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Capital Projects:

Main Cable Acoustic Monitoring

The purpose of the Upgrading of the Main Cable Acoustic Monitoring Project is to increase the likelihood of detecting wire breaks among the 11,618 individual high tensile steel wires that make up each cable.  Given the history and work carried out on the main cables, and the estimated reduction in strength that has been established, it is important to maintain confidence in wire break detection.  

The existing system was installed in 2006 and at that time it was very much a prototype system and one of the first installed on a bridge of this size.  Since then significant advances have been made in the field of acoustic monitoring of main cables and more advanced systems have now been installed on Severn and Humber Bridges.  

In order to increase confidence in the reliability of the detection of wire breaks, it is recommended that the existing system is upgraded and augmented.  It may well be that a tender from a contractor will propose replacement rather than upgrading and to ensure best value for the Authority all compliant tenders will be considered.  The estimated cost of the project in years 2013 to 2015 is £1.010 million.


Following the discovery of corrosion within the main cables in 2004, the Bridge Authority acted quickly to commission an acoustic monitoring system to detect new wire breaks within each cable.

The system is designed to provide early warning of any potential problem areas or increases in the rate of deterioration. It uses a series of microphones positioned along the cables to listen out for the unique sound of a wire breaking. Possible breaks are recorded and sent over the internet to be analysed by experts in Canada, who report back regularly to the Bridge Authority on the timing and location of any new breaks.

What acoustic monitoring cannot tell us is how many wires had broken before the system was installed, or how many wires are close to breaking as a result of corrosion. The only way to determine this is to wedge open the cables and carry out an internal inspection.

Commissioning of the system was completed in August 2006.

Related documents:

The Bridge:

Facts & Figures

Opened 1964, 2.5 km long, Main span 1006 metres
  • No restrictions on bridge (21:21 BST 24/05/15)