Specialist contractor Ken Rodney Construction is installing a new 800mm-diameter siphon pipe in the headwall at Widdop Reservoir near Hebden Bridge for main contractor Galliford Try.
The siphon pipe is designed to carry away excess water in the event of high rainfall to prevent the reservoir overtopping its dam. Climate change, resulting in extreme rainfall events, has meant that the client, Yorkshire Water, needed a larger-diameter siphon pipe to carry these excess flows more quickly.
Installation of the siphon pipe involves excavating through the dam headwall – an operation that is never taken lightly, as Groundforce Shorco’s regional manager Danny Terry explains: “The headwall is in a bund that holds back all that water pressure – it’s a critical part of the dam.
“This is only the second time in a long career that I’ve ever seen a headwall excavated – they like to leave them alone. Water companies employ panel engineers whose main responsibility is headwall maintenance.”
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The excavation at Widdop measures 6.3m long by 3.5m wide and is 3.26m deep. Sacrificial AZ steel sheet piles support the sides, driven by an MS3 EMV excavator-mounted vibrating hammer. The sheet piled retaining walls are braced with Groundforce Shorco’s Mechshore 900 Series modular braces.
After installation of the siphon pipe is completed, the excavation will be back-filled with clay. The sheet piles, being sacrificial, will remain in situ after completion of the works.
In the centre of each of the four sides of the excavation, Ken Rodney has installed a special ‘key pile’ – a steel pile with a cruciform cross-section specially modified for this application.
“We cut some steel Larssen U-piles in half and welded the pieces to the Z-pile in our workshop to create a sort of ‘cup’ on either side of the pile,” explains Ken Rodney site supervisor Steve Hall.
“This provides an extra key in the clay around the outside of the excavation and also the clay we use to back-fill the excavation.”
The excavation is a potential weakness in the 2.5m-wide clay core that extends along the length of the dam and right down to the base. The purpose of these four key piles is to improve the stability of the structure and eliminate any differential movement that might compromise the siphon pipework.
Also vital to the stability of the structure is the nature of the clay used to backfill the excavation.
“We’ve backfilled already on the ‘wet’ side of the excavation with concrete. We’re now waiting for more clay to be brought to site by Galliford Try,” says Steve Hall.
“The clay is still undergoing tests. It has to have exactly the same characteristics and water content as the clay already here.”
Close cooperation between Ken Rodney and Groundforce Shorco has been essential to the design and execution of this delicate operation, says Steve.
“I’ve been with Ken Rodney for about 15 years now and we’ve always used Groundforce. I know Danny very well and if I need anything I only have to call him. The service is always sound and they look after us.”