Supporting a deep excavation in weak ground is inherently challenging, but when faced with an additional deluge of heavy rain, the task becomes all the more difficult; especially when the excavation has to be octagonal rather than a simple, straightforward rectangle.
This was the situation facing Impact Formwork, a specialist contractor working for main contractor Galliford Try on a waste-water treatment works near Petersfield in Hampshire for Southern Water.
Impact Formwork was required to install shuttering for a new concrete humus tank at South Harting as part of an extension to the works.
The humus tank is one of the final stages in the sewage treatment process. It is essentially a settling tank used to clarify effluent by the removing the fine humus solids that have been washed off filter media further upstream.
The formwork was to be assembled within an octagonal excavation measuring 12m in diameter and at a depth of 5.95m. To support the sides of the excavation, Impact Formwork called on Groundforce Shorco which supplied its proprietary Mega Brace system to support a steel sheet-piled cofferdam. The KD6 sheet piles were also supplied by Groundforce Shorco and were installed by Impact Formwork.
The Mega Brace is a high load-capacity modular hydraulic bracing system designed to support square, rectangular or multi-sided excavations ranging from 3.0m to 20.0m in plan.
Its inherent corner joint articulation enables the legs to be used for non-rectangular excavations such as the octagonal design employed on this project.
The system’s modular format allows for easy installation with relatively light-duty lifting equipment and the pinned joint and corner connections ensure quick assembly on site.
Once assembled, the Mega Brace units are pre-loaded with the integral double-acting hydraulic rams and secured with integral mechanical lock off valves.
The system’s modular design enabled the contractor to overcome a logistical problem on this project, namely that articulated lorries were not allowed on site due to space restrictions and ground loading limits.
Because the Mega Brace breaks down into easily-handled components, the system could be delivered to site on smaller rigid trucks.
The excavation was supported by 80 No. 6.5m-long KD6 sheet piles and braced with 92m of Mega Frame installed on two levels. Groundforce Shorco also supplied a variety of ancillary products, including edge protection, lifting chains, restraint chains and ladder platforms to ensure a safe and efficient installation.
“From our perspective, this was a straightforward contract as all information required was promptly and correctly supplied by the customer,” says Groundforce Shorco area manager Liam Monnery.
The Groundforce equipment started to arrive on site in October 2023 and work has continued through the winter, including the wettest February on record.
“The main challenge for this site was flooding,” says Liam. “The area in which this excavation has been placed is at the bottom of a series of hills near a farm. During the wet winter we have had, lots of surface water has run into the excavation and required to be pumped back out and up a hill to another area.”