ELI5: How do tunnels under the ocean, like the Channel Tunnel, not collapse from the weight of the water on top of it?

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I work on a tunnel boring machine currently. The channel tunnel was dug through the ground underneath the sea. The shape of the segments that slot together to make the tunnel rings are shaped so they won’t fall in. They can only be slotted together from a forward direction if that makes sense. There’s also bolts locking them together and grout sealing the outside for my tunnel. There’s a good documentary called making the channel tunnel on YouTube for free. Worth a watch if you’re interested. Goes through all this in detail with footage of it being made.


A good analogy is a tunnel dug through a mountain. The mountain can be very large above the tunnel. The weight of the mountain gets distributed over a large area. The round shape of the tunnel redistributes the load around the opening it creates. The tunnel doesn’t have to support the full load of everything directly above the tunnel, as much of that weight above will get naturally redistributed to the surrounding rock.


The tunnels under the ocean are inside the rock, under the sea floor. They’re not just in the water at the bottom of the ocean. The rock can support its own weight. The tunnels don’t collapse for the same reason the seabed *anywhere* doesn’t collapse. Edit: there are also submersed-tube designs that are just built really strong then sunk. See the reply to this comment explaining them.


At its lowest point, the channel tunnel is 250′ below the surface. Water pressure (the weight of the water) goes up by about 0.5 psi / foot. So the water pressure is ~125 PSI maximum. This is well within what we know how to build steel structures to withstand. For example, long distance oil pipelines operate at between 600 and 1000 PSI. The math is a bit complex, but a rough calculation suggests if they build the tunnel out of ordinary steel about 1.5″ thick it should handle this load. I am sure they built the tunnel to handle far more than the calculated load, so, if my math is right, they would have used a thicker then 1.5″ steel liner, or used a higher-strength material, or both. TL;DR: this is not an difficult engineering problem to solve.


Take an egg in the palm of your hand, wrap your fingers around it evenly, equally. Now squeeze, try to crush it. With equal pressure you can’t. Now squeeze it with only one finger. Now go make some scramble eggs.