NATM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 648

Figure 20.31 Tunnels in Stuttgart constructed in the Lias α formation (Wittke & Züchner 2008)

Increased horizontal in-situ stresses Δσ H exceeding the stresses resulting from lateral

constraint in the order of 1 - 2 MPa acting in the claystone layers were first seen during
excavation of the construction pit at Station University (Section 16.7.2). The magnitude
of these stresses was confirmed during the construction of other tunnels located in the
Lias

formation (Wittke 1991). On the basis of stress measurements using triaxial
cells, displacement measurements carried out in the monitoring pit and back analyses
of monitoring results for the alternating sequence, somewhat lower stresses

α

Δσ H of 0.3
- 1.3 MPa were derived (Grüter 1988, Wittke 1991). The additional horizontal stress of
Δσ H was found to be dependent on the topography, the degree of weathering and the
water content of the claystone (Section 16.7.2, Fig. 16.33).
Figure 20.32 (left) shows the structural model of the Lias derived on the basis of ex-

perience gained from the tunneling projects mentioned above (Wittke & Züchner 2008).
The discontinuity system comprises an orthogonal system of horizontal bedding-par-
allel discontinuities B and steep to vertically dipping joints J1 and J2 (Sections 12.6,
16.7.2 and 19.2). The characteristic rock mechanical parameters (Fig. 20.32, right) were
specified on the basis of the results of field tests and back analyses of monitoring re-
sults (Sections 12.6, 16.7.2, 18.1 and 19.2, Grüter 1988, Wittke 1991, Wittke et al. 2002,
Wittke et al. 2003).
The permeability tests carried out yielded an anisotropic permeability of the alter-
nating sequence with equivalent horizontal and vertical permeability coefficients of
k H = 5 · 10 -5 m/s and k V = 1 · 10 -6 m/s, respectively (Fig. 20.32, right). An isotropic
permeability coefficient of k = 10 -7 m/s was derived for the claystone (Wittke et al.
2002).

α