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The government drive to improve the quality of UK watercourses has seen the Dwr Cymru Welsh Water Capital Alliance employ an innovative technique on two new waste water treatment tanks using steel piling at its site in Fairbourne, near Dolgellau in Wales.

GallifordTry, a partner of the Welsh Water Capital Alliance is responsible for the design and construction of the tanks, one for storm water attenuation and the other to house the Membrane Bio-Reactor (MBR) treatment plant.

The purpose of the scheme is to provide first time secondary wastewater treatment, in accordance with the Urban Wastewater Treatment Directive for local communities.

Site investigations revealed poor ground conditions of layers of peat with a high water table.

The Cambrian Coast Railway runs alongside one boundary of the treatment works site and needed to be taken into account when considering the design.

GallifordTry approached Corus Hydrobarrier to develop a a sheet-piled cofferdam that acts as a permanent and temporary retaining structure for both tanks. Hydrobarrier had already successfully completed the construction of a sealed sheet-piled attenuation tank in Exeter for South West Water.

By using temporary and permanent retaining structures, temporary ground support measures were removed from critical path activities. An on-site reinforced concrete box construction would have required extensive temporary support measures and a de-watering system.

For a relatively dry excavation the sheet piles were designed to act as a cut-off by extending into underlying low permeability soils and had a sealant system installed in the leading pile interlocks.

For the wastewater treatment tank Hydrobarrier provided the sheet piles with a workshop applied temporary sealant installed in the interlocks.

This means the tank walls are resistant to water until the interlocks can be seal welded. The sheet piles forming the walls to the storm water tank had a workshop applied permanent sealant in the interlocks, since a lower degree of water resistance was required for this structure.

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