At 08:40 hrs on Thursday 31 December 2015, subsidence of Lamington viaduct resulted in serious deformation of the track as the 05:57 hrs Crewe to Glasgow passenger service passed over at a speed of about 110 mph (177 km/h). The viaduct spans the River Clyde between Lockerbie and Carstairs. Subsequent investigation showed that the viaduct’s central river pier had been partially undermined by scour following high river flow velocity the previous day. The line was closed for over seven weeks until Monday 22 February 2016 while emergency stabilisation works were completed.
The driver of an earlier train had reported a track defect on the viaduct at 07:28 hrs on the same morning, and following trains crossed the viaduct at low speed while a Network Rail track maintenance team was deployed to the site. The team found no significant track defects and normal running was resumed with the 05:57 hrs service being the first train to pass on the down line. Immediately after this occurred at 08:40 hrs, large track movements were noticed by the team, who immediately imposed an emergency speed restriction before closing the line after finding that the central pier was damaged.
The viaduct spans a river bend which causes water to wash against the sides of the piers. It was also known to have shallow foundations. These were among the factors that resulted in it being identified as being at high risk of scour in 2005. A scheme to provide permanent scour protection to the piers and abutments was due to be constructed during 2015, but this project was deferred until mid-2016 because a necessary environmental approval had not been obtained.
To mitigate the risk of scour, the viaduct was included on a list of vulnerable bridges for which special precautions were required during flood conditions. These precautions included monitoring of river levels and closing the line if a pre determined water level was exceeded. However, this process was no longer in use and there was no effective scour risk mitigation for over 100 of the most vulnerable structures across Scotland. This had occurred, in part, because organisational changes within Network Rail had led to the loss of knowledge and ownership of some structures issues.
Although unrelated to the incident, the RAIB found that defects in the central river pier had not been fully addressed by planned maintenance work. There was also no datum level marked on the structure which meant that survey information from different sources could not easily be compared to identify change.
As a result of this investigation, RAIB has made three recommendations to Network Rail relating to:
- the management of scour risk
- the response to defect reports affecting structures over water
- the management of control centre procedures.
Five learning points are also noted relating to effective management of scour risk.