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Limerick bypass on road to completion
22 November 2009 By Nicola Cooke
The €660 million Limerick city bypass is the largest infrastructure project in the country, with more than 600 people working to ensure it is completed next year.
The opening of the tunnel will have a major impact on traffic in the city and will further cut journey times between the south and the west of the country.
The tunnel, which is part of the Western Corridor route, is fully built, and electricians, plumbers and engineers are putting the finishing touches to it.
The two toll plazas - through which the tunnel can be accessed from the west side of Limerick city and fromtheN7 andN20 - are also complete, along with ten bridges crossing creeks and rail lines.
The bypass, which is the second phase of the Limerick south ring road, comprises almost 10 kilometres of dual carriageway, 2.3km of single-lane dual carriageway, a 750m causeway over Bunlicky Lake, the twin-bore immersed tunnel, ten bridges and six underpasses.
Direct Route, the consortium behind the tunnel, includes AIB, Meridiam Infrastructure, Sisk, Roadbridge, Lagan Holdings and Austrian company Strabag.
Work began on the bypass in August 2006, and its completion date is September 2010, but management hope it will open next summer. Direct Route owns the concession rights on the tunnel for 31 years from its opening, and will make the project profitable from tolls - which are likely to cost motorists €1.90 - during this period.
When the tunnel opens, it will have the capacity to handle 40,000 vehicles a day. It has already been inspected by EU officials to ensure that it adheres to strict conditions under the EU Tunnel Safety Directive.
Sophisticated systems in the tunnel detect temperature, liquid, spillages, fires, air quality and even the presence of humans or animals.
Information from a control tower on top of the tunnel will be fed into the mainline administration building, which will have 24-hour surveillance. An emergency response team will also be on call at all times for any incidents.
‘‘We are happy with the overall project and believe it will be of major benefit to traffic flow in Limerick and boost the region as a whole. It is also a testament to the skill and feats of engineering," Direct Route general manager Tom King told The Sunday Business Post on a visit to the site.
King explained how the tunnel was sunk into the Shannon, and the challenges encountered during construction. The 675-metre twin bore tunnel - designed by British-based Kapita Symonds - was constructed in a purpose-built dry dock area on the northside of the river.
Dutch dredging specialists Van Oord dredged out half a million cubic metres from the Shannon riverbed to make a channel - and pumped this into a holding area on the northside of the Shannon, which has formed a series of lagoons. The Shannon estuary is a particularly sensitive ecological area, and is also a Special Area of Conservation (SAC).
The lagoons ensure this dredged material does not make its way back into the river.
The tunnel is made up of approach ramps, cut-and-cover tunnels and an immersed tube tunnel. The tube tunnel is made up of five sections, which the immersion crew personalised and christened Liz, Grace, Sarah, Chantal and Brigid.
‘‘Each tunnel element was the length of a football pitch, the height of a two-storey house, 25 metres wide and weighed 22 tonnes," said King.
Following preparatory work, each tunnel element was then winched into position across the river ‘‘at a rate of 3 metres per hour, by hydraulic winches, positioned on both sides of the river," he said.
Shannon Foynes Port Company (SFPC) authorised the closure of the Shannon estuary to ships for 72 hours, for each immersion. Survey stations erected across the river allowed surveyors to identify the precise position (vertical and horizontal alignment) for each tunnel element immersion.
‘‘The tunnel immersions were completed about a year ago, but prior to then we did come up against some unexpected challenges," King said.
When the workers were doing soil investigations of the Shannon before construction, they found that the aquiclude, which is an impermeable layer of sediment that stops the river coming in on top of the workers, had been disturbed during the construction process.
‘‘We had to hire German mapping specialists to find the source of this, and then we got British grouting specialists to grout it. ‘‘It was all done very quickly," he said.
Another unusual aspect of the bypass construction is that the ‘mainline’ - or ten kilometres of dual carriageway that leads to the tunnel - is built on 95 per cent imported materials.
This material was used to form three-to-eight metre high embankments on the natural low-lying ground to elevate this ‘mainline’ to protect against the risk of flooding. According to King, the design criteria was for a one-in-10,000 year flood. Until recent times, designers would have catered for a one-in-100 year storm event.
‘‘It takes account of global warming. I guess it’s becoming patently clear to world scientists that that phenomenon is becoming a certainty that must be catered for," he said.
The road building on the southside of the tunnel is ‘‘95 per cent complete’’ - the black-top bitumen coating will begin in the new year. Progress on the northside is less developed, because embankments there have yet to be fully settled.
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