Civil Engineering Reference
In-Depth Information
Simulation of tunnel driving by means of three-dimensional analyses
During tunnel driving the stress flow takes place via the rock mass to be excavated in
front of the temporary face, via the rock mass to the sides of the excavated cross-section
and via the support which has already been installed. In this way critical loading of
the rock mass in the vicinity of the temporary face can arise and deformations of the
rock mass in front of the temporary face and in the non-supported section of the tun-
nel occur before the support is installed. Thus, a three-dimensional state of stress and
deformation exists at the temporary face of a tunnel.
To investigate the stability of a tunnel, the tunnel driving should be analyzed separately
inside and outside the sphere of influence of the portal (Fig. 10.21). Inside the sphere of
influence of the portal the tunnel driving should be simulated in consecutive steps starting
from the portal. Outside of the portal's sphere of influence the simulation of the tunnel
driving can be started with any tunnel section and then be carried on in consecutive steps.
This is normally accomplished by the so-called “step-by-step” method (Fig. 10.21).
Figure 10.21
Three-dimensional simulation of tunnel driving (Wittke 2000b)
Figure 10.22 shows an FE-mesh appropriate to simulate a tunnel driving by means of the
step-by-step method. It represents a tunnel section outside the portal's sphere of influence
in which the tunnel's cross-section, the height of overburden, the in-situ stress state and the
ground conditions remain unchanged in the tunnel's longitudinal direction. The dimen-
sions of the computation section are selected large enough to inhibit significant changes
of stresses and displacements due to tunnel driving at the boundaries. The dimensions in
the tunnel's longitudinal direction depend on the sphere of influence of a pullout in front
of and behind the temporary face. For an isotropic, elastic rock mass the influence on
the stresses and displacements at the temporary face due to tunnel driving reach approxi-
mately as far as two tunnel diameters, as shown in Fig. 10.23 (Wittke 1990). The distance
between the temporary face and both the front boundary and the back boundary of the
computation section should therefore amount to at least two tunnel diameters.
When discretizing the computation section, the tunnel's geometry, the planned support
measures and the planned partial excavation cross-sections need to be considered. It has
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