At the TBM face, there is a pressure chamber that uses bentonite slurry as the working fluid. The slurry pressure in this chamber is adjusted carefully to equal

At the TBM face, there is a pressure chamber that uses bentonite slurry as the working fluid. The slurry pressure in this chamber is adjusted carefully to equal the local hydrostatic head. This seals against the axial forces on the machine's face. Also, the TBM's tail shield (armored cover for the ring building arm+trailing gear, aka the TBM's skin) is completely surrounded by a set of stiff steel brushes that are impregnated with heavy grease. These brushes are called "tail seal grease brushes" and have dimensions ~1ft x 1 inch x 1 ft. They protrude radially from the surface of the machine to bear against the walls of the tunnel (perpendicular to tunnel wall at every point). This is probably the seal implementation that you're wondering about. These things are only true for slurry-face TBM's. There are other sorts of TBM's out there, but the Seattle one is a SFTBM.