Closed Face Mean?
Closed face is one of two classifications of tunnel boring machines (TBM). Closed face TBMs can be further broken down into two types: Earth pressure balance (EPB) and slurry shield (SS). Closed face machines operate much like single shield TBMs and are specially suited for soft ground drilling and situations with unstable earth formations.
Trenchlesspedia Explains Closed Face
Both types of closed face TBMs rely on concrete segments to produce thrust.
EPB machines use excavated material to maintain a balanced material at the excavation face. The excavation rate is controlled by an Archimedes screw and is used in conjunction with the advance rate to control the face pressure to avoid perturbation. EPBs are best suited for poor ground conditions with less than seven bar of pressure.
SS machines use a slurry mixture with very high water pressure which makes the use of an Archimedes screw impossible. A pressurized slurry mixture is pumped to the cutter head to maintain hydrostatic pressure at the excavation face. SS machines aid in the evacuation of the cuttings via a vacuum.