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Irish drama unfolds

Foundations Dublin Port Tunnel

Work on one of Europe's biggest holes is well under way, the Dublin Port Tunnel. Diarmaid Fleming met the team of French, English, Welsh, Scottish and Irish doing the digging.

If you were to ask Irish people:

'What's the biggest hole in Dublin?', you would probably get answers regarding pubs or certain suburbs. They would all be wrong. What must be the biggest hole in Ireland is being gouged out of the ground on the country's largest civil engineering project, the IRú353M ($400M) Dublin Port Tunnel.

Peering over the top, the great shaft hole looks almost like a huge Roman arena, with workers scurrying far below like gladiators in battle. And drama will be heightened soon, when the giant 11.8m diameter Herrenknecht tunnel boring machine is dropped in to start gritting its teeth in anger.

While the Irish have dug holes all around the world, it is perhaps a little ironic that few of the accents to be heard around Ireland's biggest are Irish. English, French Welsh and Scottish tones can be heard in abundance, with even an example of Welsh graffiti at one corner of the site.

But the international dimension is no surprise considering that contractor Mowlem-Nishimatsu-Irishenco has engaged a joint venture of Mowlem Piling & Foundations and French ground specialist Intrafor to work on diaphragm walling for the TBM shaft, reception chamber and around 300m of associated cut and cover tunnel. The collaboration is known as the Mowlem Piling Intrafor (MPI) joint venture.

Even for experienced hands, the size of the work provokes excitement.

'At 56.6m in diameter, it must be the biggest shaft in Europe at present, ' says MPI director Chris Macdonald. The 30m shaft is enough to fit a 10- storey building inside.

The decision to use diaphragm walling stemmed in part from MPF's collaboration with Intrafor on work over the past four years.

'In this line of work, the French do nothing else but diaphragm walling, so with Intrafor we are not trying to reinvent the wheel every time we win a job, ' says MacDonald.

The work is principally divided into two. The launching shaft is towards the northern end at Whitehall, while the receiving chamber and 300m of cut-and-cover tunnel is at the southern end in Fairview. The TBM will drive 2.2km to the receiving chamber, before turning around to complete the other bore back to the launching shaft.

Concreting of the walls for the shaft has been completed, so work is focused on excavation, to be followed by pouring of a reinforced concrete floor. The shaft has been socketed up to 3m into limestone rock using chisels and grabs where the TBM will break into the shaft at the start of tunnelling. At present, the more exciting action is to be seen at the receiving chamber, using the same slurry wall method employed at the circular shaft.

The only shuttering evident on site is that needed to construct a 1.5m deep wedge-shaped concrete 'guide wall' at the top of the excavation, 1m wide at the top and tapering down to 500mm. Then the excavation work begins. A massive crane operated grab takes out 1.2m wide 'bites' of ground in 2.8m long bays, down to the full 30m depth of the excavation of the receiving chamber.

'One problem is that the grab can move. So to ensure that excavation is vertical, we survey regularly to check that the grab cable is in the centre, ' says MPI project manager Daniel Altier.

The bentonite slurry flows continuously from a reservoir pool on site into the excavation and then back to the pool where it is recycled to bring the soil content down to acceptable levels of 2%. 'When we are excavating in clay, there's not much recycling needed. But in sand it is different, and can take up to three hours, ' says Altier.

While Dublin's soil is predominately stiff boulder clay, the site throws up interesting geotechnical challenges. The giant shaft at the northern end is in boulder clay, but the ground at the receiving shaft end is much poorer, ranging from alluvial material and made ground including contaminated material from an old medical dump which has had to be removed.

'At Fairview, we found the bentonite was disappearing and it was difficult to keep it level. So we have had to pre-treat the excavation using a mix of bentonite and cement, which we dig out 24 hours later, ' says Altier.

A series of bites completes the excavation in 7.2m wide panels, formed with steel profiled stopends which accommodate a water-stop, with panels excavated consecutively. An excavation rate of around 6m bay taking up to two and a half panels to complete including concreting.

Beside the excavation, an army of steel-fixers work in a yard preparing the huge reinforcement cages, which are lifted into place by a Sennebogen 4400 140t capacity crane.

The cages, with reinforcement bars up to T40, weigh in at 32t, providing around 130kg of steel per cubic metre of concrete - 'the chunkiest I have seen, ' says Macdonald. The unsurprisingly 'busy' laps are welded, with around eight or nine workers needed to fix one cage to another as they are dropped into the excavation.

Concreting is completed with two tremie pipes, each with a vermiculite plug, with very high 175mm slump concrete, supplied by Readymix.

'We need around 70m 3/h to ensure we have enough. It is a matter of good practice to keep the concrete flowing - you don't get a second chance with a diaphragm wall, ' says Macdonald.

The completed circular shaft in clay reveals a surprisingly good finish while in the poorer ground scabbling may be needed.

MTI hopes to finish its -21.5M portion of the job by June.

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