Highways England is in the final weeks of a major operation to tackle the threat of corrosion on the M48 Severn Bridge’s critical cables.
The public body and its team are carrying out work on several sections of the two main suspension cables as part of an ongoing investigation after problems were found on similar bridges in America.
Problems found on parallel strand cables in the US in 2004 prompted Highways England to carry out inspections on the M48 Severn Bridge – one of the UK’s only parallel strand bridges. Wires forming this type of bridge cable do not tessellate and therefore have natural gaps between the wires making it susceptible to water ingress. This is in contrast to a more modern [structural cabling] locked coil cable which closes the gaps and is therefore more impermeable to water and consequently corrosion.
Work now centres on the Severn Bridge, whose first inspection of the issue in 2006 revealed unacceptable levels of corrosion of a number of the parallel strands that form the main 511mm diameter cable.
Site workers are now progressing through the third phase of work to arrest the corrosion.
Inspections are focused on the lower central area of the suspended cable midway along the 988m long main span of the bridge, as this is where the most water ingress is expected and consequently where the worst of the corrosion may exist. Cables are also being checked at the upper levels to ensure that this is the case.
Contractor American Bridge and consultant Aecom are carrying out the programme of works, which involves stripping back the cables’ white protective polyethylene sheaths, before removing the original red lead paint to expose the 8,322, 5mm diameter parallel high tensile steel strands which make up the overall main cable.
Wedges are then hammered into the centre of the cable at eight points around the circumference to separate the strands, enabling the team to inspect the surface for damage and corrosion.
“[Site workers] drive wedges into the cable to inspect a face in the middle of the cable … by hammering a wedge in at the right point,” Highways England structures team leader Mark Maynard told New Civil Engineer. “From the top it’s relatively easy, but driving [wedges] in from the bottom requires significant man power.”
Cross sections of the cables are then produced, detailing the condition of the cables and the number of broken wires. By carrying out a statistical analysis on the gathered data, the condition of the whole cable can be determined.
Sensors embedded in the road surface help the team compare actual loads on the bridge to the revised capacity of the cables, based on the corrosion of the cables.
The work has also involved repairing broken strands. Where broken wires have been identified, in each individual case, a section 6m long is cut out and replaced by feeding both ends into turnbuckles, which are then tensioned to match the original state.
After the inspection has been carried out, strands are recompacted and bound together with a 3.78mm wire tight wrap, before the white polyethylene wrap is reinstated.
Work on the downstream side is almost complete and work on the upstream side is planned to be completed by October before the closure of the Severn Tunnel for electrification works later this year.