Geology Reference
In-Depth Information
Liner segments erected within rear of shield
and usesd to jack forward TBM
a) SLURRY
shield
Bentonite slurry in
Excavated soil and slurry out
b) EARTH PRESSURE BALANCE
additives as ncessary
Archimedes screw used
to balance pressure
spoil
conveyor
c) SHIELDED ROCK
conveyor
Figure 6.14
Schematic diagrams of shielded TBMs. a) Slurry machine; bentonite slurry is pumped to
plenum chamber and mixes with spoil cut at the face. Mixture is removed for separation and
treatment before recycling. b) Principles of EPBM. Cut soil (with additives as necessary) is removed
from the face is mucked out and TBM pushes forward against the liner erected to the rear of the
shield. Other rock TBMs use grippers pushed against the walls of the tunnel and use this as the
reaction force for advancing the TBM.
options include freezing the ground and grouting the ground to stabilise it
to allow the TBM to be withdrawn (NCE, 19 January 2011).
Where the materials to be excavated include strong and weaker
material, this is known as mixed-face conditions. For stability, the
major issue concerns relative mobility of the materials rather than
just strength. A mixed face of strong boulders and hard clay
presents problems in terms of rate of excavation, but generally
not in terms of heading stability. However, a combination of
strong, stable rock with a more mobile material, such as
flowing,
rapidly squeezing or fast ravelling material, provides conditions
where the overall stability of the heading can be very dif
cult to
control as well as dif
cult to excavate. Shirlaw
et al
. (2003)
provide examples of major in
ows resulting from the use of con-
ventional rock tunnelling methods too close to the transition from
rock-like to soil-like conditions. Ironically, this particular type of
mixed-face condition has become even more problematic with the
introduction of modern tunnelling technology.
