In the video two comments up, they explain that a different kind of TBM is used for waterlogged boring. Bentonite is used to keep pressure on the tunnel face in

In the video two comments up, they explain that a different kind of TBM is used for waterlogged boring. Bentonite is used to keep pressure on the tunnel face instead of the bored material, the slurry is then separated with a centrifuge and piped back to the face/out of the tunnel.