NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 651

temporary invert of the vault could not be designed to withstand the water pressure in an
economically justifiable way. A full-face heading with a stepped temporary face was there-
fore planned and carried out to achieve a closing of the supporting ring as soon as possible.
This was favorable for static reasons as well as in terms of tightness since this kind of head-
ing allows water inflow only through the temporary face, that is, a comparatively small area.

20.3.5 Stability Analyses
Two- and three-dimensional FE analyses were carried out for the design of the shot-
crete membrane and the internal lining using the program system FEST03 (cf. Section
10.7.1). These analyses were based on the characteristic parameters specified in Fig.
20.32 (right). In the Lias

α , increased horizontal in-situ stresses of Δσ H = 1.5 MPa (in

claystone) and Δσ H = 0.5 MPa (in the alternating sequence) were taken into account.

Figure 20.34 Location of analysis cross-sections (Wittke et al. 2002)

Further analyses were carried out in which no additional horizontal stresses are as-
sumed and in which larger horizontal stresses of 1 and 2 MPa are accounted for in
the layers comprising predominantly claystone. Figure 20.34 shows the location of the
investigated analysis cross-sections in a longitudinal section.
Figure 20.35 shows the computation section, FE-mesh, boundary conditions and
ground profile of a three-dimensional analysis carried out for analysis cross-section 4A
that is located at the runway (Fig. 20.34). The tunnel heading was simulated using the
step-by-step method (Section 10.5.2, Wittke et al. 2002).