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.

The 1960s saw the construction of two major suspension bridges in the UK. With a span of 1006 m, the Forth Road Bridge was a traditional design, whereas the M48 Severn Bridge at 988 m span was a highly innovative design using a steel box deck for stiffening. After nearly 40 years of heavy use the owners of Forth Road Bridge decided to carry out an internal inspection of their cables, which revealed unexpected results of corrosion, broken wires and a loss of strength. A review of how these problems were addressed and solved around the world was undertaken and cable dehumidification was seen as the best option. Installation of the Forth dehumidification system is nearing completion and sets new standards in terms of its method of installation, efficiency, and web-based control system. An inspection of the Severn Bridge cables followed the Forth inspections and somewhat worse results were revealed, which prompted the need to urgently install a cable dehumidification system. A partial system was rapidly designed and installed on the most deteriorated sections of the main cables, followed by a full system on the remainder of the cables. The full system is now commissioned and operating.

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