UK Rail Accident Investigation Branch Press Release: Investigation into the partial failure of a structure inside Balcombe tunnel, Sussex, 23 September 2011
The RAIB is carrying out an investigation into the partial failure of a structure inside Balcombe tunnel, Sussex, which resulted in the closure of the railway between Three Bridges and Haywards Heath from about 05:30 hrs on Friday 23 September 2011 until the following morning.
At 05:24 hrs on the morning of 23 September the crew of an engineering train, which was passing through Balcombe tunnel on its way to sidings at Tonbridge after overnight work near Hove, reported to the signaller at Three Bridges that there appeared to be a problem with a structure inside the tunnel.
There are five steel structures attached to the brick lining of the double track Balcombe tunnel, which span both tracks. The structures are intended to catch water running from ventilation shafts and other sources, and divert it into the tunnel drainage system. Each structure consists of a deck, supported by longitudinal beams which in turn rest on transverse girders which are about six metres long and located at approximately five metre intervals. Plates welded to each end of these girders are attached to studs embedded in the tunnel lining.
The line was closed following the train crew’s report and an inspection found that, on one of the structures, three adjacent transverse girders were detached from the tunnel wall at one end, and had dropped about 0.5 metres. A structural engineer from Network Rail confirmed that, although the failed structure was not fouling the path of a train, the tunnel was not safe for train movements. It remained closed to trains until 03:30 hrs on Saturday 24 September while Network Rail engineering teams carried out remedial work, which involved the installation of brackets to support the detached girders.
The structure which failed was installed about 12 years ago. It had dropped because twelve of the studs, which had been inserted into holes drilled in the tunnel wall, had become loose and fallen out. Other studs, securing nearby girders, also showed signs of looseness.
Network Rail is carrying out a programme of work to make permanent repairs to the water catchment trays in Balcombe tunnel.
The RAIB’s investigation will include an examination of the reasons for the failure of the fastenings, the design and installation process for the catchment trays, and how Network Rail managed the inspection and maintenance of the structures inside the tunnel.
The RAIB’s investigation is independent of any investigations by the safety authority (the Office of Rail Regulation).
The RAIB will publish a report, including any recommendations to improve safety, at the conclusion of its investigation. This report will be available on the RAIB website.