How Tunnels are built Underwater
When tunnels are built underwater, it’s mostly because a bridge or ferry is unavailable, or to reduce the usage of an existing bridge or ferry because of competition or relief.
History of the Underwater Tunnels
In the early 19th century in London, the world’s first underwater tunnel was built through the Thames river. There was a lot of congestion, and they needed a way to increase trade without increasing river traffic. One of England’s most intelligent engineers in 1818 designed a device called a tunnel shield that would keep mud and water from seeping in. Rather than using a machine to drill, the tunnel was created by people chipping away at rocks and sand at the front of the shield. From the back of the tunnel shield, other people would be stacking bricks. There were two outlets for disposal; one for water, and one for sand and mud. The tunnel took 18 years to build and is still an important piece of London’s Overground railway network.
Tunnels can also be built before they are inserted into the ground as-fabricated concrete segments. The segments are placed into a long cavity which was dredged and graded prior to the segments being made. Once they are built, they are placed together in the cavity to form the tunnel. Water between the joints is hydraulically pumped out so the tunnel can become sealed.
There are some disadvantages to building tunnels in a prefabricated way even though they are cost-effective. Even a slight misalignment can create devastating consequences, and engineers need to be careful in sinking the segments. There is also the effect it has on the environment and ecosystem.
Drilling is the most convenient method, although the drill needs to be extremely large and is the most expensive method. There also needs to be more than one drill to create the tunnel and each one can cost millions of dollars.
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