Broadmeadow viaduct

The present structure is the third on this site. The first was built in timber for the Dublin and Drogheda Railway in 1844. Its stability was badly affected by erosion of the river bed around the piles and, after short-term remedial work, it was replaced in 1860 with a new structure of wrought iron spans on masonry piers. This sufficed until the early 1930s when new locomotives (4-4-0 Compounds) required strengthening of the piers and continued maintenance. Additional ballasting of the piers was also needed. The sea air caused deterioration of the wrought iron and these spans were replaced during 1966–1968 with the current prestressed concrete structure. At this time, the tracks were laid on ballast, reflecting modern practice.[1]

On Friday 21 August 2009, at 6:30pm, a 20-metre section of the viaduct collapsed. Some reports state that the collapse started while a passenger train was passing over it, others say shortly after. The driver of the Balbriggan to Dublin Pearse service was passing over the viaduct and noticed the section crumbling away. He coasted the train (not increasing the engine revs in case the vibrations made it collapse while the DMU was on it) across it. The train was not derailed and no one was hurt: the driver raised the alarm when he arrived at Malahide. The repairs took roughly three months, with curtailment of all services north of Malahide.[2] The competence of Iarnród Éireann's maintenance regime has been questioned and there have been previous critical reports of the viaduct's structure.[3] Iarnród Éireann stated that the structure had been inspected three days previously.[2] Erosion had been reported by an Irish Sea Scout leader who later opined that a superficial inspection might have missed scour damage below water level.[4]

Official estimates of three months to repair the viaduct (by reinforcing the seabed and replacing the collapsed pier) and restore rail services were accurate[5] and Iarnród Éireann announced that the bridge would reopen on 16 November after "round the clock" repair works that reportedly cost €4 million. The bridge reopened on the scheduled date.[6] Other piers have been strengthened and the riverbed weir restored.[7] This is despite much more pessimistic unattributable comments by structural engineers shortly after the collapse.[8] Such pessimism arose because the estuary is a Special Area of Conservation (SAC) and a Special Protection Area (SPA) and it was suggested that a full environmental impact statement (EIS) would be necessary and, if a planning application were required, obtaining the necessary permissions alone could take more than three months. In addition, the stability of the remaining structure needs to be proven. A long term solution, perhaps a new longer span bridge, will be needed.[8]

An independent report found that spans 4 and 5 of the viaduct began to collapse as the 18.05 Balbriggan to Pearse train crossed the bridge at 18.23.[9] The report said that staff had followed proper procedures.[9] The design of the viaduct meant that the piers did not go down to the bedrock but instead joined with a man made causeway underneath, making the structure vulnerable to scour erosion.[9] The causeway between piers 4 and 5 were particularly badly eroded and erosion had increased in part due to climatic, oceanographic and hydrographic factors.[9]

Visual checks and inspections had not led to questions about the structural integrity of the viaduct.[9]

Access to original construction drawings and historical documents was limited.[9]

The replacement pier 4 is founded on piles and the remaining piers were retrofitted with piles.[9]