Storm water tank installation is walk in the park for Bachy Soletanche
Specialist geotechnical contractor, Bachy Soletanche, along with the project team of Severn Trent Water, main contractor, Forkers and the client’s 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 the underground tank in the middle of street in a residential cul-de-sac.
The 12.5m diameter, 12m deep shaft was constructed using 72, 880mm diameter cased CFA piles. With the high water table and granular soils it was imperative that the piles interlocked over their full depth of 18.0m. Bachy adopted the cased CFA option instead of traditional rotary methods.
However the 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 of settlements to surrounding buildings and to avoid the ingress of water during the casting of the base slab.
In conjunction with Mott MacDonald, Bachy Soletanche’s Anchor, Grout and Mini-pile division 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.
It was critical that the grout overlapped with no gaps to make sure that the base was watertight. To monitor this, a state-of-the-art geo-physics system, Cyljet, operated by sister company EDG, was used to check the quality and actual diameter of the jet grouting using a soil resistivity survey after the grout columns were installed.
Further testing on samples cored from the jet grouted soil confirmed that the base plug had a permeability in the region of 1x10-9m/s, which is a level normally associated with cut-off barriers.
Steve Mallinson, Contract Manager for Bachy Soletanche Limited explained:
“Using the cased CFA technique is a much faster and leaner alternative to traditional LDA piling, reducing programme time and cost. 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.”
The piling section of the project took place over 5 weeks and was completed in November 2009. The jet grouting works followed immediately after completion of the secant pile wall and were completed in 6 weeks.
The storm water tank, located next to an existing sewer overflow on the boundary of Ward End Park, Foxton Road, will provide 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.