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CROSS Safety Report

Hidden defects in railway masonry arch viaducts

Report ID: 764 Published: 1 April 2019 Region: CROSS-UK

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A reporter raises concern about the sell-off of spaces under railway arches, as it may become difficult to carry out future inspections and maintenance.

Key Learning Outcomes

For asset owners and managers:

  • Regular inspections and maintenance can help keep a structure safe before it deteriorates beyond repair

  • Be aware that safety critical defects may be hidden behind linings

For all built environment professionals:

Full Report

Find out more about the Full Report

This section contains the Full Report submitted to CROSS and describes the reporter’s concerns or experiences. However, the text has been edited for clarity, and identifiable details have been removed to ensure anonymity and confidentiality. If you would like to know more about our secure reporting process, or submit a report yourself, please visit the reporting to CROSS-UK page.


A reporter is concerned about the sell-off of spaces under railway arches. This was brought to their attention by an article in Issue 852 of the Rail Magazine. If space under railway arches is sold off, then it becomes almost impossible for inspection and maintenance to be carried out. The article says ‘the risks of something really disastrous resulting from this sale cannot be discounted’.

Selling off the leasehold for 125 years is likely to lead to some major difficulties in a few years, according to the reporter, since there may be defects already hidden behind linings. The reporter is aware that many inner-city arch viaducts, where the space below the arch is used for small businesses, have not been inspected for years. For those few that have been inspected, defects have been found that have required immediate remedial action. In some cases that has not always been successful.

The freight loading on some of these structures is such that the joints between the bricks open and close repeatedly as the train passes over. Small amounts of mortar are lost occasionally, leading to greater movement over time.

The freight loading on some of these structures is such that the joints between the bricks open and close repeatedly as the train passes over. Small amounts of mortar are lost occasionally, leading to greater movement over time

The reporter believes that this problem needs to be addressed if a potential collapse is to be averted and thinks that the process could best be started by raising this issue with CROSS.

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

Railway engineers have a long tradition of monitoring, maintaining and keeping safe old masonry bridges. There is also a long tradition of business premises being located underneath bridge arches and rail authorities have powers to access these as they require. There are occasions when defects are discovered during inspections so the ability to gain access for visual sightings to look for problems and precursors of failure is important.

The reporter is right to raise a concern and the matter has been considered by the railway authorities. An article in Railway Technology from September 2018 states ‘Network Rail has sold the sites on a leasehold basis with plans to retain access rights for using the property for railway operations in future’.

Can the inspection and assessment requirements be met?

It is understood that the railway arch sell-off is supported by robust measures to ensure that the asset management inspection and assessment requirements for the loadbearing railway arch structures can be met. The new regime will of course need to demonstrate this will remain the case.

A more recent approach is to install long-term monitoring systems in old bridges to evaluate the deterioration of the structure and to determine the effectiveness of previous maintenance measures. This may include the following measures:

  • Sensors to measure distributed deformation

  • Acoustic emission sensors at specific damaged locations

  • High sensitivity accelerometers to measure the dynamic response of the structure due to vibration caused by trains passing over

The sensing data is then interpreted remotely. However, concerns have been expressed to CROSS about the ability of even the most advanced accelerometers to measure critical movements for masonry bridges. In long viaducts, where there can be many tenanted arches, an additional challenge is to decide which arches to monitor.

Network Rail’s challenge statement on tenanted arches

As part of Network Rail’s research and development (R&D) programme, they publish challenge statements to raise industry awareness of their priority challenges and to promote research and development into new ideas and technologies to solve them. They have published a challenge statement on tenanted arches and are inviting the supply chain to help with R&D activity for remote condition monitoring, hidden inspection techniques and improved detection of defects by way of example.

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