Pont Briwet Bridge Replacement Scheme

CEEQUAL Very Good – Whole Team Award
Version 5, April 2016 | Penrhyndeudraeth, Gwynedd, Wales

Client: Cyngor Gwynedd;
Designer: Hewson Consulting Engineers;
Contractor: HOCHTIEF (UK);

Assessors: Pat Jones (Cygnor Gwynedd) and Derek Walters (HOCHTIEF);
Verifier: Gemma Fenn (Fenn Environmental);

Project Summary

In 2015, the redevelopment of Pont Briwet, a rail and road bridge across the Afon Dwyryd in Meirionnydd, Gwynedd, was completed.  The main project goal was to “increase accessibility between north and south rural Gwynedd, and beyond, by sustainably improving local infrastructure”.

The original grade II listed Pont Briwet

Owned by Network Rail, the original Pont Briwet was a grade II listed, timber bridge built in the 1860’s. There was a 20mph speed limit on both the Cambrian Coast rail crossing and the private, one lane, toll road.  Traffic lights caused significant congestion. A 2 tonne weight limit meant that large vehicles, including emergency vehicles, had to travel an additional 16 miles along narrow roads through Maentwrog.  There was no safe provision for either pedestrians or cyclists.

Tourism businesses are major employers in this part of Gwynedd. The poor condition of the original viaduct and frequent congestion was an identified barrier to tourists. Coaches avoided the narrow diversion route via Maentwrog

Despite the restrictions on the use of the bridge, and the congestion it caused, it was a vital link for communities in rural north and south Gwynedd.  It provided access to employment, education and other essential services.

Because of its deteriorating condition, the remaining lifespan for the old bridge was uncertain and there were increasing concerns that the costs of essential maintenance were becoming prohibitive to Network Rail.  This posed a real threat to both the Cambrian Coast railway and the road link, should the poor condition of the bridge result in closure.

Following several years of consultation with local communities and major stakeholders a decision was made to build a new rail/road combined bridge. A successful bid was made in 2010  to the European Regional Development Fund  by Gwynedd Council and Network Rail.  Gwynedd Council’s previous experience of managing ERDF funded projects meant they became the Client, with Network Rail providing a technical approval role. This partnership was a first for the parties and has been particularly successful.

The main contractor HOCHTIEF (UK) was appointed in February 2013, and work commenced on site in May 2013.  HOCHTIEF appointed Hewson Consulting Engineers as their designers having worked with them during the tender period.

The rail crossing was opened in September 2014, with the road bridge opening in July 2015.

Access roads approaching the bridge have been widened and there is now a two lane carriageway across the viaduct with no traffic lights, toll or weight restriction.  Emergency vehicles can now use the viaduct resulting in improved response times.  Removing HGVs from the narrow alternative route has reduced congestion and improved safety. In addition the new shared cycle/footway provides a new link to the Wales Coastal Path and the National Cycle Network.


Challenges and achievements

Physical Resources – Use and Management

The new bridge is located in a challenging environment: narrow, constricted approach roads and restricted working space, adjacent to an operational railway over a fast flowing tidal estuary.

From the outset of the design, construction methods were reviewed to eliminate or reduce associated risks. Precasting the structural elements and combining lifting frames with temporary supporting structures significantly reduced time spent working over water which had a huge benefit in minimising the health and safety exposure of workers.

A proactive approach with daily pre-work briefings ensured that the workforce were constantly updated on all site changes.  Everyone concerned with the project is therefore extremely pleased that it was delivered accident free with no reportable or lost time incidents, despite these difficult working conditions.

The scheme set out to be economic and sustainable incorporating a number of innovative features that will considerably reduce life-cycle maintenance costs.  The viaduct is a pioneering bridge solution being the longest fully integral rail viaduct in the UK. Track-structure interaction analysis was undertaken to demonstrate that no rail breather joints were required along the entirety of the scheme resulting in significant capital and maintenance cost savings.

The change to precast U-beams, which was also a first for Network Rail underbridges, provided an extremely economical solution whilst also improving the structure aesthetically.

Whilst  the ERDF funding was essential for the project, it did impose a rigid timescale on the project, as all grant money had to be drawn down by June 2015.  A tight programme was further hampered by severe storms in early January 2014, which badly damaged parts of the railway line along the coast.  Problems with an adjacent electric pylon also impacted upon the programme and  necessitated the contractors allowing National Grid access to carry out emergency works on a replacement pylon.

Ecology and Biodiversity

As well as being in the Snowdonia National Park, Pont Briwet is situated within the Lleyn Peninsula and the Sarnau Special Area of Conservation and is adjacent to the Morfa Harlech Site of Special Scientific Interest.

The environmentally sensitive site was at the forefront of decision-making throughout the design and construction phases. Gwynedd Council and Network Rail worked closely with the Snowdonia National Park and the then Countryside Council for Wales and the Environment Agency to develop the scheme, and HOCHTIEF continued these relationships during the construction period.  Regular environmental meetings ensured that all parties were kept fully informed as work progressed.

It was decided at an early stage that the new structure would be ‘low-key’ with a shallow profile so the visual impact on the local area would therefore be similar to the existing bridge. To minimise its’ impact, the new road bridge encompasses the old bridge’s footprint, with the new rail bridge located slightly westwards.

The high priority placed on aesthetic value within the National Park is further emphasised through careful choice of form and finishes. Precasting the beams and crosshead shells provided a high quality concrete finish, and by shortening the construction programme, minimised the impact on the environment

Reducing both future maintenance costs, and also the environmental impact from maintenance work, were always high priorities for the design and construction team.  The new bridge has a minimum design life of 120 years and aspects such as eliminating rail breather joints reduces life cycle maintenance requirements.

The design for the new bridge has also significantly reduced the number of piles in the estuary hence reducing impact on flows and lessening future flood risk.

The coastal protection of the road and rail embankments either side of the bridge was re-engineered, replacing approximately 300m of steel pile retaining walls with rock armoured embankment constructed with a core of modified 6B. The hydraulically stable 6B material was placed directly into open water avoiding installation of a silt curtain in tidal conditions

People and Communities

The original intention had been to close the road completely whilst the new bridge was constructed.  However, following concern by local communities, plans were modified to reduce the road closure period.  This, together with the restricted working space over a tidal estuary in close proximity to a live railway, made the project extremely challenging.

Unfortunately, these plans had to be changed again when settlement of the bridge took place and significant rot was discovered in the timbers in January 2014.  It was no longer safe to allow trains or vehicles to continue using it, and both the road and railway had to be closed until the new viaduct was completed.

Whilst this unplanned closure caused considerable inconvenience to local communities in the short term, it highlighted the critical need for an improved crossing.

The restrictions and congestion on the old bridge created a physical and psychological barrier between north and south rural Gwynedd.  The new bridge provides a significantly improved, faster, safer, route for residents, businesses and tourists, as well as securing the future of the railway in north west Wales.

As well as improving transport links, the new 136m viaduct fits perfectly into its surroundings, sweeping gracefully over the estuary, the crossheads echoing a fleet of boats.  A small layby and access to a new viewpoint, together with the new footway/cycleway provides an opportunity for residents and visitors to fully appreciate and enjoy the stunning location of the new bridge.

How did CEEQUAL influence your project?

Environment and sustainability were important issues for the scheme due to the sensitivity of the location. The evidence gathering process for CEEQUAL has been a useful method to ensure that the original deliverables have been fulfilled.  It has provided opportunities to identify areas of improvement for incorporation into future projects to further enhance sustainability performance.

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