Bunlicky Lake and River Shannon at commencement of project
The Limerick Tunnel forms part of one of the largest and most important construction projects undertaken in mid-west Ireland to date. At an estimated project cost of around €660m, this major piece of infrastructure development is being carried out under the Irish Government’s Transport 21 Programme, and it will complete the Southern Ring Road around Limerick city while also forming a critical link in the Government’s proposed Atlantic Corridor on the west coast of the country.
On completion, this landmark project will remove some 27,000 vehicles per day from Limerick city centre, rising to an estimated 40,000 by 2028. The consequent relief of traffic congestion in the city will improve travel times, access for local and through traffic, the latter to and from Dublin, Cork, Galway and Shannon Airport. The reduced city centre traffic will also increase the safety for both motorists and pedestrians and will allow increased pedestrianisation and use of bus and cycle lanes in the city, while improving air quality by reducing emissions.
Construction started on the Limerick Tunnel Scheme in August 2006 and is due for completion in September 2010. The new road is Phase 2 of the Limerick Southern Ring Road and connects from Phase 1 (the N7) at Rossbrien on the South side of the city to the N18 Ennis Road at Cratloe Castle on the west side of the city. The 10km of new dual carriageway includes a 675m twin bore immersed tube tunnel crossing of the River Shannon, a 750m rock causeway across Bunlicky Lake, two toll plazas, four grade separated junctions, ten bridges and six underpasses, along with a further 2.3km link road to the north side of the city.
The immersed tube section of the tunnel is 500m long (with portal sections of 115m & 60m constructed on each bank of the river) and has been pre-cast as five 100m elements in a 550m long purpose built casting basin on the north side of the river at Coonagh West. In Autumn 2008, the casting basin was flooded and the five (temporarily sealed) tunnel elements were floated out one by one into the Shannon for immersion in a pre-dredged channel across the river. The remainder of the construction project will focus on the tunnel’s northern portal & approach ramp, the mechanical and electrical installations, construction of two toll plazas, plus a six span viaduct at the north end of the project and completion of road pavement and surfacing work.
Most of the project length lies in the deep alluvium of the Shannon’s flood plain and tributaries. Much of the project’s first year was taken up with excavation in this poor ground and major temporary works required to create the huge casting basin for construction of the tunnel elements and the deep cofferdams needed for dry construction of the cut & cover sections on each bank of the river.
The alluvium has also created a number of other geotechnical challenges for the project, not least the disposal of large quantities of excavated or dredged unsuitable material, and importing over three million cubic metres of suitable material to construct most of the carriageway on embankment above the flood plain of the River Shannon. Where the alluvium is too deep for removal and replacement, extensive use has been made of vertical drainage installed below embankments which were surcharged, for up to two years in some cases. The tunnel approaches, eight of the ten bridges and both toll plazas are also founded on steel piles driven through the alluvium.
In respect of the natural environment, the project passes through a number of designated areas including Bunlicky Lake, a proposed Natural Heritage Area (pNHA), and the Shannon Estuary including three tributary creeks, which are all part of the Lower Shannon candidate Special Area of Conservation (cSAC). The Shannon Estuary is recognised as a particularly important location for birds and is also covered by a Special Protection Area (SPA) designation for much of its length – including the crossing point of the tunnel. Great care has therefore been required from both Designers and Contractors to minimise adverse effects of the project works and to ensure that the stringent requirements of the EIS are met. This has involved constant liaison with the National Parks & Wildlife Service (NPWS) and Shannon Regional Fisheries Board (SRFB) and other relevant groups. Particular care had to be taken when constructing the rock causeway across Bunlicky Lake on the southern approach to the tunnel in 2007.
Vegetation surveys on the route of the new carriageway and tunnel also established the presence of three protected plant species, for each of which particular protection or relocation measures were required. One of these species, growing on the tidal fringes of the Shannon and three of its tributaries, is the Triangular Clubrush (Schoenoplectus Triqueter) which is protected under the Flora Protection Order 1999 and has been temporarily translocated to purpose built lagoons for the duration of the works.
The Limerick Tunnel Project is a Public Private Partnership (PPP) Scheme between the National Roads Authority and the Concessionaire DirectRoute (Limerick) Ltd. each of which has partly funded the project. DirectRoute is not only responsible for Design and Construction of the tunnel and associated roads, but also Operation and Maintenance of the facility for a 35 year concession period (until 2041).
RPS, in a joint venture with COWI, has been the Client’s Consultant since 1999 and has brought the scheme through the constraints, route selection, preliminary design, EIS, Oral Hearing, specimen design, PPP contract documentation, procurement, planning and advice phases – as well as project management for the client National Roads Authority (NRA) and its partner Local Authorities led by Limerick County Council.
RPS in its Joint Venture with COWI continues to be engaged by the NRA on this project, and represents the Authority’s interests on site by leading the Works Monitoring team during the Design and Construction Phase of the Project. The RPS and COWI teams also review the design and other submissions produced by the Concessionaire’s team, thus collectively ensuring that the project is designed and constructed as required by the client. The RPS team is lead by Commission Manager, Angela Grady, ably assisted by Senior Engineer, Claire Glynn in the Dun Laoghaire office, and Tom Bryson acts as the Authority’s Site Representative in Limerick.
Tunnel element being floated out into the Shannon for immersion