Eli5 how do under water structures work as there being built such as hotels or houses?

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This has always been a nagging question for awhile. How do they not collapse when being built? How can oil rigs stand as in how far deep do they go down in order to still stand? Underwater tunnels do they just dig on angle from a dry spot and go?

Any I’d also like to thank the admins for helping me out, I appreciate it. Especially sense I’m new to the group and reddit itself.

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Often they’re built on land and sunk in one piece, like a submarine with no propulsion.

Oil rigs are built out of multiple, or sometimes one massive, leg segments that get stacked on top of each other until they reach the surface. Not all of them are actually standing on the bottom though, many are actually floating platforms firmly chained to anchors on the seafloor.

The Channel Tunnel was dug by tunnel boring machines starting on the English and French coasts and meeting in the middle. There is tech being developed to construct underwater tunnels with concrete segments built on land and sunk into place, but tunnel boring is the primary method.

So there are a couple different construction techniques here. One way to build a structure in the water is a cofferdam. This is essentially sinking a metal collum and then draining the water from inside. From there you can build a concrete pylon to construct on top of. This is how the footings for the Brooklyn Bridge were dug.

But believe it or not, we don’t need to do that all the time. Modern concrete mixes can be poured into a form that is still submerged and they will form solid concrete. This is because water is actually vital to one of the chemical reactions that makes concrete solid. The more completely that reaction happens throughout the concrete these stronger the end result. The actual technique is highly specific, but just know it is possible and is often used when pouring modern bridge footings in water.

Things like oil rigs arent always anchored to the bottom with a solid structure. Some are, but it’s not possible with deep wells, but we do have a way to. Imagine a tight rope walker. They use that long pole that bends down below the line. This lowers their counter of gravity below the line. Making a pendulum. Pendulums will always try to return to vertical due to gravity pulling them down. You can do the same thing with the surface of the ocean. If you place enough weight below the surface or deep enough you can in essence create a pendulum. The waves will cause the platform to rise and fall but they won’t tilt it easily because the weight is keeping it vertical. From there, thick chains are used to hold the oil rig in place while it drills.

Underwater tunnels for the most part are dug through the sea bed itself. Though sometimes you need to build entrances into the water on the coast and that’s when you use a cofferdam.