TBM Tunneling - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 691

21.3.3 Seepage Flow Analysis, Assumptions
In Fig. 21.28, the FE-mesh and the boundary conditions for the seepage flow analysis
(Section 10.6.1) are represented. To eliminate the influence of the boundary conditions
on the analysis results, a very large computation section was selected. The distances
of the boundaries from the tunnel correspond to the expected extent of groundwater
lowering.
The vertical plane through the tunnel axis was assumed to be a plane of symmetry.
Accordingly, only one half of the tunnel tube was discretized. This plane of symmetry
as well as the planes x = 0 m and z = 0 m were assumed to be impermeable. The upper
boundary represents the phreatic surface at the beginning of the analysis. Its location
was determined by means of an iterative analysis described in detail in Wittke (1990)
and Wittke (2000). At the remaining boundaries, the undisturbed piezometric head of
150 m above the tunnel's roof is specified as the boundary condition.
The segmental lining and the shield were assumed to be impermeable. Thus, water can
only seep into the tunnel via the temporary face and the open steering gap up to 4 m
behind the face. In these areas, a piezometric head corresponding to the sum of the geo-
detic height and of the pressure head of the slurry was specified as boundary condition
for each node (Fig. 21.28).
Figure 21.28 Seepage flow analysis, FE-mesh and boundary conditions (Wittke et al. 2006)