Hold the roof up before you make it
How Do You Dig a Tunnel Without It Falling In?
The hard part of a tunnel is not removing the rock. It is stopping everything above the hole from arriving in it.
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Step 1 of 5
The ground above wants to come down.
Removing rock leaves everything over it with nothing beneath. The weight above a tunnel is measured in thousands of tonnes and it is patient.
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Step 2 of 5
An arch turns that weight into a squeeze.
A round tunnel presses its load outward into the surrounding rock as compression, which rock is superb at. That is why tunnels are circular and not square.
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Step 3 of 5
A boring machine builds behind itself.
A tunnelling machine cuts at the front and bolts curved concrete segments into a ring at the back. The lining is complete within metres of the cutting head.
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Step 4 of 5
Water is the real enemy.
Wet ground flows rather than holds. Tunnels under rivers are dug behind pressurised air or a pressurised slurry to hold the water back until the lining is in.
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Step 5 of 5
Two ends have to meet in the middle.
Tunnels are usually dug from both ends, and being a few centimetres out over tens of kilometres would be a disaster. The surveying is as demanding as the digging.
The Channel Tunnel's two halves met with an error of about a hundred millimetres sideways over thirty-eight kilometres of digging.
The short version
A tunnel stays up because its lining is built as fast as the ground is removed, and its round shape presses the load out into the rock.
Try it yourself
Push a finger into the side of a sandcastle. A round hole survives much longer than a square one.