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Malahide Viaduct - Collapse

On Friday 21 August the driver of the up 18:07 Balbriggan-Dublin Connolly noticed that the down line on the viaduct that carries the Dublin-Belfast line across the Broadmeadow Estuary north of Malahide had started to subside. After crossing the bridge he stopped his train in Malahide station and the line was closed. By this time, two spans carrying both up and down tracks had completely collapsed into the Estuary. The following trains were the last to cross the viaduct. Times are for Malahide station:

16:10 Belfast-Dublin, 230 + 8 De Dietrich, 18:04

17:48 Drogheda-Connolly, 29008+29026, 18:13

17:50 Pearse-Dundalk, 29017+29007, 18:20

18:07 Balbriggan-Connolly, 29028, 18:24

These trains would normally have carried over 1,000 passengers between them.

Malahide viaduct is made up of 12 spans and is 176m (577’) long and also acts as a weir holding in an approximately 3 miles by 1 mile body of water. This weir creates a difference of 3.4m (11’) in water levels between the Estuary and the sea at low tide. The 8th pier from the Dublin end collapsed and 15.8m (52’) spans Nos. 8 and 9 either side fell into the water. Piers are stone with concrete bed stones supporting the concrete deck. The present viaduct and its earlier versions are described in detail by Oliver Doyle in JOURNAL 143. The article also details problems encountered from scouring with falling and rising tides.

IÉ said that following reports of damage to piers the bridge underwent a structural examination by an engineer the previous Tuesday who examined the condition of the piers as visible above water and found ‘no visible structural issues, and that all markings were cosmetic’. The last full inspection of the viaduct was carried out in October 2007, with the next inspection due in October 2009. The six-monthly run by the track recording car took place the day before the collapse and would have measured any differences in levels and even minor subsidence. ‘It did not detect any deviations in track level’. IÉ said underwater inspections are carried out every six years in line with RSC recommendations, with the next one due in 2012.

IÉ and the Rail Accident Investigation Unit of the Department of Transport began investigations of the cause of the failure. On Monday 24 August IÉ said that a preliminary assessment identified significant erosion of the seabed as a possible reason for the collapse of a section of the viaduct. ‘It is believed that in a relatively short timeframe, possibly in recent weeks, a small breach occurred in a causeway plateau [weir] within the seabed. This would have resulted in changes to water flow, with increased water pressure on the area. Recent low tides, coupled with major rainfall on Wednesday, would have seen the volume and speed of water flowing out of the estuary increasing, causing water pressures to increase, with ultimately the forces of water pressure widening the breach quickly. The effect on the causeway plateau and sea-bed would ultimately result in the sudden and catastrophic undermining of the pier supports from below water level, resulting in the collapse of the pier’. Local sources were reported as saying that tides in the estuary were up to 4.5m high, which is higher than usual. High tide was at about 13:00 and low tide was shortly after the collapse.

Local Sea Scouts who organise canoeing and other water sports in the area. were reported as saying "We had noticed a massive change in the water flow over the past two months, with a third of it going through one of the arches that collapsed... There was no doubt that the flow pattern had changed" due to the erosion of the causeway beneath the viaduct, creating "rapids" under this arch right alongside the pier that crumbled.

IÉ said it was establishing a team to carry out a detailed inspection of 84 bridges and viaducts across running water throughout the network. They will look for changed water flows, which could indicate erosion of the sea or river bed. Divers will check the structure below water for signs of scouring. A committee, chaired by board member Phil Gaffney, former managing director of MTR (Hong Kong Metro) will oversee the investigation and will retain independent advisors. “The committee also includes board member Michael Giblin, external consultant from the UK, John Buxton, an expert in bridge scour, and further outside expertise,” The team dealing with the assessment and reconstruction of the viaduct is being assisted and advised by independent experts Eamon McKeogh, Professor of Hydraulics, UCC, and Dr Eric Farrell, Geotechnical Department, TCD.

Closure of the line brought significant disruption of a scale last seen with the collapse of the River Tolka Bridge in 1956. Passenger trains between Malahide and Dublin were initially suspended but DART services resumed that evening. Enterprise services operated north of Drogheda and Drogheda/Dundalk commuter trains operated at a reduced frequency north of Skerries. From Thursday 27 August pilotman working allowed these trains to run to/from Donabate using the down line. It is estimated that 10,000 passengers are affected per day.

Enterprise punctuality was badly affected and a temporary timetable was introduced from 6 September. Connecting buses departed Dublin at 07:35, 09:30, 12:55, 15:15, 17:20 and 1935 Monday–Saturday and at 10:35, 13:35, 15:35 and 18:35 on Sundays. Trains departed Belfast at 06:50, 08:00, 11:05, 14:10, 16:15 and 18:40 Monday-Saturday and at 09:35, 12:35, 14:35 and 17:35 on Sundays.

Replacement express buses were put on and Dublin Bus also put on additional services but newspapers reported these buses carried one third of rail passengers. IÉ issued an urgent notice for the supply of replacement bus services commencing 2 October. They estimated the cost range ‘between €2m – €10m’. ‘It is envisaged that the services will be required for a minimum timeframe of 10 weeks. This could change substantially as more information becomes available regarding the extent of the remedial works required’.

The sole remaining freight service on the line from Tara Mines in Navan to Dublin Port was cancelled and replaced by 60 laden lorry movements per day. In addition, access to the main suburban railcar maintenance depot in Drogheda was cut off. However, IÉ were able to operate full Maynooth and Rosslare line services as most railcars were south of the bridge. The Connolly based De Dietrich set was transferred to Inchicore on the Saturday and to Heuston for storage the following Tuesday. Additional DART services operated to/from Malahide to supplement diesel railcars continuing to operate.

 

The following were north of the failed bridge: 072 (Drogheda), 206 & 209 (Enterprise), 227 & 228 (York Road, being fitted with TPWS), railcars 2614/17, 2805/06, 2807/08, 2811/12, 29003, 29004, 29006, 29007, 29009, 29014, 29017, 29019, 29021, 29025 and 29027.

IÉ estimated that it would take ‘at least three months’ to repair the bridge. Work on re-constructing the bridge commenced in earnest on Friday 28 August when large excavators placed rock boulders into the sea to build an access road from the Malahide end. Large rocks were also brought in to fill the breach in the weir, which is believed to have been "a significant factor in the collapse". This breach is believed to have undermined the collapsed pier and when filled will "normalise" water flows and protect other adjacent piers. A large tracked crane was placed on the line south of the breach. Temporary buffer stops were placed on both lines between Donabate station and the breach and ballast was placed over the tracks to allow machinery access.

IÉ said the work would also protect the estuary environment, ensuring that normal water levels and flows are maintained and sustaining the estuary eco-system. "This will include piling for platforms for the crane required for the reconstruction work; works on adjacent piers to strengthen these and provide a 'seat' for new beams and installation of beams across the two spans, to be supported by the strengthened piers”. Newspapers quoted the cost of the new bridge at €4m.

The remainder of this article appears in IRRS Journal number 170, published October 2009.

Copyright © 2009 by Irish Railway Record Society Limited
Revised: October 30, 2009
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