Geology Reference
In-Depth Information
1,893m in 1881 from the UK side and a similar machine advanced
1,669m from its portal in France. Today there is a huge range of TBMs
available, ranging from ones speci
cally designed for hard rock,
through to ones that tunnel through soft waterlogged sediments using
pressurised slurry in front of the machine to support the soil. As
explained in
Chapter 6,
precast segmental tunnel liners can be erected
directly following the cutting part of the machine and bolted together
with gaskets to form a watertight tube. TBMs can be highly successful
with hundreds of metres advance in a single month, compared perhaps
to a hundred metres using drill and blast methods; so, for long tunnels,
the cost and possible delays in manufacturing a TBM for the job may be
justi
culties from ground
conditions that can slow them down or even stop them completely,
despite huge sophistication in their design. The author has experience
of tunnelling through weathered rock in Singapore using a speci
ed. Quite often, however, TBMs run into dif
cally
designed slurry machine, where in one section of the tunnel the TBM
was stopped because of the high strength of the rock and lack of natural
discontinuities. Elsewhere on the same drive, the rock was weathered to
a residual soil that was so clay-rich that the slurry treatment plant could
not cope, again causing delays and necessitating redesign of the treat-
ment plant. For the same machine, the machine operators had dif
culty
in selecting the pressure to adopt in the slurry. If the pressure was too
low, the ground collapsed, if too high, slurry was ejected into the street
above. Shirlaw
et al
. (2000) give examples of problems in tunnelling
through weathered rock terrain, and other examples, especially in
squeezing ground and zones of high stress, are given by Barla &
Pelizza (2000). Further case examples are given in
Chapter 7.
It is very important in tunnelling to consider all the potential
hazards that might be encountered and to make sure that the TBM
can cope, as addressed in
Chapters 4,
6 a
nd Appendix E. This is even
more important for TBMs than drill and blast tunnels because it may
be very dif
cult to modify the method of working and ground support.
Whereas in a drill and blast tunnel the engineering geologist can
examine the face and tunnel walls before and after a blast, in a
tunnel excavated by TBM, all that can often be seen is the spoil being
excavated (often contaminated with drilling mud), so it is rather diffi-
-
cult
rm that the ground conditions are as anticipated.
Engineering geologists are therefore relatively little used during TBM
construction, until something goes wrong and needs investigation.
In tunnels, key aspects to consider are safety for the workers and
public above the tunnel, the feasibility of different excavation techni-
ques, limiting water ingress, stability of the face and side walls during
construction and, in the longer term, the effects on surrounding struc-
tures (mainly settlement, undermining, vibration and noise) and cost.
Shallow tunnels and other excavations, such as underground railway
stations, may be constructed as concrete boxes in open excavations
to con
