Contractor Nishimatsu/ Mowlem/Irishenco (NMI) is busy gearing up to start work on Ireland's largest ever civil engineering project, won in the face of stiff UK and international opposition at a contract price of IR£353M (£282M). The Dublin Port Tunnel is expected to take around three and a half years to complete.
The aim is to provide a vital new artery for Dublin's crippled traffic infrastructure. Molly Malone may have 'wheeled her wheelbarrow through streets broad and narrow' as the city's anthem tells us, but little has been done to those same innercity streets since her cry of 'cockles and mussels' echoed though them in the distant past.
Trundling through those streets now are 9,000 juggernauts a day, proceeding often at no more than respectable wheelbarrow pace thanks to Dublin's chronic traffic congestion.
Massive growth has doubled port trade in five years to reach 20Mt last year. With the country's main port located almost at its centre, the tunnel and motorway will provide a dedicated route for port traffic to the M1 and connecting with the orbital M50.
Although mooted a number of years earlier, formal approval for a tunnel scheme was not given until October 1994. But the project has not been without controversy, even before a sod has been turned on site.
The route between the port and the M1 passes through suburbs. Initial plans envisaged an alignment which would skirt past built-up areas. But the scheme was rejected on cost grounds, forcing a rethink by Dublin Corporation's engineers and its consultants. A new, straighter alignment and adoption of the New Austrian Tunnelling Method brought costs down to £78M at 1994 prices.
Ironically, the approval for the scheme was granted just the day before the Heathrow tunnel disaster, which was to provoke the most vigorous technical debate within civil engineering for decades, and provided some uncomfortable lessons on the management of large scale civil engineering works. By that time, the Heathrow consultant, Austrian firm Geoconsult was well ensconced in Dublin, having been engaged by the corporation on pre-tender design and advice in joint venture with Ove Arup.
Few people initially made thelink between Heathrow and Dublin, least of all the 350-odd households in the suburb of Marino under which the tunnel would run. Fury erupted when residents eventually learned of the plan and the NATM connection to Heathrow and Geoconsult, later found guilty of safety breaches at Heathrow which the judge said could have led to catastrophic fatalities.
Work was due to start in 1997 but residents' concerns escalated into a vociferous campaign. An anticipated start in late 1999 was delayed, while major changes to the scheme were put in place to allay residents' fears, aired at public inquiries, about damage to their properties or worse. Principal among the changes was a decision to locate the tunnel deeper, from a mooted 10m below ground in 1996, to double this.
Five consortia tendered. The lowest and winning bid, despite being double the final estimate, confirmed the government's commitment to improve the infrastructure and abandoned NATM. New conditions on the contractor, stemming from residents' fears, combined with lessons learned from Heathrow, helped push up the cost, as did additional works and general Irish construction inflation.
'Normally a tunnel boring machine wouldn't be competitive with drill and blast, but because of tight, strict specifications for vibration, noise and other environmental effects, it was clear to four of the consortia that TBMs would give a competitive advantage, ' says Dr Gerald Bonner, the man heading up the joint Dublin Corporation/ National Roads Authority client team. 'The solution proposed by the successful tenderer involves use of two TBMs which minimises disturbance to local residents, and speeds up construction.'
'If you specify a modern tunnel with the required safety standards and management systems then you must expect the price to increase, ' says Bonner.
The NMI consortium plans to use two custom-built openface Herrenknecht TBMs to eat their way through the boulder clay.
Due to arrive later this year, the 11.8m diameter machines will each have two working platforms, three arm excavators to deliver the spoil, and seven breasting plates for face support. A probe drilling facility on the shield gathers data prior to drilling when soft ground is encountered.
The lining will consist of a precast ring of six 700mm segments and a key unit, each 1.7m long with injection ports for grouting.
Consultant Charles Haswell is the consortium's tunnelling designer, with the key role of highway and traffic management - given the location of the cut and cover section alongside busy roads - taken by consultant Charles Bro.
Discussions are under way on the technical details of a proposal by the contractor to lengthen the bored section. This would have the benefit of causing less disruption to residents.
It is planned to launch the TBMs from a pit midway along the bored section. One machine will drive south towards the port for nine months followed by a two month turnaround, finally driving north to complete the second bore. The other machine will follow the same pattern but driving northwards.
Cross-passages will be excavated using non-explosive Cardox for the first time in Ireland.
This disintegrates the soil with rapid expansion of carbon dioxide, causing little disturbance.
The bored tunnels, passing under 315 residential properties and 13 other buildings, will be 2.4km long, with the tunnel crown 19m-24m below ground level.
A further 2.1km stretch of cut and cover along existing road and through Fairview Park and 1.1km of surface road make up the scheme's overall 5.6km length. The ground conditions are primarily stiff boulder clay, with carboniferous limestone beneath the residential areas.
The conditions of contract are a modified form of the ICE Design & Construct for use in Irish law, with a reversed risk 'clause 12' where the contractor rather than the client takes the risk from unforeseen ground conditions. 'We have done very detailed site investigation and modern tunnelling techniques give the contractor all the reassurance they need, ' says Bonner.
Shouldering the ground condition risk may have helped put the contract cost up too, but should avert contractual wrangles and help work to run smoothly. 'It is a three-stage design process, with the contractor submitting preliminary design work first, then calculations followed by detailed design. We hope to fast track the process where appropriate, such as aspects on the critical path, which will help to complete the work on schedule. We would like to work in a similar way to the 'teaming' approach used with the contractor on Oresund, ' Bonner adds.
As for any local fears regarding Heathrow: 'There are major differences regarding what happened at Heathrow and what we propose to do, ' he says. 'The ground conditions are completely different, as is the way the project's been designed, incorporating a safe design for the ground conditions we've got here. We've placed great emphasis on strong project management of the work.' Management failures blamed at Heathrow will not happen again and a separate site supervision contract will be let to monitor the quality of work and safety on behalf of the client.
The tunnel is expected to be completed in 2004, and will carry 20,000 vehicles daily on opening, expected to rise to 31,000 by 2018. It will also connect to a new motorway along the south east, completing the link to the M50 further south. Tolls will apply, except to HGVs.
Bonner, a native Irish speaker from County Donegal, is looking forward to renewing acquaintances with some of his fellow countymen, in particular the 'Donegal Tigers' who have bored their way through many of the UK's tunnels from the Victoria Line to the Channel Tunnel and Jubilee Line. It should prove a pleasant surprise to those who come back to tunnel through Irish soil that the top tiger is one of their own.