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Bridge collapse blamed on engineering ignorance

The official report into the collapse of the Malahide Viaduct near Dublin last summer has found a complete failure of railway engineers to understand what type of structure they were dealing with.

The report, commissioned by Irish Rail (Iarnród Éireann), has found that engineers were unaware that the viaduct was in effect two distinct structures − a causeway/weir and a viaduct.

A major accident was narrowly avoided on 21 August 2009, following the collapse of pier 4 of the viaduct. The line was closed for almost three months, reopening on 16 November 2009. Scour was immediately identifed as the likely cause of collapse, and this has now been confirmed.

The report, submitted to the Rail Accident Investigation Unit and the Railway Safety Commission last month and made public today, sets out a series of maintenance failings dating back to a 1967 refurbishment of the structure.

Anatomy of the collapse

Works undertaken in 1967 on the superstructure of the viaduct also included significant grouting work, to a depth of 2m, to the causeway/weir.

These works, it was believed, would generally reduce the need for ongoing maintenance, particularly the unloading of “rip-rap” stone (large stone blocks) which had been regularly carried out to maintain the causeway/weir profile by replacing stones washed away by the tides.

Since this time, the placing of rip-rap was more limited and appeared to be carried out only to protect the piers.

Over time, erosion of a section of the causeway/weir between Piers 4 and 5 caused changes to the water flow under the structure, resulting in the majority of the water flowing in a deepened channel between these two piers, further increasing erosion.

In a relatively short period of time, the weir “crest” receded from the seaward side of these piers to beneath the span between them and, subsequently, onto the other (estuary) side of the viaduct.

Since grouting works were undertaken in 1967, engineering emphasis has been on the maintenance of the viaduct structure itself.

Official report

In the months prior to the collapse, the channel deepened further and the flow became ever stronger with standing waves and, latterly, a “piping” mechanism causing further “scour” action. Eventually Pier 4 became undermined and collapsed.

A key finding of the investigation is that since grouting works were undertaken on the causeway/weir in 1967, the engineering emphasis has been focused on the maintenance of the viaduct structure itself. However, the condition of the grouting in the causeway/weir required maintenance.

By this time, although protection of the pier foundations was still being undertaken, the importance of maintaining the weir profile was no longer fully appreciated.

Prior to the collapse, therefore, it was no longer appreciated that the structure as a whole comprised two separate components: a causeway/weir and a viaduct.

The structure is unusual in that the piers did not extend down to the “bedrock”, but are instead founded within the manmade causeway/weir formed of large rip-rap resting on the bed of the estuary, making the piers prone to erosion or “scour” damage.

Prior to the collapse it was no longer appreciated that the structure as a whole comprised two separate components.

Official report

The report said climatic, oceanographic and hydrological changes over recent decades have also increased the hydraulic “head” and hence the erosive effect of the water flowing into and, more especially, out of the Broadmeadow Estuary over the causeway/weir.

During the week before the collapse, a group leader of Malahide Sea Scouts observed that a rock at the base of pier 4 had been washed away and contacted Iarnród Éireann on 17th August to report this.

The information reported by this member of the public was dealt with in a “professional manner” by Iarnród Éireann staff, but due to a “misunderstanding” the engineer sent to inspect the viaduct on 18 August was looking primarily for cracks or missing stones in the pier structure rather than in its foundations.

He found the “dressed” stonework of the viaduct to be in need of pointing and there were some cracked stones on a number of piers.

Whilst none of these faults were of a serious structural nature, their presence appeared to him to explain the reason for the report from the canoeist. Therefore this visual inspection did not lead engineers to question the stability or the structural integrity of the viaduct.

Action taken

Iarnród Éireann said a series of actions have already taken place or are underway arising from the accident.

The replacement Pier 4 is founded on piles and all the remaining existing piers have been retro-fitted with piled foundations. The weir has been reconstructed to its original profile and an improved weir profile is being developed. A bridge monitoring system has been installed on the Malahide Viaduct.

Iarnród Éireann’s acting chief civil engineer has initiated a full review of the systems in place for monitoring structures subject to scour and has commissioned consultants to look at international best practice for this with a view to implementing system improvements.

Pier and abutment depths are being established for all bridges on the scour list wherever practicable. Where this is not possible, other mitigating measures will be implemented.

The structures standard should be revised to include more information on ‘scour’, the erosive effects of different water conditions.

In addition most of the bridges on the “scour inspection list” have been inspected by engineer divers and this work will be completed by April 2010. Following on from these inspections each structure will be given a risk rating and the inspection frequency will be based on this rating.

Trigger levels will be defined for special additional inspections of the structure as required (e.g. exceptional tides) and/or its closure when conditions deteriorate. A re-opening process for each structure is also to be documented.

Investigations have found that there is one other structure on the IÉ network that has similar foundations to Malahide, Rogerstown Viaduct. This is on the same route as the Malahide Viaduct. Pier and abutment depths have been established for this structure and found to be deeper than for Malahide and are secure.

The report also recommends that the Irish structures standard be revised to include more information on ‘scour’ and the erosive effects of different water conditions, particularly in the context of the design of remedial measures.

The revised structures standard should be supported by the running of a series of Structures Inspection Training Courses.

“Using a skilled external contractors would be useful to ask the ‘what if’ questions in relation to maintenance and justifying inspection procedures. It is a good searching technique.”

John Carpenter

The training should incorporate ‘follow up’ mentoring in the field by experienced, competent staff, it said. Roles and reporting lines for structures and track patrolling inspections should also be reviewed and a ‘handover’ process should be put in place to ensure knowledge is not lost on staff movements within the organisation or when staff leave the service.

Standing Committee on Structural Safety secretary John Carpenter said Iarnród Éireann had to recognise the problem of “corporate memory loss” caused by staff turnover. Bringing in external contractors could help raise standards, he said.

“To guard against scour, it requires an engineer to have an overall competence to make judgements without the relevant data,” he said.

“Using a skilled external party would be useful to ask the ‘what if’ questions in relation to maintenance and justifying inspection procedures. It is a good searching technique,” he said.

Iarnród Éireann issued five further recommendations cover:

  • Flood and tidal warning arrangements
  • The installation of monitoring/warning equipment to structures susceptible to scour
  • Updating the bridge card system of monitoring the condition of structure
  • Improving the process for dealing with reports from the public
  • Addressing the effects of climate change, land and leisure developments in the Broadmeadow catchment area on the railway.

In particular, it is recommended that dialogue is initiated with the relevant state agencies accordingly.

Iarnród Éireann said the recommendations will be fully implemented, as will any and all recommendations arising from the investigation of the Rail Accident Investigation Unit.

Iarnród Éireann’s investigation was independently chaired by John Buxton, Chartered Civil Engineer, and was also advised by a panel of experts led by Dr Eamon McKeogh of University College Cork in relation to the complex hydraulic and environmental issues involves.

The Iarnród Éireann report will also be considered by the Rail Accident Investigation Unit in the preparation of their independent investigation into the accident.

Readers' comments (3)

  • Since this is an engineering report, it is surprising that several terms, familiar to all engineers, have been p;aced between inveerted commas.

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  • Keith Nicholls

    Should this not be titled "Ignorance over geotechnical behaviour..."

    apologies for the use of the inverted commas!

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  • Gareth Scott

    This article could be placed in the "Structural", "Geotechnical" or even "water" sections of the website. But for some reason NCE have decide to locate the article in the "Transport" section. I can't think why because on the if you dig through all the interesting engineering in the article a train is mentioned.
    How many articles written about "Bridges" will be located under "Transport"? Come on NCE "get a grip".

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