Digging tunnels underwater is something Britain has a long history of, with the Thames Tunnel, constructed by Isambard Kingdom Brunel and finished in 1834, being the first of its kind in the world and helping inspire the later construction of the London Underground.
The tunnel is still in use today, ironically as part of the London Overground, having previously been part of the East London Underground line. This shows tunnels can have a long life, but good maintenance is essential.
Often, this can involve substantial repairs, which is particularly important when a tunnel crosses underwater, given the greater risk of water ingress. For this, treatments like concrete waterproofing can prove transformational in keeping water out and helping ensure the tunnel can operate for many years to come.
A recent instance of this occurred on another 19th century structure in London, the Blackheath tunnel. First built in 1849, it has been increasingly subjected to water ingress. It does not pass under a river, but penetrates the aquifer that covers much of south London (a factor in the Underground having fewer lines this side of the Thames).
However, a major repair operation has now been completed, with old bricks removed and new waterproofing installed, ensuring that rail services between Charlton and Blackheath can resume after a ten-week hiatus.
Older tunnels may be more prone to such leaks, both due to the effects of age and the construction methods and materials available at the time, although the deep-bore shield tunnelling system was already in use by 1849, Brunel having used it for the Thames Tunnel.
Elsewhere in London, such protection against water ingress is still needed in the deep Tube tunnels passing under the Thames or water-bearing rocks.
Some of the biggest challenges with underwater tunnels come when they pass under the sea, with the prime British example being the Channel Tunnel.There may soon be more, following the recent publication of plans to link the various islands of the Shetland archipelago by road tunnels, emulating a similar network that has transformed travel around the nearby Faroe Islands.