Structural design verifications, structural analysis of tunnels - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 114

3.7 Special applications of the FEM in tunnelling

Two special applications are now described to make clear the usefulness of the FEM in
tunnelling.

3.7.1 Modelling of deformation slots
Unfavourable geology, for a example heavily squeezing rock mass, can lead to problems
with high deformation in deep tunnels in rock. For this reason, movement slots can be
installed in the shotcrete layer parallel to the tunnel axis, which largely protect the layer
from destruction by large displacements. This was practiced for the first time in the Tau-
ern Tunnel. In the Inntal Tunnel, the process was practiced regularly and backed up with
structural verifications.
The appropriate measures and their numerical modelling are reported in [236]. Fig. 3-15
shows the discretisation of the rock mass with unfavourable geology for tunnelling; Fig.
3-16 shows the modelling of the slots in the shotcrete layer.

Figure 3-15 Numerical
model with definition of
geological zones [236].

Until the slot is finally closed, no or only very slight normal force is transferred from
shotcrete zone A to zone B. Slight transfer is caused by shotcrete residues in the slots or
over the tunnel vault. Contact elements offer the capability to specify both a gap spacing
a and an elastic stiffness in the gap. This means that force is only transferred through this
element when the gap a has been exceeded. The working curve based on this assumption
can be seen in Fig. 3-16.