Groundforce provides solution for attenuation system installation in Dublin
20 May 2019
'By opting for two adjoining cofferdams, designed to provide just enough working room around both the interceptor and attenuation tank, the excavation footprint was much smaller which meant a reduction in bulk excavation.'
Modular hydraulic shoring equipment and sheet piles from Groundforce have been used to install a floodwater attenuation system in challenging ground conditions at a recycling yard near Dublin port.
Large reinforced concrete attenuation tank and petrol interceptors
Main contractor M&P Construction was engaged by client Hammond Lane Metal Company to install a large reinforced concrete attenuation tank and petrol interceptors in two deep excavations beneath the client’s recycling yard on Pigeon House Road.
The new underground tanks will be used to store and treat surface water run-off from the yard which will be pumped into the Irish water system to prevent the pollution of sea.
Groundforce was called in by main contractor M&P Construction to propose a shoring solution on this confined site as the project faced significant challenges due to the very poor ground conditions and a high water table.
“The project took place in an area of Dublin port that lies on reclaimed land,” said Groundforce’s general manager Joseph Lenihan. “The area in which the attenuation tank was to be situated consisted almost entirely of sand.”
Support sheet piles during excavation and construction phase
Eight-metre Larssen L603 sheet piles were pre-driven into the ground to create a sealed cofferdam and two levels of Groundforce’s hydraulic Mega Brace were then installed to support the sheet piles during the excavation and construction phase.
Each level of frame required two cross struts and four knee braces with the lower frame removed once the base slab was cast. The top level of frame was kept above the proposed top of the tank level to avoid clashes with formwork.
The tank and associated equipment were installed in two adjacent excavations. “The contractor had the option of digging one rectangular excavation, to encompass both the attenuation tank and interceptors which would have been quite straightforward from a temporary works point of view, but with the ground classified as contaminated soil, the surplus dig would have had significant cost implications,” said Lenihan.
“By opting for two adjoining cofferdams, designed to provide just enough working room around both the interceptor and attenuation tank, the excavation footprint was much smaller which meant a reduction in bulk excavation.”
The loose sandy soil and high water table meant that loss of fines at the excavation corners was another potential problem. Groundforce proposed the use of corner piles to reduce this risk and help control the ingress of water into the excavation.
“The elegance of the installation coupled with the partial removals as the tank was constructed proved very efficient,” said David Walsh, project manager at M&P Construction.
‘Minimum interference from cross bracing’
“We were able to carry out the excavation and cast the tank base with minimum interference from cross bracing. All of the tank walls were cast with the top bracing in place as well as the placing of the roof slab decking.
“As the Groundforce system is hydraulic and modular we were easily able to remove the Mega Brace ourselves thereby saving on downtime. With the top slab poured the backfilling was completed and the sheet piles were extracted.
“Achieving the level of control we did while working in a sandy substrate was made entirely possible through the use of sheet piles and the Groundforce system.”
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