A lot of steps, a lot of concrete, a lot of waiting around 28 days for concrete to harden, and a lot of the tunnel cross-section used only for escape routes because they're making a square roadway in a round tunnel..
The next video explains how they keep the tunnel from collapsing behind the tunnel boring machine, and how they keep water from leaking into the tunnel by using "tail seal grease" injected between wire brushes at the rear of the TBM.
https://youtu.be/Dk-8Q42NCSQTail seal grease explained: https://www.tunneltalk.com/New-Products-Oct10-New-tail-seal-grease.php
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