Summary
• The tunnel under the River Shannon is 675m in length.
• 15,000m² of 18mm OSB were used as a base for the concrete tunnel sections.
• They will be sunk, with the tunnel structure, onto the bed of the river.
• The tunnel forms part of the Limerick South Ring Road project, which is due for completion in 2010.
Costing more than €500m, the construction of Limerick Tunnel beneath the River Shannon is one of the largest engineering projects undertaken in western Ireland.
The two-bore tunnel is designed to take up to 40,000 vehicles a day and alleviate the traffic in Limerick city centre, as well as bring a vital transport link for the socio-economic wealth of the area.
As part of the construction process, project engineer Capita Symonds specified a flat timber surface on which to construct the five tunnel sections, before they are floated out into the water and sunk into a pre-dredged channel in the river bed.
This necessitated a highly engineered product that would be able to support the 20,000-tonne sections and eliminate deviation, as a difference of just a few millimetres in the concrete casting would cause problems later in the build.
Flat and stable surface
German construction company Strabag International GmbH tested a range of panel types before settling on SmartPly OSB to give it the flat and stable surface required, with the tests approved by Capita Symonds.
“The structural capability of the product was vital,” said Strabag’s commercial manager Ulrich Hoeft. “In this tunnel base application, SmartPly OSB met all our needs in respect of both our quality and cost requirements.”
Local builders merchant Heiton Buckley Ltd supplied the 15,000m² of 18mm OSB used in the build and the product was attached using stainless steel screws positioned upwards into the concrete base, giving a solid and precise surface on which to build the tunnel.
The tunnel itself is being pre-cast in a 600x33m dry-dock casting basin at Coonagh, north of the river. This will be flooded in early September to allow the flotation of the sections into the correct position. The panels will then be permanently submerged with the tunnel and, having fulfilled its function, the non-toxic material will slowly and harmlessly degrade as motorists drive overhead.
“We have been impressed with SmartPly’s technical performance and intend to use it again on our future projects,” said Mr Hoeft.
The building of the tunnel is the second phase of the Limerick South Ring Road project, which will connect the N7 (Dublin Road) to the N20/N21 (Cork Road) to the south of the river and the N7 (Dublin Road) to the N18 (Ennis Road) near Cratloemoyle Castle in the north.
The finished second phase will provide 9.75km of two-lane dual carriageway, 2.3km of single-lane dual carriageway and a 750m causeway across Bunlicky Lake, as well as the tunnel.
In the long term it is hoped this will reduce CO2 emissions, improve air quality and allow for the development of pedestrianisation and public transport in Limerick city centre.
Other applications
The use of SmartPly OSB as load-bearing sheathing in the construction of Limerick Tunnel is the latest innovative application of the material.
Other recent uses include a contemporary art installation at the Fabrica gallery in Brighton and a project with Greenpeace providing environmentally-friendly showers at the annual Glastonbury music festival.
