Main contractor Clancy is installing a new storm tank for Southern Water at its Westbere wastewater treatment works near Canterbury.
The new storm tank will accommodate surface water surcharges during periods of heavy rainfall, helping to prevent the discharge of raw sewage into the nearby River Stour.
The tank comprises six precast concrete rings of 15m diameter installed one on top of the other to a depth of 6.5m. Each ring is made up of 16 interlocking caisson segments.
The concrete segments are made by FP McCann and supplied through heavy-side materials distributor Keyline.
To accommodate this tank, Clancy dug an octagonal excavation to a depth of 7m. To support this, Groundforce Shorco supplied 120 interlocking 6m GFI steel sheet piles driven to a depth of 6m using an excavator-mounted vibrating hammer Mueller MS4 supplied by Groundforce’s Piletec division.
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Once Clancy installed the sheet piled cofferdam, it excavated to a depth of about 1m at which point the top of the cofferdam was braced using Groundforce Shorco’s modular hydraulic Mega Brace system.
Clancy then continued to excavate to formation layer before casting the base slab for the storm tank.
"The excavation is octagonal because it makes no sense to dig a rectangular hole for a circular tank," explained Clancy project manager Oliver Smart.
"The octagonal excavation reduces the amount of muck away and requires less kit and less materials,” he adds. The excavation can follow the perimeter of the concrete tank closely while providing sufficient space to accommodate the bracing.
"It's a challenging design and excavation but the Mega Brace adaptability made it easier,” explains Groundforce Shorco area sales manager Arpad Nagy.
"The Mega Brace is a modular bracing system and is very adaptable. It’s easy to install whatever the shape of the excavation and in this case, an octagonal excavation requires less equipment than a square one would. With the octagonal shape the muck away has been reduced by 20%, in this case 200m3. The excavation has been reduced with 10lm worth of sheet piling and 30lm worth of bracing. On this case we’ve managed to reduce the carbon footprint which helps reduce the environmental affect."
Groundforce is one of Clancy's preferred suppliers of temporary works equipment.
"I've worked with them on several projects," said Oliver Smart. "The kit is great and Groundforce has a good design capability. They are providing on-going support and technical advice on this project."
