The collapse of the viaduct at Malahide severed the northern rail route – can it really be fixed within three months?
IF JOE O’DONOVAN, the engineer who designed both the Dundrum Luas bridge and the M1 bridge over the River Boyne, was still alive today, there’s no doubt he would be intrigued by the problems posed by the collpase of the Broadmeadow viaduct to the north of Malahide, Co Dublin and how they can be resolved to reinstate the severed northern railway line.
Joe died last year, long before he might have applied his mind to it. But other structural engineers in Dublin, many underemployed as a result of the recession, have gone out to the estuary to have a look at the damage – motivated by a mixture of sheer curiosity and the chance of bidding for work.
What to do next is the conundrum facing Iarnród Éireann. “It’s a very difficult problem,” says one engineer with long experience of railways. “The immediate thing is to establish whether works need to be carried out on the remainder of the viaduct, before they start looking at possible interim measures, such as a temporary bridge.
“Even if they had a spare bridge that they could stick in the gap, how sure would they be that the rest of the structure would stay up? And if it does need to be replaced, you have to remember that Broadmeadow Estuary is a Special Area of Conservation (SAC), and that would mean doing a full environmental impact statement (EIS).”
He recalled that the National Roads Authority had “horrendous problems” in building a bridge across the estuary to carry the M1. For not only is it designated an SAC under the EU Habitats Directive, but also a Special Protection Area (SPA) under the Birds Directive – mainly to protect migratory Brent Geese. Thus, any new viaduct or bridge would have to go through the planning process, even though it would qualify for a direct application to An Bord Pleanála under the Strategic Infrastructure Act. “You’re talking about a long time-scale for that, a lot longer than three months,” the railway engineer warns.
A leading structural engineer, who does not wish to be named, describes the Broadmeadow viaduct as “not dissimilar to most of the national infrastructure we’ve inherited, like all those masonry arch bridges designed for the horse-and-cart era that are still standing today – even under the weight of Glanbia tankers”.
The viaduct has a span of 182m (600ft) and was first built by the Dublin Drogheda Railway in 1844 – rather optimistically from timber. This was replaced in 1860 by the Great Northern Railway with a wrought-iron superstructure standing on 11 stone piers grounded in a seabed causeway. Ten of these are still standing. In the 1960s, the wrought iron was replaced by a pre-stressed concrete deck. It was a 20m section of this deck that collapsed on Friday, August 21st, after one of the stone piers crumbled beneath it.
As reported in The Irish Times on Wednesday, Iarnród Éireann was alerted the previous Monday by a leader of Malahide Sea Scouts that the viaduct could be in danger. As regular users of the estuary for watersports, they had noticed a “massive change in the water flow over the past two months”, one of his colleagues says. A third of the water was going through one of the arches that collapsed, because this part of the seabed causeway had been breached over the past two months – effectively creating “rapids” under this arch right alongside the pier that crumbled.
It was possibly because the damage was below the water at high tide that it wasn’t spotted during an inspection carried out the following day. Neither was any deviation from normal conditions picked up by the track-monitoring vehicle that passed over the viaduct on Thursday, just 24 hours before the collapse.
Whatever about what precisely went wrong, Iarnród Éireann’s priority now is to get the railway line opened as quickly as possible. According to the structural engineer, this could be done by spanning the gap, with or without rebuilding the missing pier. “The pier could also be replaced quite easily, if that option was chosen.”
He says this could be done by driving steel piles into the seabed, either from a barge or from the viaduct itself – assuming that the rest of it is stable. This would be “a pragmatic engineering solution” that would allow the line to be reopened relatively quickly, rather than “going for something more complex”.
Strengthening works might also be needed to reinforce the remaining 10 piers, if this was necessary. In his view, merely because they are nearly 150 years old doesn’t mean that they shouldn’t stay standing to get the viaduct “up and running and perfectly safe”. In the long-term, a replacement could be considered.
Many of the elements of a new viaduct could be prefabricated and slotted into place in Broadmeadow Estuary. The new Samuel Beckett Bridge in Dublin’s docklands, designed by Spanish architect Santiago Calatrava, was shipped from Rotterdam; cable-stayed on a pivot, its span is 123m. The Boyne Bridge west of Drogheda is also cable-stayed, but with a taller “inverted Y” pylon to support its 400m span. Another structural engineer suggested a conventional viaduct supported by fewer intermediate piers.
Any replacement viaduct would first have to be designed and then subjected to an EIS. Then it would have to go through the planning process which, as the structural engineer notes, “can’t be done overnight”.
Only after getting the all-clear could it be built. The viaduct carried 90 trains each weekday, including three with zinc ore from Tara Mines. For the mining company and for 10,000 commuters, the sooner it’s reopened the better.
HOW WAS IT FOR YOU?
Following the collapse of the viaduct, commuters were faced with some difficult journeys.
Pamela Duncan looks at how the week went
There were mixed reports from commuters, with some happy with the alternative bus arrangements - even though drivers took different routes and some had to be directed by passengers. Others, such as commuters in Rush and Lusk in north Co Dublin, found themselves unable to get on full buses.
Ruairi Hickey, who commutes to and from the city centre from Collon near Drogheda, says his week has been “pretty poor”. “I’ve been trying all the different ways of coming in. There’s a private bus from Carrickmacross – goes through Collon on the N2 – so I tried that. That was OK coming in in the morning – gets you in in about an hour but I left the office at 5pm on Monday and I didn’t get home until 7.10pm whereas if I left the office at 5.10pm to get the train I’d be in Drogheda at 6.05pm so it’s a long day. I drove yesterday because I had to work late but it’s what, nearly €2 for the toll at Drogheda and €12 for the tunnel, so it’s €14 in and €14 out – that’s €28 a day – that’s too expensive. Lifestyle-wise, I coach kids rugby two nights a week and that kicks off at 6.30pm, so there’s no way I can get back for that at the moment.”
Conor Faughnan of AA Roadwatch says that disruptions to traffic were minimal this week but warns that the worst is yet to come. Faughnan says traffic has been a “little heavier” incoming on the M1 but said that this was a “very false picture”. He says that the actual impact will not be known until the second week in September when school-goers and late holiday makers have all returned to the roads. “There is no doubt that traffic volumes are going to worsen as surely as night follows day . . . How badly this will affect the north of the city – we’ll find that out in the next couple of weeks.”
Andrew McLindon, media and PR manager for Bus Éireann, says the company has been “coping fine” with the extra volumes of passengers. “We will have to factor in that schools are back next week but we will be monitoring that situation on an hourly and daily basis.” Maria Brennan, Dublin Bus press officer, says the company had had a “fine week”. “We’re very happy with the way everything went. We’ve put on an extra 16 buses but we will continue to monitor the situation when the schools come back next week and we may put on extra buses if necessary.”
Alan Field, the founder of carpool.ie, says there has been an increase in carpool matches as a result of the rail disruption: “We’ve definitely seen an increase in the number of people along that route.”
The estimated number of daily rail journeys which have been affected by the viaduct collapse
The number of extra Dublin Bus/Bus Éireann buses which have been provided each day since the disruption