From England to France: a bridge too far?
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Is it really feasible to build a bridge across the English Channel, as recently suggested by UK Foreign Secretary Boris Johnson? We asked some engineering experts.
Emperor Hadrian has his wall, President Eisenhower a tunnel in Colorado, and now Donald Trump wants a wall too. There’s no better way for ambitious leaders to put their mark on the world and secure their legacy than by having their names linked to large and imposing structures. Even better if the structure in some way defines a new epoch, or a new way of being – say, for instance, post-Brexit relations between Great Britain and its nearest overseas neighbour.
Step forward Boris Johnson, the UK’s Foreign Secretary, who suggested in a throwaway comment in January that there should be a bridge across the English Channel to connect Britain and France.
Few have taken this seriously. Yet in politics, this is often how such things are broached. A minister mentions something, the government waits for the public response. If it is positive, things start to move forward. If, as in this case, the reaction is not so positive, the government, as Downing Street officials have already done, can always distance themselves from another one of the Foreign Secretary’s often eccentric headline-grabbing outbursts.
Johnson floated his idea for a bridge after discussing post-Brexit transport links with France’s President Emmanuel Macron during January’s Anglo-French summit. The Foreign Secretary is said to have declared that it was ridiculous for two of the world’s largest economies 20 miles apart to be connected by one railway line. President Macron was, reportedly, “interested” in the idea.
Elsewhere, the response to ‘Boris’s bridge’ has been less encouraging. Too expensive; too much disruption to shipping lanes in the Channel; we have the Channel Tunnel so there’s no need for it.
That’s not to say, however, that a Channel bridge isn’t doable from an engineering perspective.
Leading Scottish architect Alan Dunlop and Professor Andreas Kappos, the director of City University’s Research Centre for Civil Engineering Structures, both believe a Channel bridge is technically feasible. Arup’s Richard Hornby adds that it’s been feasible for 40 years, just not economically viable. Dr Kostas Tsavdaridis, associate professor in structural engineering at the University of Leeds, reminds us that much longer bridges, some over 100 miles long, have been constructed.
Boris’s bridge would ‘only’ need to be 22 miles long to cross the Channel. Later this year, the longest of all sea bridges, the 34-mile Hong Kong-Zhuhai-Macau Bridge, will be opened to the public.
So if a bridge were to be built over the English Channel, what might it look like?
Andrew Bance, a technical principal in the Mott MacDonald bridges team, envisages that there would be long lengths of low-level viaduct, with occasional long spans at a higher level to accommodate shipping lanes. “The viaducts tend to be constructed using standardised pre-cast concrete, while the long spans are cable-stayed bridges with steel decks,” he adds.
Aecom’s global long-span and speciality bridge director, Robin Sham, 2015 winner of the IET’s Gold Medal Achievement Award for pioneering innovations in bridge engineering, thinks Boris’s bridge would either be a series of cable-stayed bridges, or suspension bridges, or a combination of cable-supported bridges and marine viaducts.
“Depending on the alignment of the crossing, navigation constraints and other project requirements, the design might also be developed through a combination of bridges, artificial islands and subsea tunnels,” he says.
Wanda Lewis, professor of structural engineering at the University of Warwick, explains that both multi-span suspension bridges and multi-span cable-stayed bridges are used over long distances because they are light compared to conventional bridges and provide large areas unobstructed by pylons.
Lewis adds that many spans would be needed to build a 22-mile bridge without disrupting shipping routes, but warns that a multi-span cable-supported structure would probably not respond well to wind loading. “As the number of main spans increases, so does the flexibility of the bridge. This raises the risk of flutter, which is dangerous to structural integrity,” she says. Lewis also explains that the ends of the main cables could only be anchored in solid foundations, if islands were created in between the spans.
The process would normally begin with a multi-disciplinary feasibility study, says Sham. This, he explains, would be followed by a preliminary design, a detailed design, procurement, and then construction. The study and design phases would involve a topographical survey, a bathymetric survey, a geotechnical investigation, engineering analysis and physical testing, to name a few essentials.
Also needed, according to the University of Leeds’ Tsavdaridis, would be a detailed survey of environmental conditions in the Channel over the past 50-100 years. “You’d have to find out what is the depth and the stream of the water and also the narrowest part of the Channel,” he says.
Architect Dunlop adds that such a study would also need to find out about the sub-structure of the seabed.
“The seabed must be strong enough to support the foundations,” City University’s Kappos says, adding that steel inclusions might be needed to reinforce the seabed.
When it comes to the actual construction of the bridge foundations, Mott MacDonald’s Andrew Bance suggests engineers might use cofferdams – watertight enclosures pumped dry to permit construction work below the waterline – if they are building in shallow water. Otherwise, they would have to sink piles or precast caissons into the seabed and fill them with concrete.
Durability can be enhanced, Bance suggests, by using multiple levels of protection against water ingress, combined with low-permeability concrete.
The English Channel is known for its strong winds, turbulent seas and tidal conditions, all of which will put a load on any structure built there. The Channel is also one of the world’s busiest shipping lanes. Used by huge tankers as well as ferries and smaller vessels, around 500 ships a day pass through.
Any bridge would need 80 piers, Arup’s Hornby estimates, one every 500m. Dunlop says that building works would have to be organised so that ships could still use the straits. In places, the bridge would need to be high enough so the biggest vessels could get through. Hornby adds that each pier would also have to be strong enough to withstand a ship hitting it.
