Malahide Viaduct Collapse

The Malahide Viaduct, also known as the Broadmeadow Viaduct for the estuary in Ireland that it crosses, is owned by Iarnród Éireann (Irish Rail). How do organizations forget stuff that they knew for 150 years? It seems to happen more than it should. Malahide Viaduct (2009)

On August 21st of 2009 as an Iarnród Éireann passenger/commuter train passed over the Malahide Viaduct, the train’s engineer witnessed a section of the bridge collapse. He was able to have the dispatch center stop all traffic across the bridge, so that there were no injuries. Pier 4 was washed away, and 2 sections of the bridge collapsed.

First some history.

An 11 span timber viaduct was first built here in 1843, for the Dublin and Drogheda Railway and a problem with settlement, noticed soon after construction, was solved by depositing huge quantities of stone around the timber support piers, thereby creating a type of weir. Erosion remained a problem, despite continuous efforts to combat it and in 1860 a new bridge opened with masonry piers supporting wrought iron spans. By 1968, 12 spans of prestressed concrete had replaced the much deteriorated, Victorian iron across the 176 metres of the viaduct. The stone weir remained vital to the stability of the overall structure.

While the stone weir remained vital to the structure, that knowledge was lost, because we don’t need to manage infrastructure, the bridge is obviously fine, or so everyone thought right up until it collapsed.

Ongoing maintenance of the weir had been a thing until 1996. The Malahide Viaduct Collapse. (There is a great image of the viaduct post-collapse at the top of that article. Click thru.)

In 1846, two years after construction, a stone weir – which can be thought of as an underwater wall – was built. This weir did two things: it reduced the volume of water flowing in and out of the estuary; and it directly protected the sediment at the base of the piles – the scour forces would first need to dig away this rock protection before they could begin to compromise the structure. But this wasn’t enough – the rock protection was eroding. So to keep this rock wall in place, stones had to be continually discharged along the weir to replace those that were lost.

Then in 1860 the timber piles were replaced with masonry piers supported directly on top of the stone weir, and the remainder of the timber structure was replaced with wrought iron beams, which decades later were replaced with the post-tensioned concrete beams that were in place at the time of the failure.

And throughout all this work, management of the scour risk continued. For example, in 1922, a total of 5,200 tonne of stone was discharged along the viaduct, and in 1967-68 grout was injected into the stone weir to help hold it together. Then further stone discharges occurred in 1976 and 1996.

As that article notes, there was 150 years of knowledge of the impact of erosion. How do you lose track of that? Because after 1996 there was no maintenance of weir, despite the fact that a 1997 report called out scouring at the base of pier 4. The report by the Railway Accident Investigation Unit of the European Union is quite long, and I haven’t read all of it. It does address the loss of that knowledge. August 2010 Malahide Viaduct Collapse on the Dublin to Belfast Line, on the 21st August 2009.

There was a loss of corporate memory when former Iarnród Éireann staff left the Division, which resulted in valuable information in the relation to the historic scouring and maintenance not being available to the staff in place at the time of the accident.

And there was no system in place for capturing – even with pen and paper – the issues relating to infrastructure. Which precipitated…

The historic maintenance regime for the discharge of stones along the Malahide Viaduct appears to have ceased in 1996, resulting in the deterioration of the weir which was protecting the structure against scouring.

The next item of note is that the Malahide Viaduct was inspected 3 days before it collapsed. It was in “fair” shape according to the report.

An inspection carried out on the Malahide Viaduct three days before the accident did not identify the scouring defects visible at the time;

How could this be? The people doing the inspections had really no way to detect scour, and they were not trained to do so anyway.

There are other issues. A derelict barge was left stranded on the side of the viaduct for a VERY long time, and probably contributed to erosion. Because maintenance is not important.

The company confirmed the viaduct was inspected twice last week, and that it stood over those inspections. A full bridge inspection was also carried out in October 2007, with the next scheduled inspection for October.

Several people called about erosion. The bridge was “inspected” and deemed safe. We’ve seen that again and again in failing infrastructure, a “good” rating on an inspection right before failure.

The missing pier was replaced, the rest were reinforced, the track repaired and trains started traveling across the viaduct about 3 months after the collapse.

After this viaduct collapsed, Iarnród Éireann ran around and inspected 100s of other viaducts.

Here’s a video that covers various hydraulic effects due to stuff we build in the water. I have it cued up to a section on bridge piers and scouring. The relevant bit is about 1 minute long.