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Cross Country Runner

An iconic cable stay bridge forms the centrepiece to Ireland's privately financed Waterford bypass. NCE reports on the £480M project now underway.

Waterford in the Irish Republic is split in two by a road so busy that it is designated as part of the European Union's strategic roads network.

Traffic that ought to be on a motorway is forced to grind right through Waterford centre en-route from the south to Dublin or the port of Rosslare. The city is blighted by a non-stop conga of juggernauts.

That is set to change, though. The near completed A-frame of what will be a striking new bridge signals a major 24km bypass to the west of the city that will deliver respite from the noise, fumes and congestion of heavy traffic, and clip a good 30 minutes off time it takes to make the 200km Cork-Rosslare journey.

Design and build contractor Waterford JV which comprises Ascon and Spain's Dragados has been on site since late spring 2006. The JV is on course to finish the new road by 21 August 2010, when its contract is due to end, says Simon Meyrick, Mott MacDonald senior project manager and site representative for client the National Roads Authority (NRA).

The new dual-two lane road spurs off from the existing national route N25 approximately 5km west of the city centre and heads through fields, close to the south bank of the River Suir. On the north side of the Suir, the bypass will branch north to join the N9 to Dublin and east to rejoin the N25 to Rosslare and the coast.

The project's €600M (£480M) construction cost will be partially recouped through the collection of tolls by concession holder Celtic Roads Group.

The project involves construction of two major junctions on the new alignment and upgrading of an additional six junctions on existing roads that will link to the new route.

Building Waterford bypass involves crossing rivers, railway lines and roads, requiring creation of some 60 structures in all.

Click here for an overview of the central section of Waterford bypass

The landmark structure is the new Suir crossing – a cable stay bridge that will set records for length and height in Ireland. It will have an overall length of 475m, with a main span of 230m and a 120m tall pylon. Other bridge types would have been possible – though for environmental reasons it was desirable that there should be no structural intrusion in the Suir. For navigation reasons a minimum clearance height was imposed. A cable stay structure met the technical and environmental conditions, and also fulfilled the desire for something iconic.

Waterford JV is constructing the reinforced concrete A-frame pylon using a Doka jump-form shutter system. The pylon's section diminishes over its height, but its outer edges have been designed with a constant geometry, minimising the amount of trimming required on the shutter with each lift. Steel anchor blocks for the cable stays have been incorporated into the head of the pylon.

Dragados brought new thinking to bear during construction of the pylon foundations.

Conventionally each leg would be founded on a group of 12 to 14 large diameter bored piles.

However, because the top 15m of ground is very soft, there was a risk that the sides of the bores would collapse, compromising pile strength. Dragados instead used 400mm diameter minipiles in groups of 48.Minipiling to the required depth of 30m to 35m has only recently become possible, thanks to advances in piling rig technology and strength, which enable far more precise control of the auger. This prevents deviation, which has historically been a problem with long, small diameter piling. Piles bear onto underlying rock. Temporary casing was used to support the upper part of each pile while it was cast.

Structurally, the advantage of minipiles is that they offer a higher degree of redundancy than large diameter bored piles: "The margin of safety lost through failure of a single minipile is far less than it would be if a conventional pile failed to deliver the required bearing capacity," Mott MacDonald director John Murphy comments.

When NCE went to site the pylon was close to topping out and deck erection was just getting under way. Dragados is using an approach to deck fabrication known as "piece small", Meyrick says. The maximum weight of any component is 40t, meaning that everything can be lifted and placed using conventional mobile plant.

The deck consists of main longitudinal steel girders connected by cross girders at 5m centres. It will be built in 10m increments. All connections are bolted.

With steelwork in place, cables will be installed. Precast concrete panels covering half the deck width are then placed on the steel grillage and the cables will be tensioned to near full load. The only in-situ concrete pours required are the stitches needed to tie the panels together.

"All of the components used to build out the deck will be picked up from the ground adjacent to the north abutment and carried along the deck already completed," Meyrick explains.

Big advantages of going for prefabricated, "piece small" construction is that work is less a hostage to the elements and is inherently safer. "Lifting operations are faster, so you can take advantage of lulls during windy weather to get things in," Meyrick says. "Concrete pours are small, so you're less likely to be rained off. And you're minimising the amount of work done above the water. The stitches take place above steel girders, so there's no shuttering to erect and strike. That greatly reduces risk."

Waterford JV is expecting to complete a 10m section of deck every six to seven days. "The choice of method has reduced the cycle time by two days per unit at least," Meyrick comments.

The deck will be fixed at the south abutment, with thermal movement accommodated at the
north abutment by a concertina-type joint allowing 300mm of movement. Support will be provided by pot bearings.

The bridge will have striking asymmetric fans of inclined cables, running to the outer edges of the deck. Cables are being supplied and installed by Spanish subcontractor MK4. There will be 19 cables in each fan, ranging from 26 to 51 strands per cable – "relatively small for a bridge of this size", comments Murphy.

To resist overturning moments caused by loading of the main span, the backspan will be ballasted. The back span will be supported on piers.


Client: National Roads Authority

Local Authorities: Waterford City Council, Waterford County Council, Kilkenny County Council

Client's adviser on route selection, preliminary design, contract preparation, design review and construction monitoring: Mott MacDonald Pettit

PPP concessionaire: Celtic Roads Group (Waterford) - Bam (Ascon), Dragados & NTR

Design and build contractor: Waterford JV - Ascon and Dragados

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