Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 251

If the strength of the rock mass is exceeded in a construction stage a viscoplastic time
step analysis - also referred to as “iterative viscoplastic analysis” - is carried out. If the
displacements converge within this iterative analysis the tunnel can be considered as
stable in this construction stage. I iterative viscoplastic analysis is described in detail in
Wittke (1990) and Wittke (2000b).
The simulation of tunnel driving outside the sphere of influence of the portal can alter-
natively be carried out by an iterative procedure (Fig. 10.21). This method was devel-
oped in the late 1970s when computational capacity was still limited (Semprich 1980).
Although this method is more economic than the step-by-step method, its range of
application is limited to elastic stress-strain behavior of the rock mass and to cases in
which the tunnel's cross-section, the height of overburden, the in-situ stress state and
the ground conditions remain unchanged in the tunnel's longitudinal direction.
Consideration of construction stages in two-dimensional and
pseudo-three-dimensional analyses of tunnels
Although the construction stages in tunneling are three-dimensional, in many cases it is
sufficient to simulate them by means of two-dimensional or pseudo-three-dimensional
analyses. However, this requires that the deformations and stress changes due to excava-
tion arising in the area of the temporary face before installing the shotcrete membrane
are accounted for in a simplified way.
Starting from the primary stress state (Fig. 10.24, left) pre-deformation of the tunnel's
contour is achieved by reducing Young's modulus E RM of the rock mass in the area
to be excavated (E < E RM in Fig. 10.24, center) before the excavation and the installa-
tion of the shotcrete membrane is simulated (Fig. 10.24, right). The amount by which
Young's modulus needs to be reduced depends, among other things, on the pullout
length, the point in time of the shotcrete installation, the stress-strain behavior of the
rock mass and the shotcrete thickness.
Figure 10.24 Two-dimensional simulation of three-dimensional conditions at the temporary face,
method of preceding stress relief (Wittke 2000b)
In Fig. 10.24 this procedure denoted as “preceding stress relief ” is shown for the simple
case of a full-face excavation. A tunnel heading including partial excavations such as an
advancing vault excavation and a sidewall excavation (Section 20.2) can be modeled in
a corresponding manner by reducing Young's modulus of the rock mass in each area of
partial excavation before simulating the excavation of this area.