How are underwater tunnels built?

Tunneling Shield

Underwater tunneling remained a challenge until 1818 when British-French engineer Marc Isambard Brunel observed that the barnacle’s outer shell allowed it to bore through wood. Brunel applied the rule and increased the size of the tunneling shield.

In this case, it was a giant rectangular cast-iron cavity with the shield face resting against the earth and opening steadily to allow workers to dig through the soft soil outside. The shield face is then pushed forward with the screw jack and the process repeats. The protective shell of the tunnel is built of bricks in the newly carved space behind. This method led to the first underwater tunnel built under the River Thames in London, completed in 1842. Subsequent tunnels under the River Thames improved the method by pressurizing air at the front of the shield to Prevent flooding during construction.

Today, tunneling shields are still used, but they are cylindrical in shape and are usually made from steel. Steel is also used to create the rings that support the tunnel. Modern versions also use hydraulic jacks to push the shield forward. But when the machine is not moving, the area at the front of the shield is accessible through a door. To protect workers inside, the shield also has a cover.

Tunneling machine

Digging through underwater rock is a completely different challenge. That’s when modern technology revolutionized construction in the form of the tunnel boring machine (TBM), the machine that helped build the English Channel tunnel. The TMB has a similar function to a tunneling shield, but in this case, instead of human power, a rotating cutting head is used to dig through the rock layer ahead. The machine does so by applying pressure to the rock, causing it to crack. Instead of needing humans to move debris out of the way, the fractured rock is conveyed by conveyor belt. A total of 11 TMB machines were used to dig 3 56.3 km long tunnels at a depth of 45 m below the seabed.

Submerged tube tunnel

With the previous methods, the tunnel wall is built during the excavation process. However, American engineer WJ Wilgus developed the technique of building submerged tube tunnels. This method involves creating a tunnel from multiple sections of pipe prefabricated elsewhere, then dredging a trench in a river bed or seabed to place the tunnel. The pipe sections are floated and submerged into position, then all water is drained from the pipe. Excavated material is covered over the tunnel to bury the structure and restore the seabed.