A technical briefing produced by the Institution of Civil Engineers’ Engineering Knowledge team Long span or deep crossing? Irish Sea Crossing conundrum The technical civil engineering challenges of a crossing between Great Britain and Northern Ireland are significant. Is such a bridge or tunnelled solution plausible? This briefing sheet explores the issues. The idea of a crossing between Great Britain and Ireland (or possibly the Republic of Ireland). With Hendy Northern Ireland was previously mooted by Prime due to produce an interim report by January 2021, and Minister Boris Johnson when he was Foreign Secretary. his final recommendations by next summer, the key A tall order it may seem, but it is now set to be given engineering and funding challenges will be a crucial greater consideration as part of the Department for component of his work. Transport’s union connectivity review. Civil engineers are capable of designing and building The review, which is being undertaken by Network Rail links over or under waters with depths between 50-150m, chairman Sir Peter Hendy, will explore how connectivity as would be encountered in the Irish Sea, even though across the UK can be improved to support economic solutions of this nature are at the very limits of current growth and quality of life, particularly while the country technologies. But where would it run, how much could faces a challenging recovery from the Covid-19 pandemic. it cost, and should it be rail or road, or both? We asked two leading experts and ICE Fellows – independent Its remit includes studying the feasibility of a fixed link bridge consultant Simon Bourne and independent tunnel across the Irish Sea between Great Britain and Northern consultant Bill Grose – for their thoughts. Page 1 Irish Sea Crossing conundrum Scotland Northern Stranraer Larne 40km Ireland ROUTE 1 Portpatrick Bangor Beaufort’s Dyke E ngland Belfast 200-300m deep 80km Isle of Man ROUTE 3 Douglas 100km Morecambe 100m 50m Republic of 20m Ireland IRISH SEA ROUTE 2 Dublin 100km Holyhead Wales 0 kilometres 50 Potential routes Beaufort’s Dyke has water depths of There are three main options being explored: “200-300m, which is considerably more difficult for bridges than the 30-50m depth ROUTE 1: Portpatrick (near Stranraer) in Scotland to of the English Channel, for instance. Bangor or Larne (near Belfast) in Northern Ireland There are many major and long sea Length: 40km – the shortest route crossings globally but none in water much deeper than 50m. You would certainly ROUTE 2: Holyhead in Wales to Dublin in Ireland not consider putting bridge foundations Length: 100km anywhere near the dyke’s bombs. In fact, would you really want to build a crossing ROUTE 3: Morecambe in Lancashire to Douglas, over or under such a lethal deposit?” Isle of Man (100km), and then onwards to Belfast (80km) Simon Bourne, ICE Fellow and independent Total length: 180km bridge expert Key challenges The unexploded bombs are almost Route 1: “certainly likely to be a showstopper for either The water here is generally 50-150m deep, but the tunnel or bridge options.” route also crosses the 3.5km-wide and 200-300m deep Bill Grose, ICE Fellow and independent Beaufort’s Dyke, which is filled with 1m tonnes of tunnel expert unexploded Second World War bombs. Page 2 Irish Sea Crossing conundrum Route 2: A road alternative would require twin dual-lane tunnels This would require the longest tunnel sections ever built – with the current longest being the Lærdal Tunnel in or need bridge spans and lengths at the limits of what Norway, at 24.5km. have so far been achieved. It would connect Great Britain with the Republic of Ireland, not Northern Ireland, so The main issues to resolve for any new road tunnel would not directly fit the ambition for union connectivity. across the Irish Sea would be the provision of ventilation, mitigating the risks posed by vehicle crashes and At 100km, this would make it twice the breakdowns, and the consequent fire and safety threats. “length of the current longest subsea tunnel in the world, but that doesn’t mean it is not Both road and rail could perhaps be more readily possible. You would need to construct islands combined with a bridge option. Road bridges can generally as work platforms to reduce the length of also carry a railway, either on the top carriageway or, more the drives to 25km – long but possible.” commonly, on a lower section within the overall depth Bill Grose of the structure that is needed anyway. Still, such rail additions will always add further costs. Route 3: This would pose the same challenge as Route 2 regarding For the length required, a rail crossing length of sections and would inevitably cost more “is more likely to be feasible than a road