Client: Railway Safety Commission
The railway line between Dublin and Belfast crosses the Broadmeadows estuary immediately north of Malahide, in County Dublin. The relatively large and shallow estuary is divided by the railway causeway into two parts: the eastern outer estuary sheltered by a large sand-spit with an opening to the Irish Sea, and the western inner estuary receiving fluvial flow from River Broadmeadows. The estuary is classified as a Special Area for Conservation and an important site for wintering waterfowl and many other species of national importance.
The viaduct was originally constructed in 1844 as a timber trestle structure on timber piles. To secure the structure against scour resulting from tidal changes a large volume of rock was dumped along the line of the structure soon after the opening of the line. Rock was repeatedly dumped to make good storm damage, eventually forming a rock fill weir. In 1860 new masonry piers were constructed on the rock fill weir and new decks constructed. The decks have since been replaced but the structure retained its form with masonry piers on the rock fill weir.
The estuary has an extreme tidal range of 5m and the weir is subject to repeated high velocity flows in both directions. On the evening of the 21st August 2009, shortly after the passage of a passenger train, pier 4 collapsed bringing down the two spans bearing on it. The Railway Safety Commission appointed Atkins to review Irish Rail’s remedial design and the construction thereof on their behalf.
The reinstatement of the viaduct had two aspects to it, the urgent remedial works to safely reinstate the rail link between Dublin and the North and to provide whatever works were required to ensure the long term stability of the structure.
The remedial design comprised of two separate but interlinked parts, the structural design of the bridge and the design of the weir. The bridge design included piling for the replacement pier, the replacement pier and the two new decks. The remaining existing piers were strengthened by mini piles drilled through the piers and the rock weir. The design calculations were checked in detail and commented on by Atkins. Several of the collapsed beams were cut apart to allow the existing post stressed tendons and anchorages to be inspected. The procedures and reports of this activity was subject to Atkins scrutiny.
In conjunction with the structural design the behaviour of the weir was being examined by the flood study group at the University College Cork. This included the theoretical modelling of the river, estuary and weir together with a fixed bed model of the system and a moveable bed model of a section of the weir.
Atkins reviewed all aspects of the remedial design including the hydrology and hydraulics of the weir and the specification and materials used for the reconstruction of the breach and for the long term stability of the weir. Our expertise in coastal defence contributed significantly to the final design of the rock armour used to stabilise the weir.
All checking and reporting was done to a very demanding programme to minimise the disruption of the rail service, the priority however being the long term safety of the rehabilitated bridge and weir.
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