TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 683

Figure 21.20 Loosening of rock and increase in rock mass permeability (Lundman et al. 2009)
Thus, under the prevailing ground and groundwater conditions described in Section
21.2.2, block forming and instability at the temporary face was to be expected.
Also, the wash-out of annular gap grout can lead to seepage flow towards the tem-
porary face as a result of high hydraulic gradients (Fig. 21.17). Grouting tests con-
ducted in the past have shown that at high gradients grouting of discontinuities with
cement-based suspensions and pastes is not successful (Wittke & Breder 1984, Wittke
et al. 2006).
21.2.6 Measures for Improvement
Filling and Grouting of Annular Gap
To solve the problems regarding wash-out of the grout, a special procedure for the fill-
ing and grouting of the annular gap was developed and carried out.
At a maximum spacing of 20 segmental rings, so-called “barriers” were constructed
(Fig. 21.21). After excavation of a barrier section with a length of around three to
five rings, the TBM was stopped, closed and pressurized to around 10 bar. Then mor-
tar was injected into the annular gap, starting from the first ring installed and from
bottom to top. For each barrier an amount of some 100 m3 of injection material was
required (Figs. 21.21 and 21.22).