Removing A Tunneling Machine From The Ground Is Strangely Difficult

Tunnel-boring machines are genuine marvels of modern technology — self-contained contraptions big enough to build entire subway tunnels, a job that used to require thousands of pick-axes and the sacrifice of quite a few lives. But once construction is finished, getting the machines out can take months.


This timelapse shows the breakthrough and removal of two TBMs working on the San Fransisco Central Subway project, Big Alma and Mom Chong. Retrieval has to be planned before the project even starts, with a huge retrieval shaft dug years in advance. Once the TBMs reach their destination, they break through the wall, and are slowly disassembled piece-by-piece over the course of months to finally reach the surface again. [Central Subway Blog]

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Fun fact, on long projects using two TBMs they will send one below the other and as it intersects they basically fill in behind it and leave it forever underground. The chunnel has one buried at the midpoint.