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Second phase of Severn Bridge main cable de-humidification works due to start

Laing O’Rourke and its partners, Munters and Faber Maunsell have successfully completed the installation of a main cable de-humidification system on the lower centre spans and back spans of the Severn Suspension Bridge for the Highways Agency. The high level works are due to start at the end of the month.

The project involved wrapping the Severn Bridge main cable in polymer based material to create a homogenous seal and installation of a de-humidification plant room within the bridge. Dry air is blown into the cable to reduce humidity, dry out the wires inside and slow down the rate of corrosion.

The system has been installed on the bottom third of the cables in the middle spans and on the bottom third of the back span cables. Final commissioning took place on 11 February 2008 and the system has been operational since. High level works to complete the system are due to commence this month and should be completed by 2009.

Monitoring will continue and further intrusive cable inspections are planned for 2010.

"A programme of cable inspections was carried out on the bridge between April 2006 and January 2007 after corrosion was found in the cables of similar bridges in other parts of the world," said Highways Agency Project Manager Chris Pope.

"As a result, we now have a programme of works to install the dehumidification system to the entire length of the main cables. In addition, to keep track of the rate and location of any further deterioration, an acoustic monitoring system has been installed on the main cables."

Technical manager for the Highways Agency, Martin Lynch said: "The main challenge is that we stay on programme to wrap the cables so we can start to dry them out as rapidly as we can. If the monitoring system is working satisfactory, we should be able to reduce the humidity by about 40% which should stop the corrosion processes."

Later this year, a longer-term strategy for the bridge will be considered, including other potential traffic management measures and repair options with cost ranges.

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