The structure, which carries Grove Lane in Barrow upon Soar, partially collapsed over the Midland Main Line on 01 August 2016. The incident caused a large volume of masonry to fall onto the railway line below.
At the time of the incident, core sampling work was being undertaken by Network Rail and its contractor Construction Marine Limited to investigate localised subsidence in the footpath on the south side of the bridge. The bridge was closed to the public when the collapse occurred, but the railway lines below were open to traffic.
When the coring had reached about 1.4 metres below ground, water appeared at the surface and shortly afterwards, the adjacent wall fell away from the side of the bridge, taking with it part of the footpath, a length of cast iron water main and the core sampling rig.
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Five workers were able to get clear as the collapse occurred and no-one was injured. However, two of the four railway lines through the bridge were completely obstructed and there was debris on a third.
An investigation by the RAIB discovered the incident occurred because the bridge wall, built around 1840, was not designed to resist overturning. The water main, which ran close to the vertical crack, probably had a slow leak which was causing on-going subsidence in the footpath. Prior to 01 August, however, there was no evidence that the wall was at risk of imminent collapse.
RAIB said the coring work on the night of the incident disturbed the pressurised water main and it ruptured, resulting in the release of water behind the wall which quickly overloaded it and caused the wall to overturn about its base.
The organisation has made two recommendations to Network Rail;
• The first relates to the competence of its staff and contractors, and the availability of information to enable them to manage the potential risk to its structures from breaches of water utilities
• The second relates to the provision of appropriate engineering input to risk assessments for intrusive investigations and masonry repairs on bridges carrying water services
A further recommendation is made to Construction Marine about the improvement of processes relating to street works and the location of water services.
RAIB stated: "The report has identified a learning point to reinforce the requirement for bridge examiners to report evidence of underground services and any changes since the previous inspection to enable a possible connection to be drawn between a water main and observations of defects on the bridge."
(LM/MH)
