Water leak caused Barrow bridge failure, says RAIB
A ruptured water main is being blamed for the collapse of a bridge over the Midland Main Line (MML) at Barrow upon Soar on August 1 2016. Part of the structure fell onto the fast lines of the MML, disrupting services for three days ( RAIL 807).
The Rail Accident Investigation Branch (RAIB) released its report into the incident on June 6. RAIB says the wing wall of the 1840built structure collapsed while contractors from Construction Marine (CM) were carrying out core sampling, to identify voids under the pavement which it was believed were responsible for subsidence.
RAIB concluded that this core sampling disturbed a water main on the bridge. The damaged main ruptured, and the flow of water was compounded by a full-height vertical crack in the wing wall, which then overturned. There was also probably a slow leak from the water main, which caused the subsidence of the footpath.
Six minutes before the collapse, which took place at 2350, an East Midlands Trains London St Pancras-Nottingham train had passed under the bridge at 92mph. An approaching freight train was halted by an emergency call from the signaller when it was about 500m away.
During the core sampling, the core barrel and casing struck a hard object, but after further blows of the drop hammer, the obstruction cleared. Water then rose in the core casing and overflowed the top of the surrounding trial hole, which then emptied of water. Shortly after, the wing wall began to collapse and the sampling rig operators and their supervisor ran off the bridge to safety.
RAIB’s investigation showed that no original construction drawings of the bridge were found, and that there were few records retained of historic maintenance.
It also revealed that CM did not carry out a pre-work survey to scope the core sampling works and identify services, because a hole had been excavated in the same place in 2015 and no services had been found. The wing wall was built with a uniform vertical thickness - unlike most walls built to carry lateral loads, which are thicker at the bottom.
Network Rail investigations have shown that ground water could be ruled out as a cause, as this was only found ten metres below the bottom of the cutting underneath the bridge.
In its recommendations, RAIB says Network Rail should identify in its structures database those structures which carry water and other utilities, so that the information is readily available to asset engineers, structures examination contractors and minor works contractors. It should also provide training and guidance to asset engineers and structures examination contractors, so that they can identify the presence of water and other utilities in structures, recognise defects caused by leaks, are aware of the consequences of a major utility failure, and can decide on appropriate actions to be taken.
RAIB adds that NR should introduce a requirement in its procedures to notify utility companies about any emerging problems which could affect the integrity of a structure, in order to enable early remedial action and prevention of further deterioration. The infrastructure manager should also rebrief asset engineers and structures examination contractors on the importance of recording evidence of underground utilities and changes since previous examinations.
NR should also review how it procures intrusive works to its structures carrying water and other utilities, and verify that the process provides for input by suitably qualified engineers to assess the risk to the structure from the works.
The final recommendation affecting NR is that it should review its process for determining the appropriate level of competence for site supervision of works.
One recommendation applies to Construction Marine. It should undertake a review of its management processes for the planning and execution of works on structures that carry water and other services, including the training, competence and supervision of operatives who may be required to locate pipework.