcrete jet grouting is a replacement grouting method that can be used in a wide range of soils from clays to rock. Keller uses the system for underpinning, groundwater and settlement control.

On the Great Marlborough Street site, the ground conditions consist of up to 6m of made ground covering sands and gravels overlying London Clay.

For the underpinning work, Keller constructed 75, 1.3m diameter, interlocking Soilcrete jet grouted columns at 1.1m spacings directly beneath the footings of the existing building.

Each column was taken down to the interface with the London Clay.

Keller formed the columns using its double jet system, which works quickly without producing large quantities of spoil.

Throughout the process the contractor controlled and monitored ground movement and settlement using a precise levelling system.

The jet grout columns were used to effectively form a gravity retaining structure, which did away with the need for internal propping, freeing the site area and speeding up the overall construction programme.

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