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Scour revealed as cause of Irish bridge collapse

Scour undermining a Victorian masonry bridge pier has been identified as the likely cause of a near-disastrous collapse of a section of railway viaduct on the Dublin-to-Belfast main line.

The bridge had been given the all-clear by engineers after a warning from a member of the public just days before part of it fell into the sea.

No-one was injured when a 20m section of the 176m-long bridge over the Broadmeadow Estuary between Malahide and Donabate stations in north County Dublin broke up on Friday 21 August just moments after a commuter train passed along the twin-track line. The deck section collapsed after a supporting masonry pier underneath crumbled.

Packed commuter trains carrying hundreds of people had been using the line minutes before the structural failure.

Iarnród Éireann (Irish Rail), which is responsible for the Republic of Ireland’s railway network and rolling stock, said that a potential tragedy had been averted only by seconds. There would have been major loss of life with the likelihood of a derailed train ending up in the sea.

A raised alarm

The alarm was raised by driver Keith Farrelly, whose 6.17pm Balbriggan to Dublin Connolly train was the last to pass over the viaduct before the collapse.

Farrelly, who was due to travel non-stop south to Dublin, halted his train at Malahide station just south of the bridge after noticing serious subsidence in the trackbed as his train passed over. He was being hailed a hero after raising the alarm, although movement sensors would also have alerted track controllers as to the unfolding collapse.

The bridge had been inspected and passed as safe in the days before the collapse following warnings to Irish Rail from local canoeists who use the estuary.

The bridge had been inspected and passed as safe in the days before the collapse following warnings from local canoeists .

The viaduct (see box below) consists of a prestressed precast deck resting on masonry piers dating from 1860. A rock weir to reduce tidal flows between the piers runs along the length of the bridge.

A member of a local sea scout group, who has been canoeing in the estuary for more than 20 years, told NCE that members of group had noticed in July that a channel had developed in the weir.

“It was like it had been gouged out by a JCB,” he said. “We thought that this had been something done by Irish Rail, so didn’t report it immediately when it was first noticed in July. We assumed they were fully aware and knew what was happening.”

Photographs taken by the group in July show that the channel in the weir created a v-notch effect, leading to increased water flow and pressure.

The structure and its history

The twin-track Malahide Viaduct carries the Dublin-Belfast “Enterprise” service, connecting thetwo largest cities in Ireland, commuter services for burgeoning new satellite towns north of Dublin and beyond, as well as freight trains servicing Tara Mines in County Meath north of Dublin.

The railway bridge over the Broadmeadow Estuary originally dates back to 1844, before the timber frame was replaced with a wrought iron structure and masonry piers in 1860.

The 11 piers are gravity structures without piled foundations, with a rock weir built in between the piers to mitigate tidal flows between the elements ultimately supporting the railway line. Nine piers are centred at 15.85m, with the abutment and first and last piers at each end centred at 12.25m.

At typical low tide, the difference between the water retained upstream of the weir and the sea is around 3.5m, and greater for more extreme tides, with a waterfall effect as water cascades over the structure. On the day of the collapse, this was shortly before 6pm.

Engineering archives reveal that scour was an issue during construction and afterwards, with more than 8,000t of rock added to the weir between 1915 and 1924.

Engineering archives reveal that scour was an issue during construction and afterwards, with more than 8,000t of rock added to the weir between 1915 and 1924.

A new viaduct was completed between 1966 and 1967 during short line possessions, replacing the wrought-iron structure with a prestressed precast concrete deck, simply supported over the masonry piers which had new bedding stones added at the top to support the concrete structure.

Cementation was contracted to pressure-grout and stabilise the weir.

Irish Rail says it will reinstate the collapsed section and not rebuild the entire structure, after repairing the breached weir - work which has already begun - and normalising water flows and protecting adjacent piers.

The company said it planned to rebuild the collapsed pier and strengthen those on either side of it to provide a new seat for deck beams which will span from these, supported by the rebuilt pier. It estimates this will take three months to complete.

 

The scout member, who does not wish to be named, said it was clear to his group that the water flow under the bridge had altered considerably, prompting a call to Irish Rail on 17 August.

