Replacing Pont Briwet


Pont Briwet viaduct, Porthmadog, North Wales

A project to replace Pont Briwet viaduct, near Porthmadog in North Wales, is well underway and on target for completion in February 2015.

The £20 million development scheme began in April last year and will replace the 22-span wooden viaduct, a Grade II listed structure which was originally constructed in 1860, with a new bridge.

The current structure carries the single track Cambrian Coast railway from Dovey Junction to Pwllheli and a single track toll road across the Afon Dwyryd for cars. Heavier and larger vehicles are unable to use the existing bridge and as a result have to travel a further eight-miles in order to undertake a journey from Penrhydeudraeth to the Harlech road.

The imposed weight limit on the existing rail and road bridges, as well as the narrow width of the road, currently restricts the level of road traffic permitted to use the bridge. Over the years Pont Briwet has been partly reconstructed and repaired; however, it is now near the end of its life and is becoming uneconomical to repair.

The project has been able to commence thanks to a number of funding streams, these include Network Rail and the European Regional Development Fund who contributed over £9m to the project. The Welsh Government also made a substantial contribution of approximately £5m via the Mid Wales Regional Transport Consortium. Gwynedd Council, as well as managing the project, has also funded the project. All of these funding avenues allow for a full replacement of the existing structure rather than continuing making ongoing repairs.

Pont Briwet viaduct, Porthmadog, North Wales

HOCHTIEF Construction is the main contractor who undertook the replacement of Pont Briwet and the upgrade of the approach roads to the north and south of the crossing. Another firm heavily involved in the scheme is Merrett Surveys; Rail Construction News caught up with Peter Merrett from the firm to find out more:

We provided the survey work for the scheme; during this project we very unusually combined a number of different surveying technologies including aerial lidar survey, underwater bathymetric work, ground based laser scanning and conventional topographic work to create 3D computer modelling of the old structure and its surroundings.”

The new rail bridge is currently being built and following the completion of the build the railway will be switched over and the old bridge will be demolished. The new bridge will still contain the single track railway but will also provide a two-lane public highway and cycle route. The structure will be 18 metres wide compared to the existing 8.5 metre bridge and will be built in phases so that train services will not be unnecessarily disrupted. A temporary bridge has also been provided for vehicle crossings.

Every effort has been made to minimise disturbance and inconvenience caused to the public during the project but sometimes it is necessary to carry out works during unsocial hours – overnight and during weekends. However site working hours will normally run from 7.00am to 7.00pm Monday to Friday and 07.00am to 2.00pm on Saturdays, weather dependant.  

Pont Briwet viaduct, Porthmadog, North Wales

Mark Langman, the route managing director for Wales said:

This scheme joins a number of other rail improvement projects in North Wales that will help deliver better journeys and boost economic growth in the area. The work at Pont Briwet bridge will make it easier to travel between Harlech, Penryndeudraeth and Porthmadog.”

The schemes objectives are to create a robust inter-modal transport link which eases road congestion and improves the reliability of rail services. Other benefits of the scheme include free transport links as toll charges will be waived by Network Rail, less road congestion thanks to a two way carriageway and better transport links for heavier vehicles.

The scheme will also ensure that there are more alternative transport options for the community and tourists as a new cycle path and footpath that link will be implemented with the National Coastal Path and Welsh National Cycle Route 8. The new bridge will also increase the linespeed of 40mph which is currently only 20mph.

Pont Briwet viaduct, Porthmadog, North Wales

Merrett Survey Ltd

Merrett Survey Ltd specialise in laser scanning and have been in operation for more than 30 years. The firm has established an impressive portfolio of clients since their inception including working alongside National Rail and RWE, one of Europe’s leading electricity and gas companies. 

The company is currently working with Network Rail on a slope stability project in Devon using the latest airborne laser scanning techniques. Another of the company’s prestigious clients is the United Nations; the firm works overseas and have carried out surveys of international boundaries between nations in order to assist the United Nations.

The firm has recently been involved in the bridge replacement project at Pont Briwet and undertook the land surveyor’s role. During this project the firm provided five different types of technology after they were commissioned to the job by engineers, Hochtief UK.

The onsite team used airborne laser scanning – or LIDAR – to survey 40 sq km of the estuary in a bid to study any sediment movement. The team also surveyed the River Usk’s river bed using a multi-beam echo sounder which allows the shape of the river bed to be ascertained around the supports of the existing bridge. This process is called bathymetry.

Merrett also undertook ground based laser scanning on the existing timber bridge which helped the construction engineers building the new bridge, as well as providing 3D viewings of the bridge and its structure. The firm also carried out topographical surveying of the approach roads and photographic surveying which is merged with the laser data to create realistic 3D images and ‘fly-throughs’ of the site.

We caught up with the firm’s Director, Peter Merrett, to find out more about their involvement. He said:

We were on site for two weeks and it was extremely important to be involved because of the mix of different technologies we used and the fact that the data was use across a wide range of applications, from design, future monitoring and for the archaeological record.

We pride ourselves on the practical application of the latest surveying technologies and we achieved this on the Pont Briwet project.”


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