RAILTRACK WILL have to carry out maintenance work on around 25 bridges on the West Coast Main Line before Virgin's new high speed trains can run.
Tests have predicted that the trains - which will travel at up to 230km/h - will cause some lightweight long span bridges to vibrate in resonance, creating vertical deflections so large that the track ballast will become unstable.
Railtrack is contracted to produce a line capable of carrying Virgin's Pendolino fleet at 200km/h by 2002, and 230km/h by 2005.
But if ballast loses its compaction and becomes unstable, it can cause soft spots and sleeper movement, resulting in poor ride quality and leading to the imposition of speed restrictions.
Problems were first encountered with increased resonance on the French high speed TGV line between Lyon and Paris in 1997. Reports of ballast leaping off the trackbed led to speed restrictions being imposed.
Bridges on the line were designed using the internationally accepted value of 0.35g as the maximum vertical acceleration value for new structures.
On average, French bridges are significantly longer than the 1000 bridges on the WCML, although the problem is being taken seriously.
Consultant Mott Macdonald has been carrying out the bridge monitoring and risk assessment for Railtrack.
It has found that steel structures are most at risk because they are relatively light and have a low natural frequency of response.
Masonry arch bridges are heavily damped and therefore not affected.
The solution for steel bridges is to stiffen them and so increase their frequency of response beyond the danger zone by welding plates to the top and bottom of bridge girders.
At present, results obtained from the monitoring are only predictions. Tests are due to be carried out on the Pendolinos on the WCML next year. Work on the bridges will be done between 2003 and 2005, in time for the contract completion.
Mott MacDonald divisional director Tony Walker said the assessment work is a world first. Previously accepted acceleration values were for new structures, not for redesign or upgrade work.
Walker estimated the maximum cost of work on any single bridge would not exceed £500,000, giving an overall total of £12.5M.