The Collapse of the Malahide Viaduct

The Malahide Viaduct is owned and operated by Iarnród Eireann and was constructed in 1844 to provide a rail link across the Broadmeadow Estuary. The original design was a 176m long viaduct comprised of eleven timber spans, each approximately 16m long, and supported on timber piles driven into a estuary bed.

History of the Viaduct

Soon after it was open for operations, the structure was found to be highly susceptible to scour action. This was as a result of tidal flow in and out of the estuary. In order to address this issue, a stone weir was constructed along the length of the structure. This did not only protected the piles from scouring action but also assisted in maintaining a constant level of water in the estuary, thus reducing the volume of tidal flow. Also, in order to preserve the weir integrity, stones had to be continuously discharged along the viaduct to replace those lost through scouring action.

In 1860, the timber piles were replaced with masonry piers, supported on top of the stone weir. The super-structure was consequently replaced with wrought iron lattice girder beams. Subsequent maintenance of the structure continued in 1922, with approximately 5200tonnes of stones having being discharged along the viaduct. In 1965, the lattice girders were replaced with post-tensioned concrete beams with a specific design requirement. This was the easy removal of the super-structure handrails, to facilitate the easy discharge of stones along the weir.

By 1967, a grouting program was initiated to inject concrete into the weir, and subsequent grouting was also undertaken in 1972-1973. Further discharge of stones, however, occurred in 1976, with the final discharge of stones occurring in 1996.

In summary, the ongoing management of the structure’s susceptibility to scour action was a key requirement since its original construction, with the 1965 handrail design indicating that the bridge owners anticipated that stone discharge would continue into the future

The Collapse

At approximately 22 minutes past 6:00 pm on 21st of August 2009, a train was travelling across the viaduct, when the train driver suddenly noticed a portion of the viaduct had begun to collapse. The driver, however, safely crossed the viaduct, but stopped at the nearby station, in order to alert the “controller” about the dangers ahead. In response to these, the controller immediately sets all signals to danger, preventing all incoming trains from crossing the viaduct. The train driver walked up to the line, by then, pier 4 of the viaduct along with the post-tensioned beams of span 4 & 5 had collapsed into the Estuary.

Unsurprisingly, the investigation led by the Railway Accident Investigation Unit (RAIU), concluded that scour action was the ultimate cause of the collapse. Scour action had undermined the foundation of pier 4 leading to the progressive collapse of the viaduct.

Lessons Learnt

Despite the long history of scouring awareness and prevention program, scour indeed proved to be the technical cause of the 2009 viaduct failure. Why did the management of Iarnród Eireann not anticipate the potential for this failure?. This answer to this question is simply “corporate memory loss.”

Whilst the RIAU investigation identified many contributing factors (such as poor training of bridge inspectors), shortfalls in structural inspection standards and the management failure to develop a working scour management plan. The investigation proved beyond all reasonable doubts the role played by ‘corporate memory loss’ in hampering Iarnród Eiermann’s ability to forestall the failure.

The RAIU investigation report defined corporate memory as “knowledge and information from a company’s past which can be accessed and used for present and future company activities¹.

With respect to the Malahide Viaduct, a critical piece of “knowledge and information” was the structure’s susceptibility to scour action and its historic maintenance program. But the RAIU investigation concluded that the engineers responsible for the structure in 2009 were unaware of the issue.

At the time of the failure, Iarnród Éireann managed their asset management data in their IAMS system, which was introduced in January 2005. It was envisaged that civil asset information, such as the information contained in Bridge Inspection Cards, would be uploaded into this system for future use. However, the RAIU investigation found that this had not occurred, nor was there widespread acceptance and enforcement of the IAMS system. In the case of the Malahide Viaduct, there was no information to highlight the structure’s susceptibility to scour – the only information uploaded was the viaduct’s number and mileage. Even at a basic level, there were no construction drawings relating to the viaduct’s foundations and, in the absence of this information, the engineers erroneously assumed that the masonry piers were founded on bedrock – and were thus unaware that the integrity of the structure as a whole was dependent on the integrity of the weir.

Furthermore, the RAIU investigation found that former Iarnród Eireann staff were indeed aware of the viaduct’s scour susceptibility and maintenance regime. However, these staff members had since left the division and the two district engineers and two assistant district engineers who worked in the division between 2002 and the time of the failure informed the RAIU investigators that they had no knowledge of the scour risk to the structure.

Finally, as with all significant failures, opportunities to identify the potential for failure was missed. For example, four days prior to the collapse, on the 17 August, a group leader of the Malahide Sea Scouts, who was a regular canoeist on the estuary, noticed that some of the stones around the base of Pier 4 had been washed away². He informed Iarnród Eireann, and an assistant engineer investigated. The engineer carried out the inspection on the viaduct the following day and identified that some of the masonry pier’s stonework was missing or cracked. He assumed this was the issue reported by the Scout’s leader and consequently, reported that he found no major structural defects.

Malahide Viaduct Taken by Inspecting Engineer
Figure 1: Evidence of Erosion Around Pier 4 Three Days Prior to Collapse

However, during the inspection, the engineer took a photograph (Figure 1) which indicates the presence of some serious erosion around Pier 4. Not only does this incident highlight a missed opportunity to identify the potential for failure, but it also highlights the poor training the inspectors received in the identification of scour related issues.

In closure, this failure is a reminder of the mundane but typically critical role played by human factors in structural collapse. By 2009, it appears that the knowledge and information relating to the scour susceptibility of the Malahide Viaduct resided in the heads of a number of individuals who had left the division, rather than in a formal system that was accessible to the engineers responsible for the structure. In an era where the concept of a ‘job for life’ is becoming more uncommon, and with engineers moving evermore frequently from job to job and role to role, often taking corporate knowledge with them, this failure highlights the very real risks faced by asset management organizations, due to the threat of corporate memory loss.

Also see: Structural Failures – A Reflection of Societal Failure


1) Railway Accident Investigation Unit(2010) Investigation Report No. 2010-
R004: Malahide Viaduct Collapse on the Dublin to Belfast line on the 21st August 2009 [Online].Available at: (Accessed: November 2020)

2) Lessons from the Malahide (2013). The Structural Engineer, Professional Guidance Note. The Institution of Structural Engineers. (Accessed: November 2020).

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