The Severn Crossing - Cable Investigations
In April 2006, a programme of cable inspections began on the M48 Severn Bridge which opened on 8 September 1966. The inspections were carried out as a precautionary measure after corrosion was found in the suspension cables of bridges of a similar age and construction in the USA, Japan and many other parts of the world.
The main cables, which are each made up of 8,322 individual 5mm wires, were opened at nine test locations, and the wires inside were examined. Significant levels of corrosion were found at the mid-span, with lower levels of deterioration towards the towers (where the cables are under greater stress). Repairs were made to any broken wires that were found, and the cables re-sealed. Measures to reduce the rate of deterioration of the suspension cables have been taken with the installation of dehumidification systems.
The work was carried out in two phases and involved wrapping the cable in an impermeable material and blowing dry air into the cable to reduce humidity and dry out the wires inside. The system has been fully operational since December 2008 and the results indicate that the humidity within the cables has fallen to a level where corrosion will no longer take place.
To monitor the condition of the cables and keep track of the rate and location of any further deterioration, an acoustic monitoring system has been installed on the main cable.
A second round of inspections were carried out in 2010 to help us to further understand the level of corrosion and to verify the success of dehumidification. The results of these inspections have confirmed that, although there has been some deterioration within the main cable, the bridge remains in a safe and serviceable condition.
As a precautionary measure a restriction is in place limiting vehicles over 7.5 tonnes to one lane in each direction. This is to reduce the risk of the bridge being heavily loaded during an exceptional event, such as the closure of the Second Severn Crossing.
The M48 Severn Bridge provides a key transportation link supporting the local economies of England and Wales and the Agency recognises the importance of keeping this route safe and open to traffic. We would like to reassure the travelling public that the bridge remains safe to use.