Upkeep issues could leave Scotland-Northern Ireland bridge idea all washed up

Dr Luke Prendergast

A specialist in ageing infrastructure has cast doubt on the practicalities of constructing a bridge between Scotland and Northern Ireland.

The idea of a ‘fixed link’ between the two nations has been back in the news lately after a feasibility study was launched to assess the potential of building a bridge or tunnel.

Dr Luke Prendergast, assistant professor in civil engineering at the University of Nottingham, told Project Scotland that as well as the major barriers which have been raised previously relating to dumped munitions and the depth of water, one further factor which he believes could prove fatal to the idea is the challenge of ongoing maintenance which would inevitably be involved with such a structure.

“This isn’t a new idea (a bridge between Scotland and Northern Ireland); this has been floated many, many times in the past,” Dr Prendergast said.

“There are a couple of things of note that make it very unlikely. The munitions that are dumped in the Irish Sea at Beaufort’s Dyke make it an extraordinarily complicated construction. This is a 3km-wide chasm about 300m deep so it would have to be bridged likely using some kind of a hybrid floating bridge.

“If you’re going to have a hybrid fixed floating bridge structure, it’s going to be quite a complicated system in itself.”

Even more challenging, according to Dr Prendergast, would be the upkeep of such a bridge over the decades to come.

“Buildability and constructability on Day One is one issue,” he explained. “But then how do you keep the structure active and operational for the remaining design life? A structure of this nature would probably have quite a long design life so would need to be incredibly well constructed and resilient against the environment you’re putting it into. You’re also putting it into an offshore situation which means that waves, storm conditions – all of these issues you have which damage bridges at the best of times on land – become so much more complicated to manage offshore.

“My own background is in scour erosion, which is where soil gets washed out from around bridge foundations by the action of water. It’s a damaging action that affects bridges which are located in river channels onshore. But if you put it into an offshore situation, it becomes almost like an offshore wind farm scenario. Maintaining these are extremely challenging.

“Bridges require very careful maintenance. How do you access those foundations to carry out routine maintenance of any description in these offshore environments? How often does the bridge need to be closed to do this routine maintenance? What are the ongoing costs likely to be? If it’s closed, what alternative crossing do we have? if we’re going back to a ferry service, what’s the point of building the structure in the first place?

“If you have a particularly stormy event, you may have to close the bridge completely to traffic. How often is this structure going to be out of action just because it can’t tolerate storms? There’s a huge amount of questions around that.”

Dr Prendergast added that other factors which would need to be considered include the type of infrastructure required to allow emergency vehicles to reach potential car accidents on the bridge and also the additional infrastructure needed to cope with an increase in traffic around the entry points to the structure.

“If you look at where these bridges are being proposed, there are two locations between Scotland and Northern Ireland,” he said. “One is 12 miles wide but it’s very rural. And one is 22 miles. The infrastructure leading up to that bridge – the roadways and motorways – would require significant upgrading to deal with the level of traffic they’re expecting.

“This is not just constructing a bridge; this is constructing a significant routeway to these really regional places.”

Dr Prendergast said while there are examples of floating bridges elsewhere in the world, they’re generally much shorter than what would be required here and in water less deep. Bridges also have a ‘design life’, which means they’re not built to be permanent.

“Everything has a design life,” he explained. “When we build a building, we build that with a view to it not being a permanent structure.  A bridge can have many decades of design life, but it is designed to reach that at some point. We tend to rehabilitate structures more than replace them.

“If you look at North America, they built quite an expansive bridge network in the 1940s, 50s and 60s which are now all ageing progressively together and reaching the end of their design lives. They now have an issue with the long-term sustainability of their entire network. That’s something that’s going to become more prominent for the civil engineering community in general.

“If you build a structure, although it would not be designed to be ‘permanent’, it likely would never be removed. It would be rehabilitated, changed, upgraded over time and eventually would probably have to be replaced.”