M48 Severn Bridge – main cable inspection and rehabilitation

Opened in 1966, the M48 Severn Bridge has a main span of 988 m and represented a highly innovative and economic suspension bridge design. The cables were aerially spun and were protected in a traditional manner with red lead paste, wrapping wire and paint. Following intrusive inspections on other suspension bridges in the UK and US, the owners decided to carry out an internal inspection of the cables at a number of locations, which revealed unexpected poor results of corrosion, broken wires and a loss in strength. To provide data on the whole bridge, acoustic monitoring was installed to listen for wire breaks, with a system initially at mid-span, followed by a system covering the entire length of both cables. The deterioration process in the cables is driven by the presence of water leading to the corrosion of the cable wires, and followed by crack initiation, propagation and fracture. To arrest this process a cable dehumidification system has been installed. A two-stage procurement process was adopted to permit early drying of the most deteriorated sections of the cables at mid-span, with a second contract covering the remaining sections and integrating both systems. The complete dehumidification system was commissioned in late 2008.