... like I'm 5 years old
Imagine you're baking a cake. You've got your cake mixture ready and you're about to put it into the oven. But instead of a typical cake, you're making a donut, a circular cake with a hole in the middle. The donut hole represents the tunnel under the water, while the surrounding cake is the ground and water.
Engineers "bake" these underwater tunnels by digging into the ground beneath the water, very much like you would dig a hole in your garden, but on a much larger scale. They strengthen the "walls" of this hole with steel and concrete to make sure it's safe and secure.
If you think of the tunnel as a straw, the ground and water around it are the juice in your cup. The straw lets you drink without getting your whole face wet, and the tunnel lets you travel without getting wet.
... like I'm in College
Building a tunnel underwater isn't as simple as digging a hole and reinforcing it. There's a lot of planning and careful construction involved. Engineers often use a method called the "cut-and-cover" method.
In this method, a large trench is dug in the riverbed or seabed, and a tunnel is constructed in this trench. Once the tunnel is built, the trench is filled in, burying the tunnel under the ground and water.
Another method is the "immersed tube" method. In this method, sections of the tunnel are built on land and then floated out to the location of the tunnel. These sections are then sunk to the bottom of the water body, and connected together to form a continuous tunnel.
If we were to use LEGO bricks to illustrate how underwater tunnels are built, we could start by laying a flat blue LEGO base plate to represent the body of water. The ground underneath this water can be represented by stacking brown bricks on top of each other.
To represent the "cut-and-cover" method, remove a strip of brown bricks to create a trench, then build a tunnel with grey bricks and cover it back up with the brown bricks.
For the "immersed tube" method, you'd build sections of the tunnel with grey bricks on a separate plate, then place these sections into the trench in the brown bricks, connecting them together.
To represent TBM method, you'd need a cylindrical LEGO piece to represent the TBM. Move this piece through the brown bricks, removing them as it goes and replace them with grey bricks to form the tunnel.
In all these LEGO scenarios, the underwater tunnel allows a small LEGO figure to travel from one side of the water body to the other without getting wet, just like in real life!
... like I'm an expert
As an expert, you'd know that there are more technologically advanced methods for building tunnels under water. These methods, such as the use of Tunnel Boring Machines (TBMs), allow for the simultaneous excavation and installation of tunnel lining, making the process faster and safer.
When using TBMs, the machine bores through the ground while installing concrete segments behind it, which form the tunnel wall. This avoids the need for temporary supports during construction. The excavated soil is removed via conveyor belts, keeping the worksite clean and efficient.
The use of TBMs in combination with other methods like cut-and-cover or immersed tube can create complex and efficient underwater tunnel systems.