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Figure 20.35
Analysis cross-section 4A, FE-mesh, boundary conditions and ground profile
(Wittke et al. 2003)
Figure 20.36 shows the maximum values of the calculated stress resultants in a longi-
tudinal section. Accordingly, the loading of the shotcrete membrane increases contin-
uously with increasing distance from the temporary face. The peak values are reached
at a distance of approx. twice the tunnel's diameter from the temporary face. The stress
resultants in this section, which are decisive for the design of the shotcrete membrane,
are shown in Fig. 20.37. Because of the high horizontal stresses, the maximum normal
thrusts N exist in the roof and invert areas. The bending moments M and shear forces
S are relatively small (Wittke et al. 2002).
In addition to three-dimensional analyses, two-dimensional analyses were carried out
to investigate the influence of a variation of the parameters and of the horizontal in-situ
stresses in the Lias
on the loading of the shotcrete membrane.
According to the results of the stability analyses, the shotcrete membrane of the stand-
ard cross-section could be designed with a thickness of 35 cm for a safety factor of 1.7
without requiring more than the minimum reinforcement (Fig. 20.27). The specified
limits for the tunneling-induced subsidence and differential subsidence were not exceed-
ed either (Wittke et al. 2002).
α
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