Jet grouting speeds difficult storm water tank construction 06 July 2010

Geotechnical contractor Bachy Soletanche, along with the project team of Severn Trent Water, main contractor Forkers and engineer Mott MacDonald, have developed a novel solution for a storm water tank in Washwood Heath, Birmingham.

The million pound project consisted of a secant piled shaft and a jet grouted base plug solution to form an underground tank in a cul-de-sac.

Steve Mallinson, contract manager for Bachy Soletanche, explains that the 12.5m diameter, 12m deep shaft was constructed using 72 880mm diameter cased CFA piles, but that, with the high water table and granular soils, the piles had to interlock over their full depth of 18.0m.

He says that Bachy adopted the cased CFA option, instead of traditional rotary methods, but also used jet grouting to form the shaft base within water bearing sand soils. The plug was then constructed to allow the shaft to be fully dewatered without risk of settlements to surrounding buildings and to avoid the ingress of water during casting of the base slab.

"Using the cased CFA technique is a much faster and leaner alternative to traditional LDA piling, reducing programme time and cost," says Mallinson.

"Although a shaft would usually be dewatered prior to excavation, the ground stratum here would have required very deep piles to cut off the natural ground water flow in to the shaft. Using the jet grouting technique to form a base plug meant the piles could be shortened and ensured that the shaft could be made watertight, allowing the dewatering and excavation to be conducted in the safest possible way," he adds.

The storm water tank, located next to an existing sewer overflow on the boundary of Ward End Park, Foxton Road, now provides storage to greatly reduce the amount of waste and rain water spillage into the adjacent Washwood Heath Brook, reducing pollution and improving the quality of the brook and the receiving River Tame.

Brian Tinham

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