Definition - What does Face Stability mean?
Face stability refers to the stability of the inside working face (or shaft bottom) of a tunnel excavation. During construction, the face of the tunnel is subjected to pressure forces exerted by the soil layers in front of and above the tunnel face. The face of the tunnel, therefore, must possess enough stability, either inherently or by stabilization measures, to prevent collapse.
Trenchlesspedia explains Face Stability
Stability of the tunnel face is essential to prevent the total collapse of the tunnel due to the earth pressures being exerted. In many cases, tunnels will be comprised of several layers of loose or weathered rock and will not be strong enough to withstand these pressures.
In such cases, various types of support methods are used to reinforce the face of the tunnel to increase its stability. Some of the most common support pressure methods include bentonite slurry shields, compressed air and earth paste.