Boris Johnson’s proposal for a bridge or tunnel connecting Scotland to Northern Ireland was dismissed by a feasibility study as extremely costly – £ 335bn for the bridge or £ 209bn for the tunnel – and fraught with potential difficulties.
Released alongside a wider Union connectivity review, which called for investment in road, rail and national transport to better connect the UK’s four countries, the fixed link report found that a bridge or tunnel would be at the limit of what could be achieved with current technology.
The tunnel should be so long and deep that it could only accommodate trains for safety reasons. It would take at least 30 years for either link to open.
One particular issue, which was raised by engineers when Johnson proposed the link in 2019, is the Beaufort seawall. The seabed trench is up to 300 meters deep and over three kilometers wide at its widest point, and is also filled with millions of tonnes of munitions dumped in the 1970s.
A tunnel is expected to be built to depths of about 400 meters below water level, exerting significant pressure and requiring a 25 mile climb in either direction, given a maximum rail grade of one in 100.
For comparison, the Channel Tunnel is just over 23 miles long, with a maximum depth of 75 meters.
The Beaufort breakwater would also prove to be a “challenge” for the bridge as construction would have to go over the trench, with a span of at least 2.5 miles and foundations set back from the edge, the study said. , which was run by a small team of civil engineering experts.
No suspension bridge with such a long span has been built yet, according to the report, although there have been design concepts with similar lengths.
In an introduction to the feasibility report, Sir Peter Hendy, president of Network Rail, who led the connectivity review, said the study concluded that both projects were possible.
“A crossing bridge, however, would be the longest span bridge built to date,” he wrote. “A tunnel would be the longest underwater tunnel ever built considering the limited slopes on which trains can travel, the route it should take and the depths it should reach. “
Building a rail link would also create “some complexity” as the track gauge in Northern Ireland is the same as in the Republic but different from the rest of the UK.
Hendy, who ran Transport for London when Johnson was mayor of the city, nevertheless said it was “a great question to ask”.
He wrote: “For many decades politicians and engineers debated this proposal, but did so without evidence to show whether it was possible and, if so, what it would take to do so. . This is the first comprehensive and conclusive study on the subject since the idea was first raised over 150 years ago.
Johnson has a mixed record on the ambitious bridge proposals. As mayor of London, he spent at least £ 37million in public money on a garden bridge project over the Thames despite concerns about the funding, the project’s purpose and the lack of permits to planning one side of the river.
Among the ideas for the wider connectivity review were investing in the west coast railway line north of Crewe to provide better links to Scotland, something that goes beyond the plan. £ 96bn rail line announced last week, and an upgrade to the A75 route, which runs from the M6 near Gretna Green to the Scottish ferry port of Stranraer.