The existing sewer network in Hazelwood Avenue included a major 24” trunk sewer which was routed through private gardens from No.1 to No.20, installed at a depth of 5 to 5.5m. Ground conditions on the 250m route were particularly challenging, in addition to deep dig, were compounded by a combination of filled and sandy ground with a high water table under tidal conditions. The pipeline had been installed in the 1960’s and while remaining functioning in terms of sewerage transmission was failing severely in terms of ground water infiltration and leakage through failing joints and gaskets. In combination with the ground conditions the scheme was identified as high priority given to potential for ground settlement in the carriageway and adjacent garden walls. Following detailed site investigation including a full geotechnical analysis following rotary and percussion drilling, Ground Probing Radar GPR, slit trenches, CCTV, structural and condition surveys of adjacent structures, a detailed design and methodology proposal was agreed with Irish Water Project Managers, Louth County Council Water Services, our appointed designers Hanna Hutchinson Consulting Engineers and the Nu Line Coras enabling team. Works involved the full replacement of 252m of existing main with new 600mm concrete main, 6 new manholes and renewal of a 40m section of sewer under the main Coes Road arterial route by insertion of a 560mm HPPE sewer into the 24”.
The poor ground conditions and high water table directed the requirement to deploy innovative geotechnical solutions to facilitate works which included full dewatering by well pointing and 24 hour ground water pumping under full time monitoring. This reduced ground water over the full length of the pipeline route from a depth of 1.8m to 6.0m (0.5m below invert). The control of the ground water removed a significant safety hazard for the project which facilitating deep excavation under controlled conditions allowing rapid construction along the route. New sewer construction works commenced in late October 2018 with completion, testing and commissioning of the new sewer in early March 2019. Ancillary works to the scheme included full carriageway and new footpath construction along the route, the replacement of 110m of uPVC water distribution main with 225mm HPPE, service diversions, extensive landscaping in private property, construction of new boundary walls and renewal of domestic property driveways. A proactive approach was taken to customer engagement and the residents of Hazelwood Avenue were identified as key stakeholders with early and continual engagement and communication applied on the project. The project was delivered on time and budget and delivered on consistently high Irish Water KPI and SHEQ scoring. Key partners in the project were Irish Water, Louth County Council, Bay Estate Residents, Hanna Hutchinson Consulting Engineers, AG Wilson (HPPE Welding), JC Enviro, Trench Control, Geodynamics, Causeway Geotech, Xylem Pumps and Alba De-watering.