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Too much talk, too little action

Tunnelling - Dublin Port Tunnel

Construction of the Dublin Port Tunnel is entering its final and most tricky stages. But when will it be finished? Mark Hansford reports from the Irish capital.

The Irish are known to like a bit of talk. Indeed it was Oscar Wilde who said 'If one could teach the English to talk and the Irish to listen, society would be quite civilised'. So when it came to building Ireland's largest ever civils project directly beneath its capital, the engineers involved must have known they were never going to be left to get on with it.

And so it has proved. Dublin Port Tunnel (DPT) is intended to provide a direct link between Dublin Port and the M1 motorway, removing trucks from the city centre (see box). Right from the start, the project has faced challenges from residents, the state railway company and even government ministers.

All this has contributed to the tunnel's present situation - certain to be late and with questions still hanging over whether it should be made bigger (see box).

'It is impossible to finish on the original completion date, ' says Chris Sedman, project manager for the Nishimatsu Mowlem Irishenco (NMI) joint venture. Autumn 2005 is the current finish date, but recently Irish newspapers obtained DPT documents indicating that the project could be delayed by up to a year until July 2006. Costs were set to rise on the $480M job due to restrictions in the hours the tunnelling boring machine (TBM) can operate.

'These documents were opening gambits in our discussions with Dublin City Council, ' explains Sedman. The council (DCC) is acting as client for the job on behalf of funding body the National Roads Authority.

'So whether it is finished in January or July 2006 depends on the outcome of the discussions.

But 2005 would be a real challenge, ' he adds.

Progress slowed last year when NMI was ordered to cut tunnelling rates by 25% from 4mm/min and reduce tunnelling hours from 7am to 8pm rather than 11pm. This followed cosmetic damage to nine houses.

'We have got approval for working 8pm to 11pm as long as we are not causing disturbance, ' explains Sedman. 'But you can't get away from the fact that you can hear and feel the TBM.

'Most people, once they understand the situation, just want us to get on with it. But the Environmental Impact Statement has all the information and people who want to use it against you will do. There has been a concerted effort by a very small number of people, ' he says. 'Ireland is a smallish country and the Dail [the Irish parliament] got involved with questions asked in the house.

'DCC has been very conscious of its obligation to the public so at times working hours have been cut back to 8pm, ' says Sedman.

That particular restriction was lifted, 'but we have to suck it and see on a weekly basis, ' adds Sedman. The impact on the schedule is clear. Who will pay is up for debate.

'It is under discussion. It is clearly written into the contract but the question is what is a disturbance?' says Sedman. 'We priced on 11pm working and 8pm is restricting our hours. We can understand DCCs dilemma.'

But it is not just the bored section that has courted controversy.

Driving the tunnel beneath the main Dublin to Belfast railway in the southern cut and cover section is also a hot subject.

Twice since work started services on the main line have had to be suspended after settlement was observed, and the solution being adopted is intricate.

The tunnel's arch roof is just 2.9m below the track bed, which sits on an embankment of sandy clays and made ground.

Irish Rail ruled out slewing the tracks or a box-jack solution. It wanted its own design concept, but eventually accepted NMI's pipejacking solution as within the spirit of its design philosophy.

First, Mowlem-Irishenco created large diaphragm-walled shafts either side of the railway.

This allowed Nishimatsu to drive around 50, 1.2m diameter, 60m long steel pipes horizontally under the railway to form a three-sided arch 30m wide and 20m deep under the line.

This proved to be a tricky operation. Work had only just got under way on the six month pipe jacking operation in February 2002 when 5.9mm of movement triggered a 'red alert' alarm, telling controllers to stop trains.

Then in November that year trains were stopped again after Irish Rail inspectors found a 2m wide and 450mm deep depression in the track bed, 2m away from the tunnel.

Once finally installed, the pipes lining the sides of the arch were filled with concrete for added rigidity, and a 2m 2tunnel was mined out to allow the pipes forming the roof to be stiffened locally at third points across crown.

Further mining out below the crown allowed steel caissons to be vertically jacked into place using low headroom piling equipment.

Installing the columns provided vital information on ground conditions as well as additional temporary ground support.

'We got big information from these, ' says Nishimatsu site agent Spencer Hughes. 'They were effectively huge boreholes.'

The geology inferred was very different and far more complex than the original site investigation.

'We found two gravel aquifers separated by boulder clay and needed a very robust design to overcome groundwater and make the arch watertight.'

With the saline content of the groundwater ruling out ground freezing - liquid nitrogen would have been prohibitively expensive - the solution was a combination of jet grouting and vacuum sealing.

Tubes a manchette were used to grout tight the upper aquifer and then, following further excavation, a jet grouted cut off wall was installed sealing the lower aquifer. Horizontal relief valves were installed to provide control of the water pressure.

Once the ground water was under control, final excavation along the 52m underpass could begin. Work started last month.

'At first we just had all the columns in with supporting beams for the roof, ' says Hughes.

Each grid line has two 911mm by 356mm Grade 50 steel beams.

'But as we excavate down, the side walls would kick in. So we need horizontal props.'

Excavation is taking place in four stages with props installed at each stage. This was due to be completed late last month.

'Then it is effectively cut and cover techniques for the double horseshoe tunnel - although there is no actual cut, ' says Hughes. 'Plus we'll have to bring everything in through the supports.' Eccentric spacing will provide a gap of 3.5m by 2.4m to bring in the tunnel formwork.

The two central sections will be cast first, allowing the props to be struck away. The remaining sections should be cast by September.

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