How a Tunnel Builds Its Own WALL Behind Itself

As a tunnel machine crawls forward, a hidden robot arm is bricking up the tunnel behind it in real time.

How does a finished, waterproof concrete wall appear behind a machine that never stops moving?

The tunnel wall is not poured. It is built from curved concrete blocks called segments, cast in a factory to millimeter tolerances. Inside the tail of the machine, a robotic arm grabs each segment with a vacuum grip and swings it into place until they close into a full ring. That ring is the finished wall. But there is a gap between the concrete and the raw soil outside, so pumps inject grout to lock the ring against the ground. Then the machine pushes off the ring it just built to shove itself forward, and the cycle repeats.

Here's the strange part: the machine walks on the wall it makes. Every ring becomes the launch pad it pushes against to move forward. It literally pulls itself through the earth by its own bricks.

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