The N4 Collooney to Castlebaldwin is a 14.7km highway project, developed to replace the existing substandard single carriageway. The project consisted of 13.8 km of Type 2 dual carriageway; 0.9km of greenfield single carriageway; 12 structures including two No 20m span river bridges, four overbridges and six underbridges; two roundabouts and one new compact grade separated junction.
This case study focuses on the use of two drainage solutions adopted as part of the project that reduced the cost of construction and mitigated the impacts of the project on the environment.
Opportunity and challenges
In addition to being cheaper than traditional concrete systems, the adoption of SuDs systems and principles can be of benefit to local hydrology, hydrogeology and ecology receptors and the systems have a lower carbon footprint. The EIS design for the N4 project consisted of grass surface water channels in cuts and concrete channels on embankments.
However, in the time since the EIS had been published, the use of lined grass channels, which are sometimes used in areas of groundwater vulnerability and require a departure, had fallen out of favour with Transport Infrastructure Ireland (TII). Recently completed highway projects in Ireland have predominantly used concrete drainage systems to collect and convey surface water runoff.
The challenge in respect of the drainage design was to maximise the use of SuDs on the project in order to accrue the associated cost, constructability and environmental benefits which come with them.
Design and delivery
To maximise the use of SuDs systems on the scheme, two innovative drainage collection systems were adopted.
1.) Lined grass surface water channels
On the N4, the potential to use standard grass surface water channels, which are considered to be an unsealed system, was limited due to concerns in respect of aquifer vulnerability and the risk of karstification.
A departure from standard was applied for from TII to adopt a bespoke lined grass surface water channel detail developed for the scheme. While these systems had been adopted on other highways schemes, they had fallen out of favour with TII after investigations revealed that surface water was seeping through or past the systems.
In order to address this issue, a construction detail and methodology was developed, which ensured that the impermeable liner associated with the grass channels was sealed to the pavement using a bitumen adhesive.
The safety barrier design was also adapted to ensure that barrier posts would not be driven through the liner. In order to ensure that the system was sealed, where it was intercepted by drainage chambers, the surface water channel outlet areas were concreted and the liner was carried through into the chamber.
Following the acceptance of the departure application, the conceptual design was updated and resubmitted for approval and the detailed design was subsequently carried out.
Prior to construction, a number of trials were conducted to arrive at the best methodology and the favoured solutions for bonding the liner to the pavement. Non-destructive testing was carried out on the preferred solution, which was ultimately adopted in the works. Figure 1 below illustrates the construction detail adopted.
Figure 1: Lined Grass Surface Water Channel Detail
2.) Linear wetlands
Where the road was on embankment and grass surface water channels could not be used due to restrictions within the design standards, ‘over the edge’ drainage was typically used. Using this system, road surface water runoff drains into linear wetlands positioned at the base of embankments.
The wetlands are used to collect, convey, treat and attenuate surface water runoff. The linear wetlands typically followed the gradient of the local topography but incorporated check dams along their length to increase their capacity to attenuate flow.
The wetlands were used in areas where it had been envisaged that concrete surface water channels and associated pipe systems, which connected to traditional attenuation ponds, would be required.
In areas where it was necessary to provide sealed drainage in order to mitigate the risk of groundwater contamination or karstification, testing was carried out to ensure that the in-situ permeability of the ground, where the wetlands were placed, was sufficiently low to allow it to be considered to be sealed.
The approach adopted avoided the need to excavate out the in-situ impermeable material and replace it with plastic liners, which have a relatively short life span.
A variant conceptual design and associated technical report was developed to explain the change in the approach and the benefits that it would bring to the project. Following the acceptance of the alternative concept, the drainage design basis statement was updated, and the detailed design was progressed.
All design elements were subject to independent third-party check and subsequent acknowledgement from the Employer’s Representative. Figure 2 below provide an illustration of one of the completed wetlands
Figure 2: Example of Linear Wetland
Positive impact
The adoption of the systems described resulted in the removal of 11km worth of concrete channels, which had been envisaged, along with 9.7km of pipes and associated chambers. In addition, because the linear wetlands follow the local topography there is less ‘dead volume’ associated with them than is the case with typical ponds. The use of the systems reduced attenuation related earthworks by 76,000 m3.
In addition to the cost and programme savings of these changes, there are a number of environmental benefits which go hand in hand with the adoption or increased adoption of SuDs principles, as outlined below:
- Water
- Improved water quality;
- Reduction in the rate and volume of storm water runoff at outfalls;
- Improvements in existing catchment integrity post-construction.
- Ecology – Increase in the provision of habitat for flora and fauna;
- Climate – Reduction in the carbon footprint associated with concrete channels, pipes, chambers, the provision of drainage stone and all associated transport and construction works.
Impact on earthworks
On the N4 project, the adoption of the linear wetlands also had a beneficial impact which went beyond the drainage works. The use of linear attenuation systems facilitated alignment modifications which would not otherwise have been possible as there was no space to provide traditional attenuation ponds once alignment changes had been made.
The changes to the alignment ultimately allowed structural embankment volumes to be reduced by about 50% (1.59 Mm3 to 0.76 Mm3) from what was envisaged at the environmental impact statement stage. The reduced embankments footprint also:
- Reduced the requirement for excavation and replacement of soft materials below ground;
- Removed the requirement for materials to be imported to construct embankments;
- Afforded some flexibility in respect of the horizontal design, which allowed embankments to be moved away from areas of poor ground.
Additionally, the provision of attenuation systems along the base of the embankment freed up space which had been designated for traditional ponds. This space was used to store unsuitable material generated by the scheme, which ultimately assisted in removing the requirement to export materials.
Figure 3 below illustrates one location where a number of these measures combined to make significant savings.
Figure 3: Before and After Comparison of Earthworks and Drainage at Ch. 10+700.
Legacy for other projects
The successful adoption of lined grass surface water channels and the use of linear wetlands within areas which need to be sealed opens the door to the broader use of SuDs concepts on future projects and can be the catalyst for reversing the trend seen on other recently completed projects.
Authors: Seán FitzSimons is a Chartered Engineer and Fellow of Engineers Ireland (CEng FIEI). He is a director with Clandillon Civil Consulting and was project manager for the design team, which consisted of Fehily Timoney, Clandillon Civil Consulting and Byrne Looby. He has more than 16 years’ experience in the development of infrastructure projects and has undertaken roles as part of the contractor’s management team and as a design manager on a number of prestigious projects, including the N17/N18 Gort to Tuam scheme; the N9/N10 Knocktopher to Powerstown; and the M3 Clonee to Kells. John Duggan is a Chartered Engineer (CEng MIEI) and a director with Roadbridge Ltd. He has more than 27 years’ experience in the construction of major civil engineering projects both in Ireland and UK. His recent projects include N17/N18 Gort to Tuam PPP Scheme, new North Runway at Dublin airport and Center Parcs (Longford Forest). He was contract manager for the N4 Collooney to Castlebaldwin project.