The M17/M18 Gort to Tuam PPP Scheme forms part of the Atlantic corridor outlined under Transport 21 (Transport 21 is an Irish infrastructure plan). The scheme involved the design and construction of a new road to replace the existing N17 and N18 between Gort and Tuam in County Galway in the West of Ireland.
The 53.2km motorway standard road commences at Gort and terminates at the Kilmore Roundabout, just south of Tuam. From Tuam the road is a 4.2km Type 2 dual carriageway which bypasses the town and joins the existing N17 at Mountpotter.
The M17/M18 accommodates national through traffic flows and has significantly improved safety, reduced journey times for traffic travelling north-south along the West of Ireland, improved connectivity between the Western Region and Dublin and is significantly assisting in the economic development of the BMW (Borders, Midlands and Western) regions.
This scheme implemented the most recent versions of the National Roads Authority (NRA) Design Manual for Roads and Bridges (DMRB). The DMRB embodies the collective experience of the Overseeing Organisations, their agents and design organisations over many years. Environmental protection and sustainability of materials particular to this site are detailed below.
Soft ground comprising peat, white calcareous marl and blue lacustrine clay was present along the Northern section.
By careful examination of the test data and analysis of published material on the calcareous marl, it was judged that band drain, staged construction and surcharge techniques could be accommodated within the construction programme.
An iterative methodology was developed for the specification of each load cycle based on a predicted porewater pressure response to load, consolidation settlement and resulting strength improvement.
Construction involved using 5 No. 700mm lifts plus surcharge, with hold periods of three months, and a total construction period of 1.5 years. By leaving the calcareous marl in situ it reduced the volume of material requiring disposal by 500,000m3.
Approximately 200,000m3 of peat requiring disposal was present along the M17/M18. To dispose of this material and reduce its volume, initially a haul road was excavated. This acted as a drain and was pumped constantly.
The in situ and side cast peat was allowed to dry which reduced the volume of peat by approximately one third before the material was removed to the peat bund. Within the bund the material was drained, again reducing the volume of peat, the height of the bund reduced from 4m to 1.5m.
Specialised landscaping which binds the peat, fertilizer and lime pellets were also used to reduce the pH and promote growth, again improving the stability of the peat. The peat bunds could be trafficked before road completion.
The overarching objective of the Atlantic Economic Corridor was to maximise the region’s assets and connect economic hubs, clusters and catchments of the region to generate a value proposition of scale which would attract investment and support job creation, improving the quality of life for those who live in the region. Improved rural connectivity is a pillar of the Government’s Action Plan for Rural Development and this is exactly what the M17/M18 delivers.
Connectivity is essential in the development of rural communities – the provision of a high quality, inter-connected system of transport in rural Ireland is key to realising the social and economic potential which exists in rural areas. Improving rural infrastructure and transport links maximises the potential of our rural communities. The new road has also dramatically improved connectivity and reduced journey times between the Western Region and Dublin.
The Nanny River is a Special Area of Conservation (SAC). The Environmental Design Report included the following restrictions to design & construction: No instream works to take place between June and September. The existing riverbank was retained to allow for the passage of mammals beneath the bridge.
Our team ensured that the existing flow regime and channel dimensions were maintained for each watercourse to mitigate any possible effect on fish and river ecology. The design ensured, even in the most difficult ground conditions, that the river was maintained undisturbed.
Large rock cuts throughout the project allowed for the re-use of the rock material in flood embankments and as road construction material where rock was crushed to adhere to the grading envelope of various class 6 materials and Clause 804, on this project, rock was also crushed to produce the aggregate in the cement bound materials (CBM), the CBM batching plant was on site which reduced the impact on the existing road network.
Ireland’s first ‘green bridge’ was constructed over Galway’s M18 motorway in a bid to conserve an internationally important colony of Lesser Horseshoe Bats.
As part of the agreed environmental impact mitigation measures, a number of wildlife-friendly features have been incorporated into the design of the M17/M18 motorway project.
The design ensured, even in the most difficult ground conditions, that the river was maintained undisturbed.
Karst is an area of land made up of limestone. Karst landscapes feature caves, underground streams and sinkholes on the surface. Where erosion has worn away the land above ground, steep rocky cliffs are visible.
The M17/M18 was an extremely complex project due to the sheer number of karst features present in east Galway, soft ground, the variety of structures, drainage challenges presented by hydrogeology, the regionally sensitive aquifers and seasonal flooding, environmental commitments and the construction of Ireland’s first Motorway over a live Motorway.
Rathmorrissy Junction is a three-level interchange between the M17/M18 and M6 motorways. It comprises of free flow links between the M17/M18 as it passes over the M6 with a circulatory carriageway in between which is approximately 1km in length and is Ireland's largest roundabout.
The location of the interchange and the proposed geometric arrangement presented challenges in minimising material import and mitigating karst and flood risk whilst constructing in an environment with significant live services over a live motorway. In addressing the material import challenge, the design looked to select the best junction geometry and shape for the circulatory carriageway.
Multiple design iterations, looking at various shapes were conducted before the selection of an oblong arrangement. This shape allowed the lowering of the alignment to reduce embankment heights and the removal of sharp radii across structures allowing their widths to be reduced.
There are six junctions along the scheme: