Crossing No 46: River Barrow’s latest iconic bridge
09 September 2019
'A great feat of civil engineering, this iconic bridge speaks to a future of infinite possibilities and wonder. In time, it will become one of the most recognisable structures in the country.'
The River Barrow bridge – a key part of the N25 New Ross bypass scheme – has an ambitious design which sets it apart from the bridges that have gone before and, in time, the 46th crossing will become one of the most recognisable structures in the country, writes Mary Bowe.
The N25 New Ross bypass PPP scheme involves the construction of approximately 15km of N25 and N30 national route and comprises 13.6km of dual carriageway and 1.2km of single carriageway and associated works.
The scheme includes three roundabouts; one grade separated junction; eight road underbridges; three road overbridges; one railway overbridge and 16 underpasses.
An extradosed bridge crossing of the River Barrow is the main structural feature of the scheme. The bridge extends 887m in length, and its two 230m main central spans have a navigation clearance of 36m above mean high water spring (MHWS).
At its opening later this year, it will be the longest bridge in Ireland, and its two main spans will be the longest post-tensioned concrete spans of their type in the world.
The N25 and N30 are two of Ireland’s key commercial and tourist routes. The importance of the N25 Cork to Rosslare Europort route, in particular, has been recognised in national development plans, government policies and the National Roads Needs study (1998).
Delays have, however, been a common occurrence on the N25 and the N30 for many years, and the need for a second river crossing providing a bypass of New Ross has been identified in successive Wexford, Kilkenny, and New Ross development plans since 1993.
The existing N25 route at New Ross passes through the town, crossing the River Barrow at O’Hanrahan bridge, while the N30 to Enniscorthy joins the N25 at New Ross and passes through the town via an inner relief road.
If O’Hanrahan bridge had to close for any reason, traffic would be diverted through Thomastown, using the R700 and the restricted Ferry Mount Garrett bridge.
In March 1999, Wexford County Council appointed Mott MacDonald (formerly Ewbank Preece OhEocha) to determine the need for, and the location of, a second river crossing and bypass of New Ross.
A constraints study was undertaken and published in February 2001, and a route selection study, including public consultations, followed. This culminated in the selection of a preferred route, and a route selection report was published in October 2002.
Between 2005 and 2008, the scheme progressed through preliminary design, preparation of the environmental impact statement (EIS) and the statutory process.
In November 2007, Wexford County Council made an application to An Bord Pleanála seeking approval for the scheme. An oral hearing followed in April 2008, and the council received approval for the scheme and confirmation of the compulsory purchase order (CPO) in December 2008.
Public private partnership procurement
In February 2009, Transport Infrastructure Ireland (formerly the National Roads Authority) announced plans for a second Public Private Partnership (PPP) roads programme. This followed the successful award of contracts in the first PPP roads programme, which was announced in 2000.
The objective of the second programme was to deliver new road construction with a capital value of €1 billion through private sector funding under a PPP mechanism.
The N25 New Ross bypass was identified as one of the schemes to be procured within the programme. The tender process began with the publication of a notice in the ‘Official Journal of the European Union’ (OJEU) on March 22, 2010, but it was suspended later that year.
Two years later, the government announced an infrastructure stimulus package to provide investment for a range of important public infrastructure projects, including the N25 scheme.
Mott MacDonald Ireland was appointed as technical advisers to TII to progress the N25 scheme from NRA Project Management Guidelines (PMG) Phase 5 advance works and construction documents preparation, tender and award, through Phase 6 construction and implementation to Phase 7 handover, review and closeout.
The tender process recommenced with the publication of a notice in the OJEU on March 22, 2013. Four consortia prequalified for the tender.
The invitation to negotiate, including tender documents, was issued in November 2013 and, following several tender consultation meetings, tenders were returned in September 2014.
Following a technical and financial evaluation of the tenders, a contract was awarded to BAM Iridium (PPP Co), and the contract was signed on January 26, 2016.
BAM Iridium’s obligations included the design, construction, operation, maintenance and financing of the works. Responsibility for the design, construction, supervision and commissioning of the works lay with the New Ross Joint Venture (NRJV), comprising a joint venture between BAM Civil and Dragados SA.
Arup acted as the designers for the NRJV, with Carlos Fernandez Casada (CFC) providing design services for the River Barrow bridge.
Design and construction
Design and construction requirements included:
• 4km of Type 1 dual carriageway (mainline) between Glenmore, Co Kilkenny, and R733 Junction in Landscape, Co Wexford;
• 9.6km of Type 2 dual carriageway (mainline) between the R733 Junction and Corcoran’ Cross Junction;
• 1.2km of standard single carriageway forming a tie-in between the mainline and the existing N30;
• Three at-grade junctions at Glenmore, Ballymacar bridge and Corcoran’s cross forming connections between the mainline and the existing N25 and N30;
• One semi-compact grade separated junction at Landscape connecting the mainline to the R733 Arthurstown/Fethard Road;
• 37km of mainline, national, regional, local and access roads;
• Construction of in excess of 46 principal structures including three road overbridges, eight road underbridges, one railway overbridge and 16 accommodation underpasses;
• Construction of an approximately 900m-long extrados bridge with two main spans of 230m carrying the mainline across the River Barrow;
• Diversion of various services, landscaping and accommodation works.
Additional structures of note:
• Structure B01: Ballyverneen underbridge, which carries the N25 mainline over the local road (LS-7513) and the Graiguenakill stream;
• Structure B02: Ballyverneen railway bridge, which is an 80m-long three-span bridge carrying the disused, but not abandoned, New Ross to Waterford railway line over the N25 mainline. The bridge is a post tensioned, reinforced concrete structure with a main span of 38m.
• By careful management, areas of cut generated sufficient material for the necessary formation of embankments, generation of road construction product, environmental berms and landscaping areas;
• Cut slopes in competent rock were optimised by detailed design;
• Blasting was used in cuts in Glenmore, Stokestown and Camlin;
• Design in soft ground areas, located principally at Glenmore, comprised replacement with suitable material.
River Barrow Bridge construction
The River Barrow bridge comprises nine spans of the following lengths: 36m, 45m, 95m, 230m, 230m, 95m, 70m, 50m and 36m.
Three reinforced concrete pylons located in the central median support the cable arrangement. The pylon at the central river pier is 27m above deck level (60m above foundation level), with the adjacent pylons in the order of 16.2m above deck level.
Abutment 1 is on the western or Kilkenny side of the river, and the piers are numbered Pier 1 to Pier 8 from west to east, with Abutment 2 on the eastern or Wexford side of the river.
The only pier within the River Barrow is Pier 4.
The bridge is a continuous concrete box girder deck over its entire length, with the depth of deck varying between 3.5m and 8.5m and fixity at the approximate midpoint in the bridge at Pier 4.
The bridge was constructed in a series of 16 distinct sequential stages, some of which overlapped or began at the same time.
Stage one involved the construction of the foundations. On the west side of the bridge, depth to rock was shallow, so foundations consist of pad footings on rock.
On the east side, including at Pier 4, ground conditions were poor, so foundations consist of 1,200mm diameter piles, up to 40m deep and socketed 10m into rock. The pile cages were in the order of 40m in length. Pier 4 foundation had 43 x 1,200 diameter piles.
Stage two allowed for the construction of the eight intermediate piers and the two end abutments using incremental lifts to deck level.
This was achieved through the use of a climbing formwork system, effectively an external skeleton or cage, which enabled the next concrete stage to be poured while the system rested on the stage below.
The entire cage rose up a level and the next stage was poured. This continued until the section was complete.
Temporary piers were built to assist with the construction and support the deck at the three central piers (piers 3, 4 and 5).
All the piers have bridge bearings to support the deck, with the exception of Pier 4 where the pier is fixed or integral with the deck.
Stages three to six
Work on the western approach spans began with the construction of the central box of the deck.
This was constructed using a system of temporary steel supporting falsework that could be assembled, disassembled, moved and reused.
The floor slab of the central box was built first, then the sides, and finally the box was closed with the cast top slab.
Before the walls were poured, ducting for the post tensioning tendons was placed. The heads of piers 3 and 4 were constructed separately in preparation for the main river deck span.
Stages seven and eight
Stages seven and eight required the assembly of a form traveller system. One type of form traveller was used for the approach spans, and this was constructed on, and travelled along, the central box laying down each section of the deck edges or wings as it moved.
Once the wing form traveller completed its work on the western approach spans, it was dismantled and transported to the eastern side to complete the eastern approach spans.
A separate type and pair of form travellers were assembled together on Pier 4 in preparation for the construction of the main deck spans. These main form travellers allowed for the simultaneous construction of both the box and the wings.
Pier 5 began construction on the eastern side during this stage, and the eastern approach spans proceeded from Pier 5 in a similar fashion.
Stages nine to 11
Stages nine to 11 began approximately halfway along the structure at Pier 4. Two construction techniques were used to construct the main deck spans – a balanced cantilever method on Pier 4 and a cantilever method at Piers 3 and 5.
The differences between the two techniques are worth noting.
At Pier 4, two separate form travellers progressed in tandem away from the pier, casting deck sections on opposite sides simultaneously.
This allowed the weight of the deck to be balanced at all times, ensuring that unwanted rotations were avoided.
At Piers 3 and 5, one form traveller progressed in one direction only toward Pier 4. In this instance, the cast cantilever sections were balanced by the previously constructed approach spans on the opposite side of the pier.
At this stage, the pylons above the main piers were constructed incrementally once the corresponding form traveller began to move away from the pier. The pylon construction progressed in parallel with the deck below, and the cables were installed in unison.
