TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 667

Figure 21.5 Mix shield, principle (Wittke et al. 2006)
Mix shields are applied if the ground surrounding the excavated cross-section and the
temporary face have to be supported or if inflow of water needs to be avoided in highly
permeable ground.
The extraction chamber, also referred to as the “pressure chamber”, located behind the
cutterhead, is shut off against the tunnel by a pressure wall (Fig. 21.5). The supporting
slurry pressure has to at least balance the horizontal ground pressure and a potential
water pressure.
Figure 21.6 shows a shielded TBM with earth pressure face support, a so-called “earth
pressure balanced” (EPB) shield. The supporting pressure of an EPB shield is obtained
by the resistance of the excavated ground which is transformed into an earth mud. The
latter is formed by excavation and mixing tools installed at the temporary face and in
the extraction chamber, potentially with the aid of liquid or foam conditioning agents.
EPB machines are frequently used if tunnel sections have to be driven in both soil and
rock. They are also applied in formations with different and frequently changing ground
conditions (“mixed-face conditions”) or in heavily weathered rock. Therefore, in addi-
tion to the cutter discs, drag picks are mounted as excavation tools at the cutterhead.
The extraction chamber of an EPB shield, similar to a mix shield, is shut off against the
tunnel by a pressure wall (Fig. 21.6). The supporting pressure has to at least balance the
horizontal rock mass pressure and a potential water pressure.