Any deep-bore tunnel will need to have some means of preventing water ingress, but this is most important where the tunnel passes under open water. Any breach there will see large amounts of water pouring in, with the potential for catastrophic flooding.
This means concrete waterproofing systems need to be particularly effective and well maintained whenever a tunnel passes underwater, such as a river or even the sea.
Tunnels under rivers are common across the UK, with two on the Glasgow Subway and both road and rail tunnels in Liverpool. London has a large number of tunnels, with the Tube, mainline railways, Elizabeth line, Docklands Light Railway, the Blackwall and Silvertown Tunnel and the Greenwich Foot Tunnel all passing under the Thames.
However, one particularly exciting idea is to have undersea road tunnels connecting the different islands of the Shetlands. Consultants Norconsult are now set to carry out a feasibility study for connections between Yell and Unst, the latter being the northernmost land in the UK. It will carry out the study for the Unst and Yell Tunnel Action Groups.
Graham Hughson, joint chair of Yell Tunnel Action Group, explained that Norconsult will carry out a seismic survey, and work on the tunnel alignments, with the survey due to be completed in 2026. The project to build them will cost £500 million.
Co-chair Alice Mathewson said: “We are absolutely delighted to have reached this breakthrough stage in the project’s development, and to finally be laying the groundwork to make things happen.”
By using tunnels to link the islands, connections will not be dependent on the weather, with rough seas often making ferry crossings impossible in winter.Another archipelago to the north of Britain, the Faroe Islands, already has a network of road tunnels, with four of the 21 passing under the sea to link different islands together. The connections even include an undersea roundabout.