Main contractor Clancy is installing a new storm tank for Southern Water at 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 15-metre diameter installed one on top of the other to a depth of 6.5 metres. 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 has dug an octagonal excavation to a depth of seven metres. To support this, Groundforce Shorco supplied 120 interlocking six-metre GFI (Groundforce Interlocking) steel sheet piles driven to a depth of six metres using an excavator-mounted Mueller MS4 vibrating hammer supplied by Groundforce's Piletec division.
Once Clancy installed the sheet piled cofferdam, it excavated to a depth of about a metre, at which point the top of the cofferdam was braced with 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 added. 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," said 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 200 m3. The excavation has been reduced with 10 lm worth of sheet piling and 30 lm worth of bracing. On this case we've managed to reduce the carbon footprint which helps reduce the environmental impact."
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