CONTRACTOR JOHN Laing is considering plans to finally bring a failed maintenance monorail beneath the Second Severn Crossing into service - six years after the bridge opened.
Dubbed the RAT train - Rapid Access Transport - the monorail has been out of operation almost since the £300M crossing opened in 1996. It was taken out of service after cracks were found in welds on the 5.2km long underdeck support rail.
John Laing is responsible for the gantry as part of the Severn River Crossing operating consortium. It commissioned consultant Gifford to investigate the problem and come up with a solution.
Consultant Flint & Neill is now monitoring the project on behalf of the Highways Agency.
Associate Jerry Cutter said the problem appeared to be a design fault in the rail suspension system.
'The rail itself basically consists of two standard I beams side by side, welded together longitudinally top and bottom, with the RAT car supported off the outside bottom flanges, ' he explained. 'But the design of the hangers seems to be causing excess transverse bending stresses in the box section, resulting in weld failures.'
Although the crossing also has three massive underdeck access gantries, these are complex and time consuming to deploy. The four passenger RAT train was designed for easy access, with stations at 500m intervals giving access into the concrete box girders of the approach viaducts and their piers, and to the underside of the central cable stay steel span.
No one at Laing was available for comment as NCE went to press.
However, it is understood that Gifford's proposals include replacement of some of the existing hangers, most of which are at 4.6m centres. This will ease the transfer of load and minimise transverse stresses, Cutter said.
Key to the final cost and programme will be how many work gangs are used on the massive retrofit job.