TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 717

Figure 21.62 shows examples for damages that can occur if a segmental ring is only
partly bedded. This reveals the importance of complete grouting of the annular gap
directly at the tail skin (Wittke et al. 2006, Wittke-Schmitt & Lorenzo Martin 2006).
Figure 21.62 Consequences of ring deformation due to missing bedding (Wittke 2012)
As highlighted in this topic, by means of numerous examples, sedimentary rocks fre-
quently exhibit persistent, horizontal, bedding-parallel discontinuities of low strength,
and vertical joints terminating often on bedding planes. As an example, Fig. 21.63 shows
the temporary face of a tunnel which was excavated in such a rock mass. If the horizon-
tal in-situ stress component normal to the tunnel axis is small, no arch can be formed
above the roof and as a result a large rock wedge may be formed above the shield. The
segmental ring is then loaded by the weight W of this rock wedge (Fig. 21.64). Since the
horizontal loading of the ring is small, the segmental ring is loaded by an almost uniax-
ial stress state which has an unfavorable effect on the design of the segmental lining and
normally does not occur in soil.