Monday 27 July 2020
Octagonal cofferdams star in Co Sligo.
Octagonal-shaped cofferdams, provided by specialist construction supplier Groundforce, have helped save space, time and money on Irish Water’s €9.9 million upgrade of two sewage treatment works.
Stormwater holding tank
The 15m internal diameter cofferdams were required for the construction of a stormwater holding tank and clarifier for the wastewater treatment plants at Ballymote and Collooney in Co Sligo.
They are part of Irish Water’s €5.5 billion programme to meet increased demand, comply with Irish and European Union law, and reduce flood risk.
Currently, sewage from Collooney and Ballymote is treated by an old, sub-standard treatment system which has no stormwater storage capacity. As well as providing this, the upgraded wastewater treatment plants will improve the quality of the final effluent leaving the plants and the water quality in the Owenmore river.
Phosphorus removal facilities
In addition to upgrading the existing wastewater treatment facilities to cater for an equivalent population of 3,100, the project also comprises the construction of phosphorus removal facilities and primary and secondary treatment plants.
Designed to enhance critical local infrastructure and remove impediments to social and economic development in Ballymote and Collooney, the project is being delivered by Veolia, with specialist temporary works contractor Carty Group installing the Groundforce cofferdams.
Company owner Iarla Carty said: “The existing services and tankage had made it impossible to use traditional square cofferdams for excavations so in conjunction with Groundforce we came up with two 15m internal diameter octagonal sheet piled structures which use a smaller site envelope. This provided adequate excavation protection for the construction of a circular precast storm tank and clarifier.
"This solution proved both more economic on available land usage and also for volumes of backfill material used on completion of the structures.”
Reduce excavation footprint by 30%
Groundforce general manager Joe Lenihan explained that the octagonal cofferdams reduced the excavation footprint by almost 30% – a great benefit on a congested site. They also allowed for significant cost savings compared to traditional square or rectangular cofferdams.
He said: "The tank excavations were completed very successfully with the octagonal shape of the Groundforce cofferdams providing great benefits in space saving and a reduction in excavation and backfill costs on what is a tight site.
“As the strength of the system is dependent on the shape, an accurate installation is very important, Thankfully the right crew were on the job. Carty Group are a very experienced contractor and executed the installation perfectly.”
The octagonal system is one a range of solutions Groundforce can offer customers looking for a clear opening within which to construct their structure.
“This is becoming ever more important with tank capacities increasing as Irish Water continue to invest in the upgrade of water treatment plants across the country,” said Lenihan.
Veolia claim the work will benefit 73,000 people in the midlands and northwest of the republic.
Both Ballymote and Collooney plants are expected to be in operation by the end of 2020.