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Whipping up a storm in Birmingham

A novel solution for a stormwater tank in Washwood Heath, Birmingham was the brainchild of specialist geotechnical contractor Bachy Soletanche, Severn Trent Water, main contractor Forkers and the client’s engineer Mott MacDonald.

A novel solution for a stormwater tank in Washwood Heath, Birmingham was the brainchild of specialist geotechnical contractor Bachy Soletanche, Severn Trent Water, main contractor Forkers and the client’s engineer Mott MacDonald.

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

The 12.5m diameter, 12m deep shaft was constructed using 72 80mm diameter cased CFA piles. With the high water table and granular soils it was imperative that the piles interlocked over their full depth of 18m. A cased CFA option was adopted instead of traditional rotary methods.

An unusual feature of the project was the use of jet grouting to form the shaft base within water bearing sand soils. This “plug” was constructed to allow the shaft to be fully dewatered without risk to surrounding buildings and to avoid the ingress of water during the casting of the base slab.

In conjunction with Mott MacDonald, Bachy Soletanche proposed this alternative solution which required the installation of 76, 1.6m to 2.8m diameter single and double jet grouted columns. These were drilled within the confines of the secant pile shaft wall, forming a 5m thick plug of jet grouted soil just below the 12m deep shaft excavation.

The storm water tank, located next to an existing sewer overflow will provide storage to greatly reduce the amount of waste and rain water spillage into the adjacent Washwood Heath Brook.

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