Specialist geotechnical contractor, Bachy Soletanche, is working with main contractor KMI Water, a joint venture between Kier Construction, J Murphy and Sons and Interserve Project Services, on a project for United Utilities to treat the water that goes into the River Ribble using a storm water shaft solution involving deep Diaphragm wall shafts and Cased Secant Piling (CSP).

The project, which started in May of this year, is due to run until October and is worth an estimated £4.5m for Bachy Soletanche, and £70m in total for KMI Water. A 27m diameter shaft, adjoining 22m deep 50x20m screening chamber and a 17m diameter shaft constructed by Diaphragm Walling will connect with nine additional Cased CFA shafts on the North side via a 3.5km tunnel constructed 30m below the ground by KMI Water and will pave the way for a cleaner river.

One of the south diaphragm wall shafts is 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 ensure maximum water tightness between adjoining panels. Panels are dug under a bentonite suspension, then reinforcement cages are placed and concrete is tremied into the trench. All trenches are going to be approximately 20m deep.

The screening chamber will only be used in the case of extreme flood conditions when the shafts can’t manage the load of the storm water in need of treatment. The 12m deep shafts on the North side of the River Ribble will take up the dirty water, feed it back under the River and into the treatment works on the South side.

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, due to its flexible and compact operation. It brings with it the following advantages:
* Better tolerance than an uncased CFA secant wall
* Better ultimate appearance and reduced overbreak than an uncased CFA secant wall
* Speedier operation than the more traditional ‘kelly / casing’ rotary bored pile method
* Reduced risk of ‘flighting’ in poor soils than a standard CFA system
* As it is a cased system; it reduces the risk of distress to adjacent structures in certain soils such as sands or gravels

Steve Mallinson, Contract Manager for Bachy Soletanche Limited, said:
“The large southside shafts are more than 20m deep, hence the use of the diaphragm walls which involve less construction joints than bored pile walls. On the northside 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. Water companies are heavily investing in waste water treatments at the moment so the Preston Shaft project has been a great one to work on and one we’re sure will make a difference to the quality of water going into the River Ribble.”

Email: amy@manifestcomms.co.uk