The challenge
First opened over 150 years ago, Barmouth Viaduct is a Grade II* listed single-track wooden railway bridge, one of the oldest of its type still in regular use. To ensure longevity of the structure, Network Rail took the decision to run a major programme of upgrade works to completely restore the viaduct. These works aimed to maintain the bridge as a viable crossing for Cambrian line trains, as well as being sympathetic to the bridge’s great heritage and listed status. In keeping with this, components had to be swapped out on a ‘like for like’ basis.
The first stage of these works, carried out in 2020, involved an innovative bridge lift to facilitate the replacement of 24 of the bridge’s crossheads. Alun Griffiths carried out these works, and we provided a bespoke jacking scheme to temporarily lift two deck spans of the bridge off its central support, while the crossheads were replaced.
Speaking about the second phase of the works, Steve Richardson, Operations Manager, at Alun Griffiths, said: “As part of the ongoing restoration and renovation programme, Network Rail wished to replace the two original 160 tonne metallic spans of the bridge. As with the previous work carried out, everything had to resemble the original 19th century structure, with the new hogback trusses being specially fabricated.
“Considering the complexity of the project, we decided to carry out a test run of the proposed method of installing these new spans. This test was conducted around half a mile away from the site, meaning we also had to devise a way to move the whole system from the test compound to the bridge, without damaging the new main spans.”
Our solution
To facilitate this, a temporary works design was engineered by Tony Gee & Partners, with us designing the temporary jacking arrangements to simulate the actual system on the structure, as Colin Campbell, Senior Engineer at Mabey Hire explained: “To prove the proposed system would work, it was important that it was tested. To do this, Tony Gee & Partners designed a temporary works scheme featuring our bridge panels and corner brackets to create an 11-bay truss. This was then connected directly to the bridge’s new trusses, which were placed on seven rail bogies, with our hydraulic jacks placed underneath the trusses. This whole system was then pushed 50 metres along the mock rail, proving that it did indeed work.
“The second part of the test involved energising the jacks to lift up the edge beams and, using a slider system, move the main spans across onto mock piers, made using our Mass 50 propping and Hymat jacking solutions. We then de-energised the jacks to lower the spans back down, simulating the actual installation.”
With one of the main challenges being the movement of the trusses onto the bridge, our hydraulic jacks were crucial to these works, accounting for the undulations anticipated in the tracks during transport. With the jacks pre-set to accommodate for the weight of the beams, they acted almost as a suspension system, ensuring there was little to no movement during transportation.
Another issue that Alun Griffiths had to consider was the positioning of the finished trusses, as Steve explained: “With the plan being to move the new trusses onto the bridge inside of the existing spans, we had to allow for 75mm clearance on either side of the whole system. This would allow the new trusses to clear the originals and then be easily placed into their footprint. Once the new trusses were in position on the bridge, the old supports were cut into smaller pieces and lowered onto pontoons on the estuary below, where they were taken away. The new trusses were then installed using a push-pull jacking system, which was mounted on a cantilever framework designed by Tony Gee & Partners.”
Steve concluded: “The project went well, with the temporary works system doing its job and successfully supporting the new bridge trusses. Having worked with Mabey Hire on the first phase of the renovation programme, we knew they had the capabilities to supply what we needed and they were very good to work with onsite.”
As a result of the Network Rail programme, the bridge now has an estimated further lifespan of 120 years and was reopened to trains in early December 2023.
Photo credit: Mulholland
