Groundforce

Mega Brace Cofferdam Support

Thurles Regional Water Supply Intake Works: delivering a secure solution to a sensitive water scheme

By: Groundforce

Published: October 1, 2017

Thurles Regional Water Supply under construction - Courtesy of Groundforce

The new intake works on the River Clodiagh near Holycross, County Tipperary 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 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.

Undertakings

The project included a new intake chamber and pumping station on a site directly alongside the River Clodiagh, upstream of the R661 Rathkennan road bridge. Groundforce supplied an intricate cofferdam solution to allow the construction of a new pumping station on the river.

The new facility also includes an underground holding tank with associated pipework, power supply and service ducting, as well as landscaping around the installation.

Approximately 50 (No.) 15m long precast concrete piles were required to support the concrete structures of the pump house, gravity main and intake chamber.

Cofferdams

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.

Mega Brace

Support for these cofferdams was provided by Groundforce’s heavy duty hydraulic frame, Mega Brace, with 150-tonne capacity HSK150 hydraulic props installed as knee-braces across the corners.

In the deep excavation housing the holding tank and pumping station, Groundforce provided three levels of Mega Brace and eight knee-braces. Once the concrete piles had been installed and the base slab cast on top, the contractor was able to remove the two lower braces, giving a clear opening of almost 13m x 13m.

Mega Brace heavy duty hydraulic frame – Courtesy of Groundforce

The excavation linking the gravity main and the pumping station was also supported with two levels of Mega Brace, with 4 (No.) 80-tonne capacity HSK80 hydraulic props used to provide lateral support.

Groundforce also supplied 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.

The contractor’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.

Summary

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.

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.