One such project is under way at Barrow-in-Furness, Cumbria, where geotechnical contractor Bachy Soletanche is building a diaphragm wall to create a shaft for a storm water tank in the heart of the picturesque town.
Barrow is on the outskirts of the Lake District, an area which the Met Office describes as one of the wettest place in the UK with average annual rainfall totals often exceeding 2000mm. As part of major improvements, United Utilities has a £14M sewer flood alleviation project in the centre of Barrow.
Work for the project began in April with the subcontractor starting work on a circular concrete shaft for main contractor KMI Plus. The 23.6m internal diameter shaft, which forms the outside of the storm water tank, is built using a diaphragm wall technique, rather than the more commonly used secant piled wall.
"We employed the diaphragm wall technique to construct the shaft to the required depth of 30m." says Bachy contracts manager Barrie Arkwright. "By using a crane-mounted grab, we have the ability to build a deeper shaft than is possible with a traditional piling rig."
The diaphragm wall was installed using the slurry trench technique. This involves excavating a narrow trench that is kept full of bentonite mud. This fluid exerts hydraulic pressure against the trench walls and acts as a shoring to prevent collapse.
The construction process begins with the trench being excavated in panels. The shaft at Barrow consists of 12 panels, each one 1m thick, which are built using the rope operated grab. Each panel consists of three bites of the 2.8m wide grab – a virgin bite, a stop-end bite and a centre bite. When these three are complete, each panel is 6.4m wide.
A 1m deep concrete guide wall was built at platform level to maintain the accuracy of the shaft and support the reinforcement cages.
To ensure accurate control between panels, the grab alignment is guided by clamping on to a stop-end. The stop-end is placed vertically at each end of the primary panel to form joints for the adjacent secondary panels. A water bar is incorporated into each stop-end to mitigate any water flow between panel joints.
Once excavation of a panel is complete, the bentonite support fluid is cleaned within the panel to reduce its density ready for concreting.
However, before the concreting can begin, three steel reinforcement cages are tandem lifted by two cranes and placed in the panel. Each reinforcement cage is 30m long, lifted in two sections and then spliced together during installation into the panel.
Once reinforcement cages are in place, concrete is poured into the panel through tremie pipes, which are removed as the concrete begins to fill the excavated panel.
"For the Barrow project each panel takes approximately 200m3 of concrete and takes about six hours to cast," says Arkwright. "The concrete displaces the bentonite mud, which is then pumped from the top of the panel to the de-sanding unit where it is recycled, de-sanded and cleaned for re-use on subsequent panels."
Six reservation tubes, with an inside diameter of 160mm, are then placed into each panel. These tubes form voids so a 4m socket can be drilled into the rock, removing the need to go through concrete and reinforcement.
A T40 8m long bar is inserted into the rock socket. The rock socket and reservation tube are then filled with cement grout and slightly pressurised to seal the area between the toe of the diaphragm wall and the rock.
Before completing the wall, Bachy installed four pressure relief wells inside the shaft. These wells help to relieve water pressure and maintain the stability of panels when closing off the shaft, and later, when the inside of the shaft is excavated to its 30m depth.
Bachy completed the shaft in August, however, there are still several stages to complete for the Spring 2009 project deadline. The circular cement shaft is being excavated to form the storm water tank.
Approximately 12m down the shaft a new tunnel will be constructed to enlarge the sewer drain that will run into the tank. At this point, a tunnel boring machine will be lowered down into the shaft to begin work.
The area where the new storm water tank is located was formerly a bus station and landscaped area, and as part of the project specification this area will be restored once the scheme is complete.