National Cycle Route saved by innovative bridge abutment replacement

Full Moon Bridge is a single-span pedestrian bridge in Crosskeys, South Wales, that also carries Route 47 of the National Cycle Network across the River Sirhowy. The bridge was designated for full replacement following extensive flood damage sustained to the Victorian stone masonry abutment in 2020.

Damage left the bridge unsupported for over 50% of the abutment length, and it was immediately closed to members of the public for safety reasons.

Through discussions with the client, EDS proposed that the existing abutment could be replaced in place by propping the bridge from a temporary structure positioned beneath the bridge deck in the river channel.

Key Challenges:

  • Designing and fabricating a temporary steelwork support structure capable of carrying the bridge’s weight and resisting erosion and impact from debris in flood conditions
  • Accommodating the temporary structure in a narrow river channel while diverting river flows to dewater the working area for a new reinforced concrete abutment
  • Working around a structure with reduced support and managing risks to operatives during installation of the temporary steelwork support

Solutions:

  • EDS and temporary works sub-consultant Pebble Engineering Ltd. designed and fabricated the temporary steelwork, with manufacture completed in our in-house workshop
  • A trial erection of the support structure was carried out to ensure it could be installed safely in the channel once the foundations were placed, minimising the need for operatives to work beneath the bridge before stability was reinstated
  • A survey control system was established to monitor bridge movement, reducing risks during both temporary and permanent works
  • Construction began in April 2021. Low river flows initially allowed the installation of pollution control measures and a temporary river bund to divert water and dewater the area for precast concrete foundations

  • Unfortunately, flooding in May 2021 overtopped and damaged the bund, leaving it no longer watertight or structurally sound. The immediate priority became securing the bridge’s stability, as further floods risked losing the remaining support from the failed abutment

  • EDS developed alternative methods for placing the precast foundations without dewatering, drawing on our in-house commercial diving capabilities. Using the damaged bund to create calmer conditions, a cofferdam was installed in two sections around the foundation locations. Our divers then cast a 200mm-thick underwater concrete slab using a 32m boom pump and tremie method. To achieve accuracy in nil visibility, a pre-installed screed rail system guided the diver during placement
  • After curing, precast slabs were set down and resin-fixed. With a solid foundation, the steelwork was lifted into position, aligned and secured. Excavators carried out the lifting, while support cross beams, hydraulic jacks and Lindapter clamps were used to hold the bridge. Once supported, the damaged abutment was demolished under close monitoring, with jacks adjusted as required
  • From there, construction progressed quickly: reinforcement, formwork and concreting to cast the new abutment base slab and stem wall. Unlike traditional bridge builds, the abutment had to be cast up to the level of the original bearings before the bridge deck was lowered onto it
  • Final activities included constructing the approach retaining structure, backfilling and compaction, scour protection with rock armour and rip rap, reinstating the footpath, installing a safety fence, and landscaping

EDS’ innovative approach not only saved the bridge from collapse but also eliminated the need to remove the deck for abutment reconstruction. By adapting to site conditions with underwater diving techniques, we reduced weather-related delays and reopened the bridge for public use in August 2021.