“It was no longer appreciated that the structure as a whole comprised two separate components”

Yesterday RTÉ reported the publication of the independent report [pdf file] into the collapse of the Broadmeadow rail viaduct near Malahide last August. As the Irish Times reports today, following work carried out in the period 1966-1968, “It was no longer appreciated that the structure as a whole comprised two separate components: a causeway/weir and a viaduct”. From the Irish Times

The report said the structure of the viaduct was unusual because the piers holding it up did not extend down into the bedrock of the sea. Instead, they sat into a manmade causeway made of large stone blocks which rested on the bed of the estuary. This made the piers vulnerable to erosion. It said in 1967 grouting was carried out on the causeway and it was believed this would reduce the need for ongoing maintenance. Since then, engineers had focused on the foundations of the piers, replacing stone blocks to protect the piers, but not the entire causeway.

“It was no longer appreciated that the structure as a whole comprised two separate components: a causeway/weir and a viaduct,” the report said. The importance of maintaining the causeway “was no longer fully appreciated”. In the months prior to the collapse, the channel between pier 4 and pier 5 deepened and the flow became ever stronger with standing waves, the report found. Eventually, pier 4 became undermined and collapsed.

And from the report [pdf file]

3 The collapse of the structure was due to the undermining of one pier’s foundation caused by ‘scour’ erosion. 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 stone blocks (rip-rap) resting on the bed of the estuary. Thus the viaduct piers were prone to erosion or ‘scour’ damage.

4 Maintaining the causeway/weir was of paramount importance to ensure the integrity of the viaduct structure itself. In 1967 the superstructure of the viaduct was replaced and significant grouting work was undertaken to the causeway/weir, extending to a depth of two metres into the structure, to stabilize it. These works, it was believed, would generally reduce the need for on-going maintenance, particularly the unloading of rip rap stone 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.

5 Over time, erosion of a section of the causeway/weir plateau between Piers 4 and 5 caused changes to the water flow under the structure, resulting in the majority of the water flowing in the 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. 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.

6 A number of days after the collapse of the viaduct, as the initial investigations proceeded, engineers established that the first challenge to be faced in rebuilding the viaduct was stabilisation and re-instatement of the weir, before any work on replacing the collapsed structure could commence.

7 A key finding of this investigation is that since the grouting works were undertaken on the causeway/weir in 1967, the engineering emphasis has been focussed on the maintenance of the viaduct structure itself. However the condition of the grouting in the causeway weir deteriorated over time and eventually 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. [added emphasis]

And

12 Malahide Viaduct had received routine two yearly ‘thorough’ inspections by IÉ in 2005 and 2007 and a ‘special’ underwater inspection by a specialist company in 2006. No serious faults were found and it was recommended that the piers should be re-pointed when convenient, as the mortar loss was not in need of timely repair. It was further recommended that the substructure units be inspected underwater at intervals not to exceed six years and soundings taken after exceptional occurrences. It appears that none of the inspectors had any detailed knowledge of the particular foundation arrangements, although such information is often not available for a structure of this age. [added emphasis]

And from the report’s recommendations

Recommendation 1

Complete all actions in “Action taken or in progress since the incident” section of this report.

Recommendation 2

The structures standard should be revised to include more information on ‘scour’, the erosive effects of different water conditions (e.g. standing waves), particularly in the context of the design of remedial measures.

Recommendation 3

The introduction of the revised structures standard should be supported by the running of a series of Structures Inspection Training Courses. The training should incorporate ‘follow up’ mentoring in the field by experienced, competent staff.

Recommendation 4

Roles and reporting lines for structures and track patrolling inspections should be reviewed and a ‘hand-over’ process should be put in place to ensure knowledge is not lost on staff movements within the organisation or when staff leave the service. [added emphasis]