‘Depending on the alignment of the crossing, navigation constraints and other project requirements, the design might also be developed through a combination of bridges, artificial islands and subsea tunnels.’
Kappos thinks the bridge would need at least one major crossing at least a mile long, for the really big ships. He cites the Oresund Bridge between Denmark and Sweden, where the 7.8km cable-stayed bridge has a 490m central span to allow the big ships through, as well as a 4km tunnel and a 4km artificial island connecting the two.
The Oresund Bridge is 204m high at its tallest point. In the English Channel, the pylons would need to be 500m high, says Lewis. That’s 150m supports to allow 70m ships to pass safely underneath, with the rest above to support the cables. The highest bridge pylons built to date are just over 300m.
“You’d need to assess how the wind would affect the pylons and cables and carry out a vibrational analysis of the bridge’s structure during high winds,” Lewis, of the University of Warwick, adds.
She also explains that a relatively large number of pylons would be needed to prevent the cables from becoming overstressed. This, however, would conflict with the need to have unobstructed space on the water for ships to pass through. It would also, Lewis believes, require lighter, stronger materials than the carbon fibre composites currently available.
“It may be possible to combine a cable-supported structure, for the part of the bridge where a clear sailing space is needed, with a conventional column bridge style,” she says.
Lewis points out that the Danyang-Kunshan Grand Bridge in China has such a design, but that the short distance between the columns on this bridge would not work in the busy English Channel.
There has long been talk of building a bridge across the English Channel. As far back as 1889, the Spectator magazine wrote that a huge iron bridge was possible, but not desirable, because it would be 10 times cheaper to build a fleet of steam ferries.
Throughout the 1960s and 1970s, engineers pitched ideas for a bridge to the authorities without success. In the 1980s, the Thatcher government looked at proposals for a 67m-high, 21-mile-long toll bridge. “It consisted of a crossing built in three sections, with a central tunnel 21km long, flanked by two cable-stayed bridges at each of the English and French side,” says Aecom’s Sham. “The three structures were to be connected by artificial islands. In the late 1990s there was a second study, which included further investigations into multi-span long-span bridge schemes.”
These schemes were abandoned as they were considered too expensive. Then the UK and French governments decided to go with the Channel Tunnel. Sham adds that since the 1990s, engineering experts have gained significant experience in fast-track marine construction, mitigation measures for seismic and aerodynamic effects and material durability in operation and maintenance. All this, he says, makes building bridges across the sea more feasible.
Tsavdaridis points out, though, that bridges built years ago are still standing – even those built close to seismic faults. “We try to improve things rather than say that things we have done so far don’t work,” he says. “Current research is about understanding how designs might work under certain circumstances – how to make materials lighter, resilient and more environmentally friendly.”
However much Boris Johnson might like the idea of putting his name to the first overland connection between England and France since the natural land bridge between them was washed away after the Ice Age, it looks as if economic and political considerations will once again put a stop to the idea.
Imagine public reaction if the government decided to spend £120bn (Dunlop’s estimate) on a Channel bridge in the same year as cutting eligibility for free school meals, having seen £46m wasted on Boris’s other bridge, the one he wanted to build over the Thames.
Hornby believes that in the UK there’s neither the demand nor the finance for a Channel bridge. “The Channel Tunnel still hasn’t covered its costs and is not used at full capacity so why do we also need a bridge?” he asks.
Dunlop adds that it’s easier for the one-party Chinese government to build such a large and expensive bridge, because it doesn’t have to take democratic processes into account.
For Kappos, it’s a simple equation. “Does the expected cost benefit of having a bridge exceed the cost of building it? If not, you shouldn’t build it.”
Of all of our experts, only Sham is upbeat about the idea. He argues that a bridge would leave a legacy for engineering excellence and that any bridge’s real economic value should be worked out throughout the whole of the bridge’s lifespan, not just from how much it costs to build it.
Might Boris Johnson’s people be picking the phone up to Aecom HQ once the proposed talks between French and English experts about ‘joint projects’ are complete?
There is, of course, another logistical problem: should such a bridge ever be built. The English drive on the left side of the road and the French on the right. Where would the crossover point be? Perhaps a 50m-tall statue of Boris Johnson could mark the spot. Bronze, of course.
Bridging Scotland and Northern Ireland?
In March, an official spokesman said the Scottish government is ready to discuss a bridge connecting Scotland with Northern Ireland. Northern Irish politicians also seem open to such discussions, after architect Professor Alan Dunlop revived an idea, originally conceived in the 1890s, which appeared in the 2015 Democratic Unionist Party’s election manifesto.
Dunlop says he has looked into the idea, particularly the geological challenges, and believes it is feasible.
He explains that the best route in terms of access would be between Larne in Northern Ireland and Portpatrick in Scotland – a distance of 26 miles. This would take the bridge across Beaufort’s Dyke, a 300m-deep sea trench, into which the Ministry of Defence has been dumping thousands of tonnes of munitions waste since the Second World War.
“The shortest connection from the Mull of Kintyre to the Antrim coast is 12 miles and potentially more economically viable,” Dunlop says.
For now, he envisages a suspension bridge with a floating island over Beaufort’s Dyke, adding that such a bridge would only become a potentially serious proposition after a feasibility study is done, which is the next stage, he says.
Richard Hornby of Arup is not convinced, however. “You are trying to connect eight million people to 51 million people. Anyone who has taken the Stranraer ferry to Northern Ireland will tell you that it takes three hours just to get from Glasgow to Stranraer.”