Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 278

The effect of drainage holes can be simulated by specifying the piezometric head of the
drainage level at individual nodes. The piezometric heads of a fully effective drainage
with drainholes inclined below the horizontal are equal to the geodetic height at the
heads of the corresponding boreholes. At the corresponding nodes water can leave the
computation section with flow rates that can be calculated from the gradients of the
computed piezometric heads.
The effect of wells with constant pumping rate can be simulated by specifying flow rates
at the corresponding nodes.
Furthermore, the piezometric heads at two nodes initially unknown but equal to each
other can be specified as boundary conditions. Such boundary conditions are used for
so-called “pseudo-three-dimensional seepage flow analyses”.
Figure 10.54 Seepage flow analysis for a tunnel, FE-mesh and boundary conditions (Wittke 2000b)
The required size of the computation section for a seepage flow analysis depends
on the range of influence of the groundwater flow initiated by the structure under
construction. This range, depending on the permeability of the rock mass, is usu-
ally considerably larger than the range of stress changes in a stress-strain analysis.
As an example, Fig. 10.54 (upper) shows the mesh and the boundary conditions
for transient seepage flow analyses carried out for a tunnel. These analyses are
described in Section 10.6.3. Because of the expected range of the groundwater low-
ering due to tunnel heading, the required size of the computation section is 400 m