Page 56 - Numerical Analysis and Modelling in Geomechanics

COMPRESSED AIR TUNNELLING 37

Figure 2.5 Typical output of the 3D model showing the deformed groundwater surface.

the perimeter walls will be negligible. However, for the cases where shotcrete is
used as a temporary or permanent lining, the air losses from the tunnel walls will
be significant. The computer model can consider the stages of excavation and the
corresponding excavation times.
The permeability of shotcrete to air (or water) is not constant and decreases to
a certain value during curing. From observation of the field data from a case
study, a logarithmic relationship has been used to describe this change as:
(2.8)

where
k a, sh = permeability of shotcrete to air
A, B = constants
t = time in days
Parameter identification, based on back-analysis of the field measurements from
the Feldmoching tunnel in Germany, has been used to identify the parameters A
and B. Initially the tunnel longitudinal profile is divided into a number of
excavation stages and the corresponding time for excavation and installation of
the lining for each stage is evaluated from the anticipated rate of tunnel advance.
This evaluated time is used to estimate the permeability of the shotcrete at the