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.

Severn Bridge cable investigationThe 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 theSevern Bridge cable investigation 2 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.