Restoration of Timber Rail Viaduct
Network Rail’s bridge stock in Mid Wales includes nine remaining under-line bridges which were constructed in timber over rivers and estuaries on the Shrewsbury to Aberystwyth and to Pwllheli Coast routes.
The spans are generally similar in length at about 6m and their construction is also of similar form but has variations in the assembly of the timber baulks.
EDS was commissioned by Alun Griffiths (Contractors) Ltd. on behalf of Network Rail to provide specialist dive and underwater engineering services to replace six timber piles on separate trestles on the 11-span Caersws timber rail viaduct on the River Severn in Mid Wales over two engineering possessions (blockades).
Working from floating pontoons, the sequence of works first involved the removal of the trestle diagonal and waling timber so the pile could be replaced. Working in accordance with the lifting plan and equipment (chain blocks and slings), timbers were lowered onto pontoons and temporarily transported away from the working area to allow the existing pile to be cut out above water level.
The location for the new pile splice joint to be cut into the existing pile stub had to be located below the lowest recorded river level to prevent long-term exposure to the cyclical effects of wetness and drying. EDS Divers carried out all underwater works using a combination of hydraulic tools to cut the new splice joint and to secure the new pile onto the prepared splice joint. Once the new pile had been installed and contact checked at crosshead level the existing trestle diagonal and waling members were reinstated.
The team were met with various site and weather challenges. On some piles to be replaced there was insufficient distance between the riverbed and lowest recorded river level, which meant hand excavation with underwater tools by our Divers in nil visibility. Weather wise, the second possession coincided with the wettest May on record, with the River Severn in flood at the beginning of the blockade. There was a slight delay to the commencement of the works, and the programme adjusted for the team to begin work on the Piles in a different location to the original plan.
Despite the challenges, EDS completed all works ahead of the original programme allowing Alun Griffiths to hand back the track to Network Rail on time.