This project involved the design, build and 20-year operation of two new wastewater treatment plants (WWTPs) and associated sewerage networks to serve the towns and surrounding areas of Dungloe and Glenties in County Donegal, Ireland. Works included the design and construction of two wastewater treatment plants including a foul gravity network to allow for the ultimate Phase 2 expansion to cater for a population of 3,200 in Dungloe and 2,400 in Glenties. Networks comprised of an inlet sewer, a local collection network and a new gravity sewer.
The works comprised of the design, specification, procurement, supply, construction, testing and commissioning of the complete, fully functioning WWTPs, main lift pumping stations, gravity sewers and rising mains. The works included all civil, mechanical, and electrical operations and were carried out at ground level.
The following deep wells, tanks and chambers were constructed:
- The main treatment plant works in Glenties was within a water retaining in-situ reinforced concrete structure 26m by 20m by 6m deep, and had a capacity of 3,000m3
- Two sewerage pump stations with associated stormwater holding tanks
Other works included the construction of five pumping stations. On commencement of the works, we carried out our own site investigation works and discovered up to 14 metres of soft peat in the location of the main sequencing batch reactor (SBR) tank. Our initial proposed method of works had been to excavate the soft peat in the tank location as the initial design for the tank showed the tank base level almost four metres below ground level. When it was discovered that up to 14 metres of peat were present, we re-designed the whole works to raise the level of the tank to sit on Tiroler Rohrer (TRM) piles at ground level. The hydraulic model of the sewerage network allowed the level of the treatment works to be altered. TRM piles are circular ductile iron piles and we designed this piling system and installed the piles using our own piling plant. As the tank was raised up out of the ground, flotation was no longer a concern and the wall thickness was reduced, saving on concrete and reinforcement. The load on the piles was also reduced, which allowed for increased spacing of the piles and therefore fewer piles. Due to the depth of peat on the site we elected to construct the 200-metre-long access road into the treatment works using a ‘floating road’ design, which incorporated geogrid and geotextile products.
The main treatment plant works in Glenties was within a water retaining in-situ reinforced concrete structure 26m by 20m by 6m deep, and had a capacity of 3,000m3. Because of the ground conditions at the site (an average of 4m depth of soft material on top of the rock), the water retaining structure was designed to be found on top of the bedrock, and the top section of the structure was above ground level. There had to be enough room around the tanks for the sides of the excavations to be battered and stepped back for health and safety reasons and the method statement was strictly followed in this regard. Where there was the insufficient area available around the tank to batter the excavation, sheet piles were installed. Sheet piling was used at the along the access road and at Dungloe where the excavation was close to the river. Temporary groundwater drainage sumps were positioned around the edges of the excavation to deal with groundwater ingress. Groundwater was pumped to a temporary silt settlement bund away from the excavation before returning to local drainage. Two types of reinforced concrete structure were designed for the five pumping stations. These designs were fully developed following access to the site once the contract had been awarded. In the three cases where the sides of the excavation could be battered back, bespoke precast tank units were designed such that they could be lifted into place in one piece. Directional drilling was used in the Dungloe and Glenties Sewerage Scheme for the installation of two 450m long pipelines of 180mm O/D HDPE rising main along the Pole Road in Dungloe.
CCL was responsible for all aspects of testing and commissioning and is responsible for the operation and maintenance of the WWTPs, pumping stations and all associated rising mains and gravity sewers for a period of 20 years.