Irish Railway Record Society

JOURNAL 171

Malahide Viaduct - Reinstatement

Following the collapse of the viaduct on 21 August, IÉ instituted extensive replacement bus services for commuters. Many were express services and used the Dublin Port Tunnel for quick access to the city. Enterprise services ran to/from Drogheda with bus connection to/from Dublin. A revised timetable was introduced from Monday 7 September after it proved impossible to maintain punctuality and allow locomotive fuelling at Drogheda during the day. Weekday trains departed Dublin at 07:35, 09:30, 12:55, 15:15, 17:20 and 19:35. Departures from Belfast were at 06:50, 08:00, 11:05, 14:10, 16:15 and 18:40.

Work on removing the collapsed pier and concrete bridge beams commenced immediately. Temporary buffer stops were erected on both lines on either side of the worksite and an additional ballast layer was laid to act on top of the track as a temporary roadway. This allowed a large tracked crane and other machinery access via the trackbed.

IÉ said analysis of the area in the vicinity of the collapsed pier indicated that the likely cause of the collapse was due to the pier being undermined by the scouring action of the tides. Scouring resulted from a breach of the weir across the mouth of the estuary under the viaduct.

The weir distributes tidal movements evenly through each opening under the viaduct. This ensures that the entire waterway under each span is used to handle the flow thus reducing the velocity of the water. Minimising the velocity of the water reduces the risk of scour, thus protecting the structural integrity of the viaduct.

The breach in the weir reduced the level of the top of the weir at the fourth pier from the north end. Water flowed to this artificial low point, greatly increasing the speed of the flow under one span of the viaduct. The increased speed of the water resulted in local scouring around the base of one pier, eventually triggering its collapse. The flow through the scoured section was equivalent to 70% of the entire flow into and out of the estuary.

The first action was to reduce the velocity of the water through the weir to prevent further damage and loss of water from the inner estuary. Five and ten tonne stones were placed in front of the weir and back-filled with smaller stones. This allowed the water level in the estuary to rise to normal levels by mid-September, in time for the migrating bird season.

Having stabilised the weir, a 4m access causeway was constructed along the weir parallel to the line using hundreds of tonnes of rock. This causeway was used to allow access for heavy plant and machinery. Pipes have been put in to ensure a normal flow of water through the Broadmeadow Estuary.

IÉ evaluated options and decided to re-build Pier 4 and replace the concrete beams linking it to both adjacent piers, as well as strengthening the remaining piers with pile driven supports. By early September, work commenced on preparing a foundation for the replacement pier. Piles were driven 20m deep to bedrock to support the rebuilt pier. The collapsed pier was completely re-built in concrete and new concrete spans installed to replace those that collapsed into the estuary. Other work included strengthening of all other piers on the viaduct, including piling and grouting.

All works were designed by IÉ who retained Roughan O'Donovan, Consulting Engineers, to review all design decisions. IÉ were advised on the causeway by Dr. E. McKeogh, Department of Hydrology, University College Cork, who directed the construction of a physical model of the site and compiled a computerised model of the water movements. It is understood that the causeway will be retained. Dr. Eric Farrell, Trinity College Dublin, advised on geotechnical aspects of the work. IÉ also engaged Dr. M. O'Sullivan, UCC, as environmental consultant to advise the project on methodologies to avoid adverse long term environmental impacts. Dr. O'Sullivan assembled a team to prepare an environmental report on any impacts resulting from the repair works. Liaison with the National Parks and Wildlife Service also took place.

Following re-instatement of track, testing using laden Tara trains commenced on Thursday 12 November. The first train to cross was an empty Enterprise set from Dublin to Belfast, 208 + 8 De Dietrich, which crossed at 10:00. Fifteen minutes later, 072 + 12 Tara wagons arrived from Drogheda. The Tara train remained at the Malahide outer home signal north of the viaduct for forty minutes before moving on to the bridge, stopping with the locomotive clear of the bridge and with just the rear two wagons on the new section. The train was divided, with 072 + 6 moving into Malahide station where it reversed out on the down line to stop on the viaduct with again only the two rearmost wagons on the new section. The procedure was reversed at 14:30 and at 14:50 the recombined train moved off to North Wall.

The following day, 081 and a train of track panels crossed the viaduct shortly after 10:00 en route to Platin. 075 + 12 laden Tara wagons arrived in Malahide at 10:43 from North Wall. It departed north to the bridge at around 10:55 where it was divided. 075 + 6 then went to Drogheda to run round and returned to the up line. The train of 075 + 12 Tara later passed Raheny at 17:03 en route to North Wall. On Saturday, 072 ran up and down light-engine to clean the rails from Malahide.

Work was carried out within the original three month estimate provided by IÉ and the line re-opened on Monday 16 November. The direct cost of repairs was reported be €4.5m - 5m, but the full cost including replacement bus services and loss of revenue was estimated at €10m. IÉ said the number of passengers on the Enterprise service fell by 60% with the drop in commuter numbers reported at 40%. Translink estimate at it would lose £1m – 2m in revenue from the disruption.

Copyright © 2010 by Irish Railway Record Society Limited
Revised: January 04, 2016 .