Geology Reference
In-Depth Information
Figure 7.3
EPB
Machine in cross
section. The
pressure at the face
is maintained by the
rate at which
broken down
material is removed
in the Archimedes
screw.
6
-Mixing
Blade
Lining segments
Man Lock
1-fixed
Blade
Plenum
chamber
Horizontal Gate
Screw Discharger
Erector Equipment
3-slime Injection
Pipes
Tail seals
24-shield Jacks
8100
the east bank of the Singapore River to Clarke Quay Station on the
west bank. The total length of each drive was only 70m. A single 6.53m
diameter machine
(Figure 7.3)
was used to construct the tunnels with
the TBM turned around after the
first drive. The tunnelling passed
from weathered sandstone to marine clay on the southbound tunnel
and the reverse on the northbound tunnel. To maintain the face
pressure in the weathered sandstone, the EPB shield needed to
be operated at almost maximum torque, which resulted in extensive
wear to the cutting tools and dangerously high temperatures in
the slurry and spoil. Almost all of the discs had to be replaced after
70m before re-launching the shield for the northbound drive.
Furthermore, the sandstone was broken down to abrasive slurry,
which, in EPB mode,
fills the chamber and the area between the cutting
head and the excavated face causing further damage to the machine,
including the screw conveyor. In a separate case, for an EPBM tunnel-
ling mainly through Old Alluvium in Singapore, the pressure bulkhead
(40mm thick steel), became so abraded that it failed, resulting in a
major loss of ground (Marshall & Flanagan, 2007). The incident
caused about 280mm of settlement and closure of a lane of the central
expressway for 12 hours (Shirlaw, personal communication).
Similar problems have been reported and should be anticipated for
other soils and rock with high silica content, including granite and soils
derived from granite, and can be predicted using Cerchar Abrasivity
Index tests. The abrasion can be lessened by appropriate use of additives.
In the example above, for the southbound and most of the northbound
drives, either water or polyacrylamide were used as a conditioning agent.
For the last 10m of the northbound tunnel, the contractor switched to
foam. This had a dramatic effect in reducing the torque required to
rotate the cutting head, and the temperature of the spoil.