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History repeats itself in Dublin

SITE REPORT

The biggest bridge in Dublin has become so important that they're building another almost exactly the same beside it.

And the head of the original contractor's team over 10 years ago is back - this time as the client's eyes and ears.

Diarmaid Fleming went to see how practice makes perfect

ON A SCALE rather larger and more robust than your usual Blue Peter artefact, project engineer Mike Broderick can honestly claim 'Here's one I made earlier', pointing to a bridge nearly 400 m long teeming with motorway traffic high above the River Liffey.

Alongside, only a short distance of 4.5 m from its older neighbour's parapet, work is well advanced on the new Second Liffey Valley West-Link bridge.

There can't be many civil engineers who return years later to the site of a major structure to start building another one which is largely the same. Some locals could be forgiven for thinking that Mr Broderick spends his life building balanced-cantilever bridges, having headed up the contractor's team when the first WestLink bridge was built in the late 1980s.

'On the first bridge I was contracts manager for IrishEnco - who were building the bridge with Diwydag. It just happened that I was around when this one started, except I was working with Arup, ' says Mr Broderick, a veteran of civil engineering projects in Africa and overseas before returning to his native land.

Second time around, however, his role is considerably different. As project resident engineer for Arup, consultant to private sector client National Toll Roads for the state National Roads Authority in a pilot Public Private Partnership project, his job is to ensure that the works proceed in accordance with the design. The knowledge and expertise picked up on the original contract placed him in a position worth its weight in gold - he had seen it all before.

Few contractors, such as the Irish/Austrian John Sisk-Strabag joint venture over the Liffey, will have had the experience of dealing with a counterpart on the client's team with such intimate knowledge of a job.

Knowing the tricks of the trade, like a poacher-turnedgamekeeper, would be invaluable to any client.

But while good contractors and poachers have nothing in common, for some, the same flush of worry would course through the veins on learning that one of their own is on the other side of the fence.

'At the beginning of the contract, they were a little worried, ' says Mr Broderick, wryly. But that didn't last long. He adds: 'It has worked out pretty well and it has been a team effort in which we have worked together to get the job done.'

Knowledge of the tricks of the trade has helped solve problems rather than uncover red faces guilty of any sharp practice on site. Similarly, the contractor hasn't had to worry about being faced with a rigid, impractical engineer working to specifications more suited to the design office where they were drawn up than the battleground of the site.

Mr Broderick plays down the notion that he is there to police a contractor's armoury of tricks. 'There aren't really any tricks you could play, even if you wanted to. If anything, with my experience as a contractor on the job, I would have a more practical view and a more sympathetic ear to the contractor, providing that does not adversely affect the project. If you can make things easier for the contractor, then it is better for the job providing you don't compromise the contract, ' he says.

Sisk-Strabag project manager Seamus Sorohan says: 'Essentially the same techniques are being used as before and, of course, things were learned from the way the job was done before. Naturally there are occasional differences in opinion but our relationship has been good: from a construction point of view he has helped in a big way, and where input was needed he has offered it, ' says Mr Sorohan.

Mr Broderick's experience, however, was not confined to construction knowledge. A contractor's alternative proposed and accepted on the building of the first West-Link has followed through to the construction of the second. Proposing the first alternative 14 years ago has given him intimate familiarity with the design.

As well as being a major benefit to the smooth running of the project, it is again a comparative rarity to find an engineer as comfortable talking both design and construction.

'The original design back on the first bridge was for 3.8 m cantilevered segments. We put forward an alternative using 5 m cantilevers and had to redesign the pre-stressing and reinforcement for the approval of Arup which was also the engineer then, ' says Mr Broderick.

'We had to go through the whole design process ourselves - this has proved useful because the 5 m design, which was accepted, has also been used on this bridge.

But while designs may have been similar, changes have taken place in the 14 years since work began before at the site. Then the Irish construction industry was in the doldrums, while at the start of this job the biggest boom the Irish building industry has seen was in full swing.

'Back then, we had only two expats on the job. Today there is a big crew of Austrians, which just shows you how the whole construction market here has changed, ' says Mr Broderick. Projects like this have brought many of the Irish construction diaspora home: Mr Sorohan is working on his first major project in Ireland after a career in the UK and the Far East, similar to Arup resident engineer Finbarr Kenny, who worked in the USA.

The boom has also meant more availability of site plant, such as big concrete pumps, which were not around in any numbers in Ireland previously. Other big changes include greater environmental requirements such as landscaping and pollution control. The price too is different - the first bridge was built for £5.6 million, a little over a quarter of the capital cost of today's bridge which is costing around £21 million.

Mr Broderick says he has no secrets from his days as a contractor on the job which might cause him embarrassment in his present role.

Even if he had, only two others are likely to have been to hand to relate them: two steelfixers living locally to the job are the only other veterans of both the first and second bridges.

Second West-Link bridge

BRENDAN Behan may have been the most famous thing around Dublin in the 1950s, but today it's the M50 motorway. Dublin's part-completed 'C-ring' orbital motorway - the city's 'M25' and Ireland's busiest road - crosses the River Liffey to the west of the city over a stunning valley bridged in 1990 when the first West-Link bridge opened. Increases in traffic levels, outstripping predictions, have brought about the need for expansion of the motorway, allowed for in the original design. The new Second Liffey WestLink bridge marks the first stage, expanding the capacity of the roads tolls just to its north.

The deck is 385 m long with five spans up to 90 m, which along with its predecessor, makes it Dublin's biggest, and the longest multi-span bridge in the Republic of Ireland. The in situ deck is a pre-stressed box-girder structure with internal bonded tendons on reinforced concrete piers and abutments at each end.

The deck is built by the asymmetrical balanced cantilever method: a Wito form traveller - a steel frame which runs on rails along the deck - supports the collapsible Doka formwork for the in situ deck segments before they are stripped and tendons pretensioned. The traveller moves out from one of the reinforced concrete piers to which a giant temporary 2m-diameter steel prop is braced. Anchored to the deck and foundation, the prop can take both the compressive and tensile loads generated by the asymmetrical launching, when an unbalanced deck section cantilevering from the pier pulls or compresses the prop through overturning forces.

'The concrete segments are all poured in situ. We are using 60 N concrete to give us early strength of 30 N to allow stressing of the tendons in 36 hours.

The tendons are loaded to between 200 and 300 tonnes, with loads of similar order generated in the props, ' says Arup resident engineer Finbarr Kenny.

Concrete with high slump of 160 mm for the bottom of the deck box and 120 mm for the side using superplasticiser admixtures was needed to ensure adequate flow throughout the shutter. Tendons comprised 11 300 kN Dyform strands, which were stressed using conventional multi-strand jacks by VSL. The deck is designed for standard BS5400 loading with stone mastic asphalt surfacing. Mechanical pot bearings will take loads up to 2,000 tonnes.

Conventional slipforming was used to build the four massive hollow reinforced concrete piers, which carry the deck 35 m above the Liffey. These are at slightly different locations from the original bridge for visual aesthetic effect, marking the main difference between the structure.

An incredible variation in ground conditions across the site meant different foundations for each. 'We have shallow limestone bedrock, alluvial material, cemented gravels on the northern side and boulder clay. So the foundations for the piers are a combination of spread footings, bored piles and driven steel piles, ' says Arup associate and design project manager Troy Burton from Sydney, now resident in Ireland. Piling subcontracts were carried out by Murphy and Johns Construction.

The 24-month project is due to be completed next month.