The design and construction of Rathmorrissy Junction: An analysis
17 April 2018
Design co-ordinator Seán FitzSimons of Clandillon Civil Consulting and Noel Curtis of John Sisk provide their insights into the design and construction of the interchange at the heart of the scheme
Rathmorrissy Junction – three-tier junction linking the new N17/N18 Gort to Tuam motorway with the M6 Dublin-Galway motorway
Following the recent opening of N17/N18 Gort to Tuam motorway, design co-ordinator Seán FitzSimons of Clandillon Civil Consulting and Noel Curtis of John Sisk & Sons (Holdings) Ltd provide their insights into the design and construction of the interchange at the heart of the scheme.
Project and junction background
The N17/N18 Gort to Tuam motorway consists of a 54km motorway which runs from Tuam in the north of Co Galway to Gort in the south, where it connects with the existing motorway network to Limerick. The scheme, which was procured as a Public Private Partnership (PPP) and constructed by DirectRoute (Tuam) Ltd, is part of the Atlantic Corridor route which aims to link up the western coastal cities of Galway, Limerick and Cork by motorway.
The interchange at the heart of the scheme, Rathmorrissy Junction, is located approximately 20km east of Galway city centre and links the new M17/M18 motorway with the existing M6 Galway to Dublin motorway. Given the existing east, west and southern railway connections already in place, the completion of the junction and the associated motorway will see the east Galway region including local towns, such as Oranmore and Athenry, become one of the best-connected areas in Ireland.
Rathmorrissy Junction is a three-tier junction. It consists of the existing M6 at its base, a new 1km long circulatory carriageway roundabout, which passes over the M6, and the new M17/M18 motorway, which passes over both the existing M6 and the new roundabout. The junction consists of three motorway overbridges, two motorway underbridges, four free-flow link roads and eight slips roads. More than 500,000m3 of granular fill was used in the construction of the associated embankments, which are up to 12.5m high.
Some of the challenges, which were managed by the design and construction teams, included:
• The minimisation and sourcing of materials for the embankment construction;
• The presence of live underground and overhead services;
• The presence of live traffic on the existing M6;
• The presence of karst limestone in the area; and
• An absence of surface water outfalls.
Junction design and construction
The scope of works undertaken by DirectRoute (Tuam) Construction Ltd included the detailed design of the scheme, which was carried out by CH2M Barry ARUP JV. Principal elements of the junction design included the design of the geometric layout, the design of the five associated structures and the geotechnical and drainage design. The primary goal of the design was to provide a functional and compliant design, which minimised the risk to construction workers and future road users and made the optimum use of materials and natural resources.
Oblong shape for central roundabout chosen
The design commenced with the development of the junction geometry, and the shaping of the central roundabout played an important role in meeting the design objectives. Following the review of several options, an oblong shape was selected. The oblong shape of the roundabout had two significant advantages over the conceptual true circle shape.
First, while the two high points of the roundabout, which are located over the existing M6, were relatively fixed, ‘stretching’ the roundabout from a circular shape to an oblong shape allowed the level of the two roundabout low points located under the new motorway to be lowered by 1.02m. This in turn allowed the level of the new motorway through the junction to be lowered, which reduced the volume of embankment materials required for the mainline and slip road construction by more than 100,000m3.
Second, the change to an oblong shape allowed the alignment of the roundabout to be straightened where it crosses the existing M6. As a consequence, the artificial widening for visibility around the roundabout, which had been required because of the tighter radii associated with the circular shape, was removed and the two bridges were reduced from 19m-wide bridges to 14m-wide bridges.
Minimising embankment earthworks by minimising structural depth
Like the geometry design, the structure’s design took a holistic approach for the three overbridge crossings of the existing M6 and the two underbridge crossings under the new M17/N18. This approach sought to provide a robust and efficient structural solution, while also seeking to minimise embankment earthworks by minimising the structural depth.
To optimise the design programme, the sets of underbridges and overbridges were developed as mirrors of one another.
The adopted solution for the roundabout overbridges consisted of fully integral single span structures supported with reinforced concrete skeletal abutments on pad foundations. The structures spanned approximately 34m and the superstructure comprised of pre-stressed, pre-cast concrete beams (W10) with an in-situ reinforced concrete deck slab.
The structural solution for the underbridges also consisted of single span options supported on reinforced earth retaining walls. The underbridges spanned 25m and used TY4 pre-stressed pre-cast beams. Where the new motorway crossed the existing M6, a three-span structure was adopted.
The construction of the two roundabout overbridges played an important role within the construction programme. Approximately 350,000m3 of embankment associated with the junction and a further 150,000m3 of mainline embankment lay between the existing M6 and the main Dublin-Galway rail line 2km south of the junction.