because of the overall length of the crossing. crossing. Rail is also more likely to be politically and environmentally acceptable.” Bill Grose Road, rail, or both? In tunnel, any rail option would probably require twin bores plus a service tube, similar to the design of the 50km-long Channel Tunnel. This latter system, which is on average 40m below the seabed, has two single-track bores either side of a service passage. The world’s longest rail tunnel is the 57km Gotthard Base Tunnel through the Alps. The benefits of a solely rail tunnel include being able to use clean power and to provide a controlled, safe environment where fire risk could be best managed. Lærdal Tunnel, Norway North Running tunnel Channel Tunnel South Running tunnel (7.6m diameter) (7.6m diameter) Service tunnel (4.8m diameter) Page 3 Irish Sea Crossing conundrum Bridge, tunnel, or both? Tunnel option: Any crossing would be undertaken at the extremes of Tunnels are growing in popularity and the maximum existing civil engineering know-how – still, it would length possible is increasing all the time as machinery and appear to be feasible. logistics improve rapidly, meaning greater reliability and lower costs. Bridge option: Even if the unexploded ordnance issue could be resolved Precedents for this option include the Channel Tunnel and along Route 1, a long crossing would still be needed the 54km Seikan Tunnel in Japan. Plans are also afoot to over very deep water. Typical options in such water, with build the 18km-long Fehmarnbelt immersed tube tunnel shipping to consider, are very long suspension bridges or between Denmark and Germany, which would contain cable-stayed bridges. both a twin track railway and a four-lane motorway. The longest central span of any suspension bridge built Tunnels are inherently safer in operation to date is the Akashi Kaikyo in Japan, at just under 2km, “than a bridge. They are all-weather and, I’d although proposals exist for 3km spans on the as-yet argue, lower in construction carbon and risk.” unbuilt Strait of Messina crossing between mainland Italy Bill Grose and Sicily. The longest cable-stayed span is the Russky in Russia, at just over 1km. The main challenge for such Cross-section of Fehmarnbelt bridges in water of this depth (generally 50-150m) is that immersed tunnel design Stone each pier would need to be 300-400m high, taller than protection London’s Shard, and positioned every 1-2km. layer FEHMARNBELT Options for bridge piers in such deep water could include a new floating-type solution with the piers anchored Cut & cover tunnel to the seabed using tension legs, such as those used in offshore oil rig technology. Road Rail Existing seabed Any bridge piers in deep water, or floating piers, need enormous areas of General fill “ Locking fill protection from shipping impact.” Gravel bed Simon Bourne Concept design for suspension bridge with floating piers End Side created for deep sea crossing in Norway elevation elevation Pier Pier Road deck Pier Piers anchored to the seabed using tension legs Seabed Tension legs Page 4 Irish Sea Crossing conundrum Combined bridge and tunnel: Norway has built a number of small-scale floating Crossing the Irish Sea could be achieved using a bridge crossings and is considering a much larger series combination bridge and tunnel construction. This could of schemes to cross deep fjords on its west coast in its include an immersed tube section, whereby the tunnel initiative exploring the use of an innovative submerged sections are built on land and floated into position and floating tube, or Archimedes bridge. connected underwater. Such immersed tube tunnels are typically only used in water 20-30m deep. Floating tunnels, deep enough to avoid “shipping without being on the seabed, There are precedents in Europe. In Denmark, the 18km need to be anchored down with tension legs Great Belt (Storebælt) Fixed Link crosses the strait and able to resist a shipping incident falling between the islands of Zealand and Funen. It comprises on to or against them. All achievable I am a road suspension bridge and railway tunnel between sure, but at a cost.” Zealand and the small island of Sprogø in the middle of Simon Bourne the Great Belt, and a box girder bridge carrying road and rail traffic onwards to Funen. Costs Linking Denmark and Sweden is the 16km-long Øresund Our experts have been tracking the costs of bridges and Bridge, made famous in the television drama The Bridge, tunnels from the past few decades. Their conclusion is an almost 8km-long combined railway and motorway that a crossing to Northern Ireland via Route 1 would crossing that links the Swedish mainland with an artificial cost £12bn for a rail-only tunnel, or £24bn for a island in the middle of the strait.