Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 238

Figure 10.13 Required dimensions of
the computation section, tunnel crossing
a fault zone (Wittke et al. 2006)

In general, in each individual case it should be verified on the basis of the analysis re-
sults as to whether the computation section was selected large enough.

10.5 Stress-Strain Analyses

10.5.1 Boundary Conditions
Boundary conditions can be specified as either nodal forces or nodal displacements.
When defining the boundary conditions, the forces and displacements that are im-
posed on the boundaries of the computation section have to be selected in such a
manner that the primary stress state { σ p } is correctly simulated. In a more general

sense, specified forces or displacements that are applied not only at nodes on the
boundaries but also at nodes lying inside of the computation section can be denoted
also as boundary conditions.
In the following, the boundary conditions for a two-dimensional stability analysis of
a tunnel in isotropic rock mass are considered. The computation section consists of a
vertical rock mass disk.
Force boundary conditions can be body forces, distributed or surface loads, line loads
and point loads. The self-weight of the rock mass is introduced as a body force corre-
sponding to the unit weight of the rock mass (Fig. 10.14).

In case of seepage flow, hydrostatic uplift and seepage forces are determined for each
node by means of a seepage flow analysis (Section 10.6) and then applied as equivalent
nodal forces (Fig. 10.14).

γ