“If the whole viaduct is going to have to be rebuilt, it could be massive undertaking”

While Iarnród Éireann are still predicting it will be “at least three months” before the line across the Broadmeadow viaduct near Malahide reopens, following its collapse on Friday, Translink are already talking of the line closure lasting “6 months or more”. That seabed erosion, possibly in recent weeks, is being suggested as the cause of the collapse, might point to a problem with the location itself. As the Irish Times Environment Editor, Frank McDonald, details, the viaduct has been rebuilt completely on a number of occasions since the first timber structure was built by the Dublin and Drogheda Railway in 1844. It was last “replaced in the period 1966-1968 with the current prestressed concrete structure”.From Frank McDonald’s report

THE VIADUCT over Broadmeadow Estuary has given trouble since it was first built by the Dublin and Drogheda Railway in 1844, mainly due to tidal scouring of its support piers. Indeed, the first structure – made entirely of timber – had to be replaced by wrought iron superstructure on stone piers as early as 1860.

As engineer Niall Torpey recounted in the Irish Railway Record Society’s journal, the superstructure had to be strengthened to carry heavier locomotives on the Dublin-Belfast line in 1932. The stone piers also needed regular attention because of Broadmeadow’s tidal scouring and were repointed in the 1960s.

“Finally, following the observed deterioration in the wrought iron in this marine environment, it was further replaced in the period 1966-1968 with the current prestressed concrete structure”, Mr Torpey wrote. Since then, no further major works were carried out on the viaduct, which is 180m (594ft) long.

In 1998, as Fine Gael transport spokesman Fergus O’Dowd recalled yesterday, International Risk Management Services (IRMS) identified sections of the viaduct as being among the most unsafe stretches of rail track in the country, assigning it a 60 per cent security risk on a scale where 5 per cent is “best practice”.

The IRMS report led to a major improvement in railway standards, including relaying track on precast concrete rather than wooden sleepers. Two follow-up reports, also by IRMS, in 2001 and 2003 monitored the progress on what Barry Kenny, spokesman for Iarnród Éireann, called “this massive investment programme”.

And from the Irish Times report on the viaduct’s collapse

Iarnród Éireann said a preliminary examination showed a small breach occurred in the seabed, possibly in recent weeks, which changed the flow and pressure of the water around the viaduct piers.

“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,” said Barry Kenny, Iarnród Éireann spokesman.

“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 on Friday evening.”