Finite Element Method (FEM) - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 273

Figure 10.48 Potential modes of failure of an advancing vault excavation with open invert
(Wittke 1990)
In Wittke (1990) and Wittke (2000b) measures often applied in practice to increase
the stability of a vault with open invert were investigated by means of FE analyses.
As a result, a systematic bolting of the complete arch leads to both a considerable
decrease of the normal thrusts and bending moments in the shotcrete as well as the
vertical displacements at the vault's roof and foot. Other measures such as grouting
or a local anchoring and a widening of the shotcrete membrane at the vault's foot
turned out to be statically less efficient.
Advancing vault excavation with temporary invert
The Burgberg tunnel, a railway tunnel constructed as part of the high-speed railway line
from Mannheim to Stuttgart, subsequently is presented as an example for an advancing
vault excavation with temporary invert (Wittke 2000b).
This tunnel has been driven in ground conditions in which the vault would not have
been stable without a temporary support of the invert as was shown by the results of
measurements carried out during a test excavation (Bienstock et al. 1986).
Figure 10.49 shows the dimensions of the tunnel. The overburden of 48 m corresponds
approximately to the maximum overburden. The rock mechanical parameters specified
in Fig. 10.49 were back analyzed from the monitoring results obtained during the test
excavation mentioned above. The tunnel is located in horizontally bedded (B) and ver-
tically jointed (J) sedimentary rock (“Bunte Mergel”). The joints J strike parallel to the
tunnel axis. The vault's support consists of a 30 cm thick shotcrete membrane. In order
to minimize the bending moments the transition area from the sidewalls to the vault's
invert are rounded off with radii of R 1 = 1.15 m and R 2 = 18.85 m.