A report by the Rail Accident Investigation Branch into the partial collapse of a bridge onto open railway lines last year has highlighted the need for better information to be available to Network Rail to manage the potential risk to its structures from breaches of water utilities.

The report by the Department for Transport’s Rail Accident Investigation Branch (RAIB) covers the partial collapse of a bridge over the Midland Main Line which resulted in a large volume of masonry falling onto open railway lines at Barrow upon Soar in Leicestershire around midnight on 1 August last year.

While there were no trains in the immediate vicinity at the time of the collapse, about six minutes earlier, a London St Pancras to Nottingham service travelling at approximately 92 mph and, about two minutes earlier, an empty train, travelling at approximately 96 mph passed under the bridge on the lines which became obstructed. Two of the four railway lines through the bridge were completely obstructed and there was debris on a third.

At the time of the collapse, core sampling work was being undertaken by Network Rail’s contractor, Construction Marine Ltd to investigate localised subsidence in the footpath on the south side of the bridge. Five workers were able to get clear as the collapse occurred and no-one was injured.

Lack of understanding of risk to structure and railway from work close to water main a key factor

Image showing partial bridge collapse at Barrow upon Soar

Following the incident Severn Trent Water (STW), who own and are responsible for the maintenance of the underground water main laid across the bridge, isolated the water main at 1:55 am.

The RAIB investigation found that the incident occurred because the bridge wall, built around 1840, was not designed to resist overturning. It had also been weakened by a full- height vertical crack. The RAIB concluded that the water main, which ran close to the vertical crack, probably had a slow leak which was causing on-going subsidence in the footpath. However, the report concludes that there was no evidence that the wall was at risk of imminent collapse prior to 1 August.

The report said that the coring work on the night of the incident disturbed the pressurised water main and it ruptured.

According to the RAIB, underlying the incident was a lack of understanding of the risk posed to the structure and to the open railway from coring in proximity to the water main.

Number of organisations had different responsibilities for bridge and surrounding infrastructure

Network Work was among a number of organisations with different responsibilities for the bridge and surrounding infrastructure, including Leicestershire County Council and Severn Trent Water, all of whom freely co-operated with the investigation.

The bridge carried a 4” internal diameter cast iron water main, reported by STW to be around 70 or 80 years old, with a service life of over 100 years, which had an inner cement-mortar lining was thicker at the bottom of the pipe. The report says this indicated that it had been applied in situ and had slumped before setting.

STW had no record of when the lining was applied and its records, held since 2010, indicate that there had been no bursts of the water main on Grove Lane bridge during that period. STW also had no records of any maintenance on the water main on the bridge although there had been two bursts on Grove Lane, downstream of the bridge, in April and July 2015 and water leaks in 2010 and in 2016.

After the collapse, approximately 16.2 metres of water main was found to be missing - 15.9 metres of pipe in five sections was recovered from the rubble. One of the sections of recovered pipe had a modern coupling but the water company was unable to provide any maintenance history of the repair.

Combination of underlying factors included slow leak and works activity close to water main

The pipework was forensically examined by a laboratory acting for STW under the supervision of a forensic specialist acting for the RAIB.

The RAIB concluded that the incident occurred due to a combination of factors, including a probable slow leak on the water main which was causing subsidence of the footpath. The core sampling activity in proximity to the water main also disturbed the pipe to the extent that it ruptured.

The report says that examination of the water pipes, carried out by RAIB’s forensic specialist, found that a section of water main under the area of subsidence in the footpath had corroded through the full thickness of the pipe wall. Combined with other evidence, this indicated that the pipe was probably leaking. The specialist estimated that the pipe had been leaking for approximately five years and possibly longer.

A separate report on the forensic examination of the pipes prepared for STW by its own consultant concluded that, while there was corrosion on both the inside and outside of the pipe and it was possible that the corrosion extended through the full thickness of the pipe wall, in their opinion there was no evidence of water leakage prior to the incident.

However, the report says that other evidence was consistent with a slow leak, including witness evidence from the coring activity that the ground material in the vicinity of the water main was wet.

The report states:

“A leak on an underground water pipe is a well-known cause of localised subsidence…..Leicestershire County Council’s Highways Engineer was aware from experience that settlement of this type could be caused by leakage from water pipes.”

“Although the Council could have conducted its own investigation, it tended to leave this responsibility to the structure owner. The Council was aware from the applications for road closures that Network Rail was investigating and assumed the matter was in hand.”

