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24/08/2010
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Geotechnical contractor Bachy Soletanche is using a storm water shaft, with deep diaphragm walls and cased secant piling, on a project for United Utilities.
 The project, which started in May, with main contractor KMI Water (a joint venture between Kier Construction, J Murphy and Sons and Interserve Project Services), is to treat water bound for the River Ribble. It is due to run until October, and is worth £70m in total.
A 27m diameter shaft, adjoining a 22m deep 50x20m screening chamber and a 17m diameter shaft, constructed by Diaphragm Walling, will connect with nine additional cased CFA shafts.
These are on the north side, with links via a 3.5km tunnel constructed 30m below the ground by KMI Water, and, according to the companies, will pave the way for a cleaner river.
One of the south diaphragm wall shafts has been formed by 14 interlocking panels 1m thick by 20m deep. Between each panel, a CWS waterbar is being installed to stop water leaking through the joint, and to ensure maximum water tightness between adjoining panels.
Panels are being dug under a bentonite suspension, with reinforcement cages placed and concrete added into the trench.
Steve Mallinson, contract manager for Bachy Soletanche, explains that the Cased CFA, CSP technique forms a closed structure to act as a barrier in water-bearing soils and is ideal for a city centre project, such as the Preston Shafts.
He cites benefits as including: better tolerance than an uncased CFA secant wall; better appearance with reduced over-break; speedier operation than traditional kelly/casing rotary bored piling; reduced risk of flighting in poor soils; and a reduced risk of distress to adjacent structures.
"The large south side shafts are more than 20m deep, hence the use of the diaphragm walls which involve less construction joints than bored pile walls," says Mallinson.
"On the north side verticality has also been key throughout this project due to the nature of the interlocking between the male and female piles to ensure a watertight solution is created."
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Author
Brian Tinham
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