Leixlip to Saggart Pipeline

The provision of services to our Client included definition of tender scope and management; specimen design and coordination of design submissions. All planning, archaeological and environmental constraints were required to be taken into consideration allowing for archaeological investigations and protection of wildlife habitats. Ryan Hanley was tasked with preparing a construction stage traffic management plan such was the potential impact of the construction stage traffic movements and the sensitivity of the existing highly trafficked road network.

The greatest civil design challenge was the crossing of the N7 at a location with six live carriageways. Based on extensive assessment of the crossing point options the optimum crossing location was identified which required installation of the pipeline under the N7, a major commercial enterprise and the R120 regional road. Based on the results of geotechnical investigation works undertaken, micro-tunnelling was identified as the most appropriate no-dig technology for the location and following this decision a specimen design was prepared which identified the requirement for micro-tunnelling of two 168m long, 1,800mm diameter tunnels in order to cross under the N7 dual carriageway, a major commercial property and the R120 road. A 1,200mm pipeline was then sleeved inside each of the 1,800mm tunnels.
Micro-tunnelling slurry recycling measures were designed so that the environmental impacts of the micro-tunnelling process could be minimised, in addition to which a construction environmental management plan was prepared which set out requirements for the disposal of surplus excavated materials off-site. This section of works also included two 11m wide x 11m breadth x 9m deep control valve chambers, one at the launch and one at the reception location of the trenchless section.

Accompanying works included design for scouring of the pipeline when no discharge to the environment along the pipeline route was permitted due to flood risk in the River Camac flood plain. The design solution for this comprised a combination of the new pipeline and a 1,026m long 300mm diameter scour pipeline. The additional benefit of this design element was to provide the flexibility so that the 1,200mm pipeline could be used to supply in the reverse direction to that of the design brief, thereby increasing the flexibility of the supply options provided by the Project.

Other complexities, involving extremely high groundwater levels along the route, added the requirement for dewatering while issues with the availability of the substantial volumes of potable water required for testing and commissioning also brought their own challenges.

Other design elements involved:

• Interaction with the Department of Defence, Irish Defence Forces and Irish Air Corps required specialist aviation input;
• The preparation of Wayleaves and Right of Way plans and reports in accordance with Uisce Éireann Standards;
• Detailed reports for presentation at an An Bord Pleanala oral hearing;
• The management of communication between South Dublin County Council, Dublin City Council and Uisce Éireann.

Comprehensive site management, supervision and consultation practices were engaged for the duration of the project aiding in effective communication with impacted parties along the route and external stakeholders.