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Keller completes its first UK controlled stiffness column scheme in Liverpool

Keller has helped Peel Ports overcome weak ground conditions at a site near Liverpool through the transfer of technology from its French and German businesses.

Controlled stiffness columns were used at the former warehouse site in Alexandra Dock to create support for a load transfer platform for the ground bearing slab on a new distribution centre.

The technique has been used by Keller elsewhere in Europe but this was the UK business’s first application of the method.

“This system fits neatly between our vibro techniques and piling,” said Keller operations manager for controlled stiffness columns Richard Smith. “Provided site levels allow the construction of a load transfer mattress, we can adopt this system where the soils are too soft or too organic for vibro.”

At Liverpool the columns are formed from 340mm diameter driven cast insitu piles were driven into the underlying dense sands, although 450mm diameter CFA piles were used close to a sensitive culvert.

Keller has said that finite element analysis of the settlement was carried out in order to determine the column spacings and thickness of the overlying load transfer platform.

In total, Keller installed 4,412 columns at 2.5m centres with a 500mm thick mattress of heavily rolled 6F2 stone over the top to create the transfer platform.

The very dense sub base sitting beneath the old slab was used as a piling mat following rigorous load testing carried out by main contractor McLaughlin and Harvey. Once a column was installed, the upper part of it was removed down to the base of the granular mattress and replaced with dense gravel. The mattress was then topped up to reach the required thickness.

 “In certain parts of the country the granular mattress may be too expensive, or indeed not available, and a two way suspended slab will be the optimum solution,” added Smith. “At Alexandra dock the existing granular material was incorporated into the load transfer mattress so this was a win win for the project.”

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