Since this portion of the route was effectively land-locked, it was necessary to complete the bridges over the N6 before the surplus material which was excavated to the north of the junction could be transported south and the new workfront opened. To cater for this programming constraint, the two roundabout overbridges were prioritised and subsequently opened as haulage routes.
In the photograph, right, the western roundabout overbridge has been opened to construction traffic and the embankment works on the southern portion of the junction are progressing, while embankment works in the north of the junction are well progressed.
Presence and extent of karst features in the region
The geotechnical design of the junction commenced with a thorough site investigation. Given the site is in an area where the underlying bedrock is predominantly limestone, the site investigation works were particularly interested in examining the presence and extent of karst features in the region.
Based on the results of the site investigation within the junction and across the entire motorway, a range of solutions was developed to deal with karst features which were discovered. The measures varied to take account of different karst features sizes and scenarios. In addition to the details and geotechnical layouts, which were developed following the site investigation, a site-based protocol was put in place to cater for the discovery of any unidentified karst features discovered during the works.
The geotechnical design also played a part in minimising both the extent of earthworks and the requirement to import materials to complete the embankment works. Given the extent of granular material, which was generated within the several rock cuttings along the scheme and reused at the junction, it was possible to design embankment side slopes at 1V:1.5H, rather than at 1V:2H, which is more typical for embankment construction with Class 2 cohesive materials.
The early classification of material along the route also assisted in determining how much suitable rock material could be reused to form embankments and fed into the development of a mass-haul schedule which was used to control the earthworks.
Where feasible, on smaller embankments, slopes were graded back to 1V:3H or 1V:5H with landscape fill material to provide a passively safe environment for road users and avoid the need for safety barriers.
Development of drainage design particularly challenging
The development of the drainage design within the junction was particularly challenging. Issues which had to be overcome included the presence of a high ground water table, a lack of surface water outfall locations and differing drainage requirements to the north of the junction versus those to the south of the junction – a function of the scheme having been initially developed through two separate planning processes. The drainage solutions adopted included a combination of kerb and gully and slot drain systems, which connected to sealed carrier drains.
In respect of the outfalls, as much of the runoff from the junction as possible was diverted away from the junction to outfalls to the north and south, where the problem with the high ground water table was less pronounced. For the remainder of the water collected from the junction, a design was developed that channelled the water towards the internal quadrants of the roundabout.
The ponds within the roundabout footprint consist of both attenuation ponds and infiltration ponds. On the south side of the junction the water is first held within an attenuation pond and gradually released into the infiltration pond to the west of the new motorway. Specific construction details were developed to ensure that the infiltration system employed mitigated the risk of karst features developing, along with any risk to the groundwater.
Minimal interference required with overhead HV lines and underground gas main
One of the success stories associated with the construction of the junction was the minimal interference that was required with the overhead HV lines and the underground gas main located within the junction footprint.
Although these utilities had to be considered during the design and construction phases of the works, their original diversions, carried out during the construction of the M6 motorway, had taken account of the future provision of the M17/M18 motorway. As a result, their presence placed minimal restrictions on the design solutions developed and the construction operations.
A further significant achievement is that, notwithstanding the construction of three bridges over the existing live motorway, and the construction of the eight adjoining slip roads, traffic flow was maintained throughout and there were no reportable incidents.
The N17/N18 Gort to Tuam PPP was opened on September 27, 2017, five months early, having first been tested by more than 2,000 cyclists who took their one and only opportunity to cycle the route. The cyclists have now been replaced with cars and HGVs, which can drive uninterrupted from Tuam to Limerick in a journey time of approximately 75 minutes, saving more than 30 minutes and improving journey reliability significantly.
With plans being put in place to develop the 80km M20 Cork to Limerick scheme in the medium term, it may not be long before the Atlantic Corridor is complete between Tuam and Cork. In the meantime, the completion of the M17/M18, with Rathmorrissy Junction at its heart, provides the east Galway region with a significant advantage over its near neighbours and a unique opportunity.
Authors: Seán FitzSimons, an associate director with Clandillon Civil Consulting, is a chartered civil engineer with 13 years’ experience in the planning, design, construction and administration of major construction works. He has worked both as a design manager and as part of the contractor’s management team 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.
Noel Curtis is a senior construction manager for John Sisk & Son (Holdings) Ltd and was responsible for the safe delivery of the central section of the N17/N18 Gort to Tuam scheme on behalf of Direct Route Tuam (Construction) Ltd. He has previously overseen the construction of major infrastructure works in the UK and Ireland including elements of the Crossrail scheme in central London and the N9/N10 Knocktoper to Powerstown in Co Kilkenny.