Network Rail:  investigation strategy did not cover eliminating water main as possible source of subsidence

However, Network Rail’s investigation strategy did not include eliminating the water main as a possible source of the subsidence, the report says. STW had no records of a request from Network Rail for assistance in ascertaining whether the water main was leaking and the water company was likewise not aware from its own leak detection processes of a slow leak at the location.

The RAIB considers that the partial collapse of the bridge would not have occurred without the trigger of a burst water main. Other causes have been considered and discounted.

Database of rail structures does not include information on buried services

Network Rail’s asset engineer did not investigate a leak as a possible cause of the subsidence because, during his several visits to the bridge, he had not observed signs of soil and water leaching through the structure. The report also says “he stated that he did not generally know whether there were buried services in a bridge” and that Network Rail’s database containing details of structures does not include information relating to buried services.

Contractor’s supervisor did not hold relevant street works competences

Although CML’s risk assessment for the work recognised the risk of bursting the water main, the depth of the pipe and its alignment were not identified and the CML supervisor permitted the core sampling to commence without the water main having been accurately located by hand digging.  

The supervisor stated that “he believed that the water main was protected by a concrete plinth because his previous experience was that all services were protected in this way.”

The CML supervisor did not hold the relevant street works competences including the competence relating to location and avoidance of underground services, required for the work being undertaken. He was also not supervised on site by someone with the relevant training and competence.

Risk to bridge was not understood by Network Rail and its contractor

Neither Network Rail nor CML understood the risk to the bridge structure and to the open railway from the coring works, according the RAIB.

The report states:

“An assessment of the risk involved would have required careful civil engineering evaluation in consultation with STW to minimise the likelihood of affecting the water main, but such an evaluation was not applied at any stage of the planning and execution of the investigation works.”

Network Rail project manager who procured works not a civil engineer and no knowledge of structure

The remit from Network Rail’s asset management section simply set out the objective to be achieved i.e. to carry out coring works through the footpath in the vicinity of the vertical crack to find any voids in the structure.  

According to the RAIB, no guidance was provided to the framework contractor on the sensitivity of the structure and there was no consideration of the potential risks involved in delivering the work.

Network Rail's asset manager considered that a risk assessment of the proposed works should have been managed by Network Rail’s Minor Works (Civils) section, which has access to all the bridge records and civil engineering competence within the section.

The Minor Works (Civils) project manager responsible for procuring the works was not a civil engineer and had no knowledge of the structure. Although the section has civil engineers who can be consulted on complex works, there appeared to be "no reason to commission a special risk assessment for this work, which seemed to be routine in nature", the report says.

“Impracticability of commissioning risk assessments for high volume of work items handled on a daily basis”

The report also states:

“The project engineer reported the impracticability of commissioning risk assessments for high volume of work items handled on a daily basis.”

In this instance, Network Rail placed the responsibility for undertaking the risk assessment for the works, onto its contractor CML. While the risk assessment carried out by CML’s site manager for the coring works recognised the potential for damage to a buried pipe and the need for a search to be carried out beforehand, it did not specifically refer to STW’s requirements for water pipes to be located by hand digging. The risk assessment also did not consider the risk to the bridge structure from a breached water main.

The CML risk assessment was also not shared with Network Rail before the work was carried out. Risk assessments are normally uploaded with other paperwork to a shared database by the contractor after the work has been completed and so there was no opportunity for the assessment to be reviewed by Network Rail prior to the work being done.

In the  RAIB’s view, the level of communication between the parties was such that “neither understood the real nature of the risk from the core sampling works.”

The RAIB has recommended that contractor CML should examine the way it approaches ‘street works’ on Network Rail structures to ensure that the risk of damaging water services are fully understood and its operatives are properly trained and equipped to control the risks. This should include the training, competence and supervision of operatives that may be required to locate pipework.

Network Rail database should identify structures carrying water and other utilities as readily available information 

Key recommendations included in the report say Network Rail should:

• identify in its structures database those structures that carry water (and other) utilities so that this information is readily available to its asset engineers, structures examination contractors, and minor works contractors;
• provide training and guidance to its asset engineers and structures examination contractors so that they are able to 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 decide on appropriate actions to be taken
• introduce a requirement in its procedures to notify the relevant utility company about any emerging problems which might affect the integrity of a structure, to enable early remedial action and prevention of further deterioration
• rebrief its asset engineers and structures examination contractors on the importance of recording evidence of underground utilities and any changes since the previous examination
• review how it procures intrusive works to its structures carrying water (and other) utilities, and verify that the process provides for sufficient input by suitably qualified engineers to assess the risk to the structure from the proposed works
• review its process for determining the appropriate level of competence for site supervision of the works

Click here to read the RAIB Report in full Partial collapse of a bridge at Barrow upon Soar