Underwater tunnels defy crushing ocean pressure

It seems impossible: excavating and maintaining a dry passageway hundreds of feet beneath a churning body of water. Yet, engineers have devised ingenious methods to do just that, creating vital links that defy the crushing force of the liquid above. The most common techniques involve either building the tunnel segment by segment on land and sinking them into a pre-dug trench, or boring through the seabed using massive, specialized machines.

The immersed tube method involves constructing hollow concrete or steel sections on shore, sealing their ends, and then floating them out to the tunnel site. Once positioned above a dredged trench on the seabed, these enormous segments are slowly sunk into place by flooding their ballast tanks. Divers and precision GPS guide the sections to perfectly align with their predecessors. Once linked, the water between the segments is pumped out, creating a seamless, dry passageway. The joints are then sealed, and the trench is backfilled with rock and sediment, anchoring the tunnel against buoyancy and external pressures. This method allows for a relatively shallow tunnel depth and is less susceptible to ground conditions than boring.

For deeper or longer tunnels, especially through solid rock, tunnel boring machines (TBMs) are the go-to solution. These colossal machines, sometimes larger than a three-story building, are essentially mobile factories that drill, excavate, and line the tunnel simultaneously. At the cutting face, a rotating disc armed with cutters grinds through rock and soil. Behind this, chambers are filled with pressurized air or slurry, forming a barrier that prevents water and loose ground from entering the excavation. As the TBM advances, it installs pre-fabricated concrete segments, forming a watertight ring that immediately supports the newly excavated tunnel. This 'shield' not only prevents collapses but also withstands the immense hydrostatic pressure.