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Cutting contamination

When Chelverton Properties began redevelopment of what was once the Vickers armaments factory, it uncovered a difficult ground stabilisation problem. Nearly 2,000 foundation piles had to be installed without affecting local residents, and in an environmentally sensitive manner.

Extensive research with boreholes and a dynamic probing system established that the ground consisted of, among other things, 2.5m of fill and peaty alluvium containing industrial contaminants, over a 4m layer of dense gravels.

A crucial factor was that the chalk bedrock, lying 6m to 7m below the surface was a potable water source. Whatever solution was found, penetration of the chalk was prohibited.

Engineer Roger Stagg Associates and main contractor Jarvis decided to use vibrated concrete column technology from Pennine. The concrete columns were formed in shafts created by a vibroflot, 'a vibrating unit that sinks into the ground under its own weight,' says Pennine's operations director Mike McKeown. 'Rather than remove spoil from the hole using a CFA piling system, the soil is compacted into the sides of the hole, increasing the strength of the surrounding ground. Then we pump in concrete. Because the vibroflot expands the base of the hole, the column is formed with an enlarged toe to spread the load.'

Benefits to using the system were that the process is very quiet, the only noise being the hum of the Caterpillar tractor unit. And VCCs do not create spoil disposal problems as the soil displaced is compacted inside the hole.

Consumption of concrete is also low, since vibroflotation increases the strength of the surrounding soil which allows the columns to be shorter. On the Vickers site vibrated columns could be 3m to 4m in depth compared with the 12m needed for conventional piling.

Penine formed 60 columns per rig every day. Using conventional rigs 15 to 18 piles per rig per day would have been good progress, the company says.

Fax Phil Cutting (0118) 971 2480

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