Weaknesses in Network Rail’s bridge assessment regime were to blame for the collapse of a railway bridge near Feltham last November, rail accident investigators said this week.
“Failure” to avert collapse
The Rail Accident Investigation Branch (RAIB) criticised Network Rail and the Environment Agency for failing to avert the bridge’s collapse.
The 170 year-old brick structure spanning the River Crane failed without warning on 14 November 2009, causing part of the bridge to subside.
The first indication of a problem was a track defect reported by a train driver crossing the bridge on the up line at 10.45pm.
Track maintenance staff, called to the site, immediately blocked the up line to all traffic when they became aware of a serious defect with the bridge. The down line was not blocked until 5.55am the following morning.
A total of 21 trains crossed the failing bridge between the first report and closure of both lines. There was no derailment and no injuries occurred.
Scour “went unnoticed”
The RAIB found the cause of collapse to be scour which undermined the bridge’s east abutment.
The scour went unnoticed because of “weaknesses” in Network Rail’s assessment regime, the RAIB found.
Railway officials failed to notice that part of a fallen tree upstream of the bridge had caused a change of river flow.
The altered flow had gouged a void beneath the bridge’s east abutment leaving a 5.5m long section unsupported. This equates to about 65% of the abutment’s width.
An Environment Agency inspector spotted the obstruction in October but did not report it to Network Rail. A bridge examiner working for Network Rail contractor Amey also failed to notice the obstruction.
The RAIB said the failure to check for obstructions was a probable cause of collapse. It also said an underlying factor was Network Rail’s failure to carry out mandatory underwater examinations.
RAIB’s report orders Network Rail to improve its assessment of scour risk, and its approach to underwater assessments.
“Network Rail should review its underwater examination task lists nationwide to check for further omissions, and require that underwater examinations are normally undertaken in advance of scour assessments to enable a fuller picture of a structure’s condition to be realised,” it says.
Safety reporting service Cross said the effects of scour must be taken more seriously.
Cross director Alastair Soane said that this was the third major rail bridge collapse in a year. In particular, he emphasised similarities with the Malahide collapse in Ireland where scour was also to blame (NCE 11 March).
“Scour gives no warning in a collapse so bridge inspectors must make the extra efforts to identify any possible precursors,” added Soane.
Network Rail “has taken action”
A Network Rail spokesman said it had taken action to minimise the risk of a recurrence of this type of incident in advance of receiving the RAIB report.
He added that the rail operator will “consider carefully” the recommendations RAIB has made in respect of Network Rail.
The Environment Agency said it could not comment as it was still considering the report’s findings.