Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 249

been found appropriate to divide the computation section in the tunnel's longitudinal
direction into vertical slices normal to the tunnel axis. In the area where the tunnel driv-
ing is simulated, the thickness of these slices should correspond to the planned pullout
length, which is 2 m in the example illustrated in Fig. 10.22. Outside this area the slices
may be selected thicker than the pullout length.
Figure 10.22 Three-dimensional mesh for the simulation of a tunnel excavation according
to the step-by-step method
The simulation of the construction stages in principle is carried out according to the anal-
ysis of the excavation of a construction pit presented before. Starting from the primary
state in each step of analysis the excavation of a tunnel section is simulated. Thereby, the
area next to the temporary face remains unsupported, like the slope of the construction
pit after excavation of each bench. By this means, the unsupported section before the
installation of the support is simulated. Depending on the planned concept of excavation
and support, the unsupported section can be modeled by one or more slices of the mesh.
In the following step of analysis the stressless installation of the support in this section is
simulated as described before by means of the example of a construction pit.
In the first steps of the analysis the excavation of larger tunnel sections than a pullout length
can be simulated. When the area where the tunnel driving is simulated is reached, the exca-
vation of one pullout length is simulated in each step of analysis, followed by a stressless in-
stallation of the support. In the example illustrated in Fig. 10.22, 21 pullouts, each of length
2 m, and the subsequent stressless installation of the shotcrete membrane are simulated.