Groundforce has supplied an intricate cofferdam solution to allow the construction of a new pumping station on the River Clodiagh near Holycross, Co Tipperary. The new intake works is part of Phase 1 of the Thurles Regional Water Supply Scheme for Irish Water and will replace existing water-supply sources that are vulnerable to contamination.
The main contractor Glan Agua, in conjunction with their civil-engineering Partner MEIC Ltd, has built the new intake chamber and pump house on a site directly alongside the river, upstream of the R661 Rathkennan road bridge. The new facility also includes an underground holding tank with associated pipework, power supply and service ducting, as well as landscaping around the installation.
The site is within a Special Area of Conservation with unusually high numbers of otters, crayfish and lamprey in the river and badgers in the surrounding fields and woodland. Pollution control and habitat preservation were therefore given top priority.
Approximately 50 x 15m-long precast concrete piles were required to support the concrete structures of the pump house, gravity main and intake chamber. These were installed by specialist contractor Taranto Ltd. Three interlinked cofferdams were required for the construction of the facility with a safe, dry working area inside despite the poor silty ground and the high water table. The excavations reached a depth of 6m with the water table at 1.5m below original ground level.
The cofferdams utilised 7m long Larsen L603 sheet piles to enclose the intake chamber and gravity main. The holding tank – which is located underneath the pump house itself – required 11m-long Larsen L605 sheet piles. Support for these cofferdams was provided by Groundforce’ heavy duty hydraulic frame, Megabrace, with 150-tonne capacity HSK150 hydraulic props installed as knee-braces across the corners.
Deep excavation process
In the deep excavation housing the holding tank and pumping station, Groundforce provided three levels of Megabrace and eight knee-braces. “Once the concrete piles had been installed and the base slab cast on top, MEIC Ltd were able to remove the two lower braces, giving a clear opening of almost 13m x 13m,” says Groundforce general manager Joe Lenihan.
The excavation linking the gravity main and the pumping station was also supported with two levels of Megabrace, with four 80-tonne capacity HSK80 hydraulic props used to provide lateral support. Groundforce also supplied MEIC with EdgeSafe edge protection, LadderSafe access ladders and pile-cropping equipment to trim the concrete piles ready for the base slab to be cast on top.
MEIC Ltd’s main concern was to ensure safe working and environmental protection on the small and very confined site. After consulting with Groundforce, it was decided to use sheet piling to create the cofferdams which, in the end “proved to be the backbone for the safety and success of this project,” according to MEIC project manager Eoin Delaney.
“The high water table was factored into the design of the shoring system with water ingress minimised to such an extent that is was much easier to dewater than anticipated," he added.
“There was complete confidence in the safe access provided to the excavations themselves due to the strength system. And as the sheet-piles were kept in place for approximately three months it was crucial that workers could confidently work within these underground work-zones productively and safely.”
For information, see https://www.vpgroundforce.com/ire/.