How Tunnels Are Dug Through Soft MUD Without Collapsing

They dig tunnels through soft, waterlogged mud, and somehow the wall of soil in front never caves in.

In ground that wet, why doesn't the whole tunnel just collapse the second you dig into it?

The trick is called earth pressure balance. As the cutterhead scrapes soil loose, that soil does not get thrown away. It piles into a sealed chamber right behind the face and gets churned into a thick paste. The machine keeps that paste under pressure, and that pressure pushes back on the wet ground exactly as hard as the ground is pushing in. The two forces balance, so the face stays standing. A giant screw slowly meters the paste out the back, letting just enough leave to keep the pressure steady while the machine creeps forward.

Here's the strange part: the excavated mud is the wall. The very soil the machine just dug becomes the shield holding the rest of the earth back, right up until concrete rings take over.

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