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Alarm as engineers find potential for disaster on Thelwall viaduct

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THE M6 Thelwall viaduct survived a major scare earlier this month following another dramatic failure of the viaduct's main span roller bearings, NCE learnt this week.

Engineers working on the 1.4km long four lane viaduct found that a 160mm diameter solid steel cylindrical roller bearing had cracked in half beneath the single lane of traffic that runs over the structure during bearing replacement works.

Had the failure gone unchecked, stresses in the concrete supports and steel deck could have built up to dangerous levels, triggering a chain of structural failure.

Traffic was immediately realigned to pass over neighbouring supports while emergency works to pack the failed support with steel crates was carried out.

Contractor John Martin is replacing all roller bearings with spherical slider bearings on the viaduct. This was refurbished in 1996 by Amec in a £27M project. Consultant was Pell Frischmann. The work followed failure of roller bearings in June 2002 and subsequent investigations revealed that 136 of the seven year old roller bearings were failing due to antigranular corrosion (NCE 3 April 2003).

The latest failure came to light this week after further fears were raised for structure's integrity by engineers working on the structure.

They claim the rocker bearings, which were previously thought to be sound, are also in poor condition with PTFE sheets coming free.

The sheets are essential in maintaining the low friction necessary for the joint to operate as designed. Like the roller bearings these were supplied by Italian manufacturer FIP.

The Highways Agency this week confirmed that two rocker bearings are be removed for more detailed assessment but said that inspections of the whole viaduct structure are being carried out as part of its responsible management of the repairs.

'This includes visual examination of the rocker bearings, two of which will be removed in coming weeks for a more detailed assessment, so that we have all the available evidence about the impact of the works, ' said a Highways Agency spokesman 'No decision has been taken to replace the rocker bearings at Thelwall, ' he added.

Should all rocker bearings also need to be replaced this would significantly extend the works by up to 12 months. This is currently programmed for a spring 2005 completion.

The cost, which could exceed £20M, would further impact on highways contractors in the region who have suffered a downturn in Agency workload (NCE 27 November 2003).

Current work on the viaduct totals £45M. This includes £24.1M for a systematic replacement of 117 roller bearings, £10.5M for the emergency replacement of 19 roller and four rocker bearings by John Martin and £10M in traffic management.

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