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Balcombe tunnel failure lessons revealed

15 Nov 13 Until it became synonymous with fracking, the Sussex village of Balcombe was more closely associated with a problematic rail tunnel, which partially failed in September 2011.

Balcombe tunnel
Balcombe tunnel

The Rail Accident Investigation Branch (RAIB) has now released its report into that incident and set out the lessons to be learned.

Early on the morning of Friday 23 September 2011, the crew of an engineering train passing through Balcombe tunnel observed that part of a large steel structure mounted in the roof of the tunnel, spanning over both railway tracks, was sagging down. An emergency inspection found that on one side of the structure, three supports had become detached from the tunnel lining leaving a 12 metre length partially supported.

The structure, one of six within the tunnel, was intended to catch water dripping from the tunnel roof. It was supported by anchor studs fixed with polyester resin into holes drilled in the tunnel’s brick lining. Within the tunnel, 18 studs (5%) were found to be missing and a further five studs were loose. The RAIB’s investigation has found that this connection was inadequate because the resin was not compatible with the tunnel brickwork and may have been adversely affected by shrinkage and the damp conditions in the tunnel. It is probable that the resin was selected using inadequate technical data and probable that insufficient resin was placed around some studs.

Although some railway staff were aware that studs had fallen from the structure on more than one occasion since 2008, this did not result in appropriate risk mitigation.

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This was because of inadequacies in the reporting of these events and because there was insufficient support for a member of railway staff who was managing some aspects of the tunnel maintenance but had limited experience. Inadequate access for tunnel examinations due to conflicting demands on the limited available access is considered to be an underlying factor.

The RAIB has identified three learning points from this incident: the need to consider the adequacy of information contained in manufacturer’s data sheets; the need to maintain awareness of published information; and the benefit of marking significant tunnel defects such that they are visible from track level.

The RAIB has made nine recommendations addressed to Network Rail that focus on managing existing polyester resin connections and controlling the future use of this material; confirming the compatibility of materials during design work; effective responses to defects and abnormal events; competency of staff managing structures; access for examining structures; the examination process for structures in tunnels; and retention of records relating to structures.

  1. Network Rail should, where failure could result in risk, identify where polyester resin anchors have been used to support structures (including overhead electrification and signalling equipment), and develop an appropriate regime to detect loose fixings including tactile testing where appropriate.
  2. Network Rail should implement procedures to prevent the use of polyester resin anchors in circumstances where dampness or shrinkage may affect the safe performance of an asset.
  3. Network Rail should review, and if necessary amend its processes, such that designers of structures are required to positively confirm the compatibility of materials with their intended application and environment, including fixing metallic structures to masonry, if the application is safety critical.
  4. Network Rail should review and, if necessary, modify the management arrangements that are now in place to provide an appropriate engineering response when structure defects are reported. This should include assessing the risk in the period prior to rectification, the means to verify that work requested has been carried out, and whether the reported defect is an indication of a wider problem.
  5. Network Rail should undertake a comprehensive review and, if necessary, implement a time-bound plan to modify its levels of staffing and competency requirements so that all technical tasks associated with the management of structures are performed or checked in a timely manner by sufficiently qualified and experienced staff.
  6. The intention of this recommendation is to improve the effectiveness of Network Rail’s investigations when abnormal events are reported. Network Rail should revise its arrangements for the briefing of staff or contractors who are sent to investigate reported defects, so that all relevant available information is provided, and correct any deficiencies found in those arrangements.
  7. Network Rail should review, and if necessary amend, its processes to include adequate safeguards such that sufficient track access is provided for the examination needs of all structures in a manner commensurate with the risk they pose to railway safety.
  8. Network Rail should clarify arrangements, including its relationship with its contractors, for examining structures which are within tunnels, but are not fully encompassed by the normal tunnel management regime.
  9. Network Rail should review, and if necessary improve, arrangements for recording, storing and retrieving data so that all relevant information is readily available to staff undertaking the examination, evaluation and maintenance of structures.

The full report can be seen here:

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