The cables were anchored to the deck either side of the pylons and passed through the pylons using a saddle system. 18 cables anchor the deck at Pier 4 while eight cables were used at Piers 3 and 5.
Separately, the eastern approach spans continued to progress towards the end abutment with the form traveller forming the wings as it moved along using the same method as on the western side.
Stages 12 to 16
Stages 12 to 16 allowed for the three separate bridge constructions to join together as one.
The cantilever decks from Piers 3 and 5 progressed towards the balanced cantilever deck from each side of Pier 4 until they met at the connection. The form travellers were then dismantled, and the finishing works on the bridge deck could commence.
There are 39 individual deck segments in each of the main spans. Each deck segment required the completion of steel fixing, pouring concrete, post tensioning and the installation of cables before the form traveller could move to the next segment.
Unlike the approach spans, the full cross section of the deck on the main spans was constructed in one single stage for each segment. The deck included longitudinal and transverse post tensioning tendons.
Once the concrete was strong enough, the tendons were passed through the ducts that had been set into the concrete during the pouring stage. Hydraulic jacks tensioned the tendons, which were then anchored to the deck and bonded to the concrete via grout injected into the ducts.
The bridge’s slimline profile was achieved using high strength concrete in key areas. The concrete possessed compressive strengths of up to 95N/mm2, which is more than twice the strength of standard concrete mixes.
While the concrete allowed for better resistance of high compression loads and thus a narrower visual profile, the low water to cement ratios used in the mix meant the resultant material was less workable. Special mixtures were added to improve workability, but greater care and accuracy was needed in construction.
The deck is designed to allow internal access to provide inspection galleries for future maintenance. Secure doors at each end within the abutments provide access inside the bridge.
Clear access is provided throughout the entire deck, and as the floor of the deck inclined, suitable walking platforms and railings are installed. Lighting and a power supply are also provided.
The bridge could not be called an extradosed bridge were it not for its cables. The cables have between 113 and 127 wire strands in each cable, with an overall cable diameter of 400mm. In total, approximately 500km of strand was used in the cable system.
Cable testing was undertaken by Chicago-based company, CTL, which owns the largest cable testing facility in the world. The test specimen comprised of a 127-strand arrangement, each 15.7mm in diameter with a Guaranteed Ultimate Tensile Strength (GUTS) of 279kN per strand. The overall cable GUTS is 127×279 = 35,433kN or 3,543 tonnes.
Two tests were undertaken:
1.) A fatigue test whereby the system was subject to two million stress cycles (limited to two per cent wire breaks);
2.) A static tensile test whereby the system tensile load is increased and must achieve 95 per cent GUTS.
The design and construction teams’ use of Building Information Modelling (BIM) allowed potential issues to be addressed before they arose and provided visibility on how the different elements of the bridge interacted.
An analysis of the complex spatial relationships of different levels of steel reinforcement and post-tensioning ducts in the bridge deck, for example, allowed the team to detect any possible clashes before the actual build.
The teams also used 4D modelling, intelligently linking the 3D bridge model to the time dimension and enabling the model construction sequencing to be presented and reviewed against construction timelines.
To complement the slim design of the bridge selected architectural lighting will be used to illuminate the bridge at night. Discrete feature lighting will illuminate the pylons and stay cable arrays with spill controlled uplighting. A continuous row of LED luminaires will illuminate the bridge deck edge.
Navigational lighting will provide safety and security to ships sailing below and aircraft flying above. There will be no road lighting at deck level.
A suite of structural monitoring equipment has been installed on the bridge to monitor wind, temperature, movement and so on.
• Anemometers at four locations;
• Load indicators to record stay cable loads and bearing loads at each pier;
• Movement indicators to record movement and rotation of bearings;
• Temperature sensors to record the temperature of various elements.
The River Barrow bridge is the 46th crossing of the River Barrow, but its bold, ambitious design sets it apart from the bridges that have gone before.
The project team’s willingness to push the boundaries of engineering and work together in the true spirit of partnership is evident in the magnificence of the structure.
A great feat of civil engineering, this iconic bridge speaks to a future of infinite possibilities and wonder. In time, it will become one of the most recognisable structures in the country.
As the date of the scheme opening approaches, we look forward to the N25 New Ross bypass delivering safe, shorter and more reliable journeys for commuters and improved quality of life and economic prospects for all living in New Ross town and the wider region.
Author: Mary Bowe is a senior engineering inspector with Transport Infrastructure Ireland (TII). She is responsible for the delivery of national road projects in the south-east region and is TII’s representative on the N25 New Ross bypass PPP scheme. She has more than 25 years’ experience in the planning, design, construction and administration of major road projects. A native of New Ross, she is a past chairperson of Engineers Ireland’s South East region and has served at both council and executive within Engineers Ireland. A chartered engineer, she graduated from UCD in 1993.
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