Two massive metal cans are currently being assembled deep under west London to allow HS2’s tunnel boring machines to drill through water-saturated ground. HS2 is using four Tunnel Boring Machines (TBMs) to dig two train tunnels between West Ruislip and Old Oak Common. They set off from either end last year and will meet roughly in the middle of the tunnels at Greenford.
To remove the TBMs at Greenford, HS2’s contractors have dug two large shafts down to the tunnel depths to lift them out, and the shafts will be then be reused to provide ventilation for the live railway. While the project is already complicated, a problem with the soil deep underground makes it more so.
It’s wet. Very wet. That makes digging a shaft down a bit of a pain, but one that is well-understood and manageable. However, these two large shafts are also where the TBMs will meet and be removed from the ground.
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https://www.ianvisits.co.uk/articles/engineering-marvel-how-hs2-is-using-giant-metal-cans-to-overcome-groundwater-hurdles-73801/