In a written statement to NCE, the company said it had been alerted to concerns about “markings” observed on the piers and erosion to the piers.

“Iarnród Éireann responded promptly to this call by arranging for an engineer to inspect the viaduct and its piers on Tuesday 18 August. This assessment identified that there were no visible structural issues, and that all markings were cosmetic,” the company said.

It added that the marks were unrelated to the collapse, and that a track monitoring vehicle also travelled the line on 20 August, the day before the collapse, indicating there were no deviations in the track.

Crucial assistance

In its statement the company said that the estuary users “have been of great assistance to Iarnród Éireann’s investigation, and their input concerning their observations of the water in the viaduct area” was “crucial in helping to establish our primary line of inquiry” focusing on a “recent and significant erosion of the seabed” near the pier which collapsed.

“It is believed that in a relatively short timeframe, possibly in recent weeks, a small breach occurred in a causeway plateau within the seabed. This would have resulted in changes to water flow, with increased water pressure on the area.

“It is believed that in a relatively short timeframe a small breach occurred in a causeway plateau within the seabed.”

Iarnród Éireann statement

“Recent low tides, coupled with major rainfall on Wednesday [19 August], 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 seabed would ultimately result in the sudden and catastrophic undermining of the pier supports from below water level, resulting in the collapse of the pier on Friday evening [21 August],” the IÉ statement said.

Irish Rail has established an investigation committee, chaired by company board member and former managing director of the Hong Kong Mass Transit Railway Phil Gaffney. Other members are fellow Irish Rail board member Michael Giblin and UK consultant John Buxton.

Assisting the investigation are University College Cork professor of hydraulics Éamon McKeogh and Dr Eric Farrell of the geotechnical department at University College Dublin.

Detailed inspection

The company has also ordered a detailed inspection of all bridges and viaducts spanning running water across the Irish rail network, which covers 2,288km of track.

Investigators into the Malahide collapse are also to examine tidal and climatic issues. Ireland has seen record rainfall over the past three years, with July the wettest month on record. Inspection and maintenance procedures and reports are also to be examined (see box).

The Irish minister for transport, Noel Dempsey, expressed his relief that no one had been killed in the incident. He also demanded urgent information on similar structures and the cause of the accident.

Inspection regime

Iarnród Éireann (Irish Rail) says that in addition to drivers being required to report any unusual track conditions, the viaduct is subject to thrice weekly inspections by patrol staff walking the line, examining the track and track bed “as well as monitoring the condition of the viaduct as visible from the track”.

A track-monitoring vehicle − the EM50 − travels every section of track across the network every six months, while all bridges and viaducts are subject to a full inspection every two years. The last such inspection of the Malahide Viaduct was in October 2007 and the next is scheduled for next month.

Scour inspections by engineer divers at viaducts take place every six years, with the last inspection at Malahide in 2006 revealing “no scour issues”, according to Irish Rail. The company said the next diving inspection was not due until 2012, a schedule it says is in accordance with the Irish Railway Safety Commission’s requirements.

The bridge was designed for 20t axle loads, the largest the structure encounters from the Enterprise service between Dublin and Belfast.

All bridges and viaducts are subject to a full inspection every two years. The last such inspection of the Malahide Viaduct was in October 2007.

A 1998 safety examination report covering the entire network in the Republic of Ireland by International Risk Management Services for the Irish government gave the viaduct a 40% “safety inadequacy” rating and the embankment a 60% rating, making it one of the least safe stretches of infrastructure in the country. Since then, a major infrastructure investment programme has been under way.

A report into a cement train derailment at Cahir in October 2003 by the Railway Safety Commission cited serious shortcomings in Irish Rail’s asset management and maintenance procedures relating to the accident at the time. The report said that detailed records did not exist for the structure and that some maintenance procedures resulted from informal instruction.

The commission also said the network’s EM50 track inspection vehicle gave inaccurate readings, including under-measuring track cant by up to 50%. However, it indicated that a number of the shortcomings identified had been rectified after the accident.

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