FunWhy don't the tunnels collapse?

Why don't the tunnels collapse?

The drilling of tunnels on hard rock and free of joints or faults does not require any type of support, but for safety reasons a concrete lining is installed.

Obviously, in tunnels practiced in soft ground this inner lining is always necessary, to avoid their collapse. Since ancient times, the interior of the tunnel was lined with masonry or bricks; subsequently, cast iron segments were used, which have been replaced by precast concrete pieces. During drilling, operators use mobile steel frames , called screens, which prevent any landslides.

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