Geology Reference
In-Depth Information
from the ground surface (maybe 50mdeep). Immersed tube tunnels are
formed from boxes of concrete constructed on land and then
oated by
barge to position where they are sunk to the prepared river or seabed
and bolted together.
In bored tunnels in soil at relatively shallow depth, the main concerns
will usually be stability of the soil in the face, in
ow of water and
settlementofadjacentstructures.If necessary, the ground can be pre-
treated with cement grout, and frozen or compressed air can be used to
restrict water in
ow and stabilise the ground. When using compressed air
there are considerable health considerations and regulations and there is
danger of a blowout occurring, particularly where running close to some
pre-existing structure such as a well or borehole (Muir Wood, 2000).
In relatively shallow depths in tunnels in rock, the prime considera-
tions will be blocks of rock falling into the opening, or encountering
faults which may be full of water. Small failures are generally to be
expected in drill and blast tunnels or are protected by the shield in TBM
operations. Generally, in fractured rock, shotcrete is applied quickly,
together with steel mesh and rock bolts as necessary to stabilise zones of
potentially unstable rock. Similar temporary support systems are some-
times used in tunnels through soil, although the principles are different,
as discussed in
Chapter 6,
and there have been many failures in soil
tunnels when trying to adopt an observational approach to temporary
support (essentially the New Austrian Tunnelling Method).
Tunnelling inevitably disturbs the
in situ
stress condition. Existing
stresses in the ground have to
ow around the created void. As discussed
in
Chapter 6,
depending on the ratio of
σ
1
(maximum principal stress) to
σ
3
(minimum principal stress), tensile zones will develop. Generally,
these will not cause any great problem other than some minor cracking
and possibly some water ingress at shallow depths. In deep tunnels,
however, the concentration of compressive stress in sidewalls to a level
of the intact rock strength, combined with lack of con
ning stress, can
lead to spalling off and rock bursts (Carter
et al
., 2008). Such phenom-
ena are not really a problem for most tunnels but can be signi
cant for
those constructed deep through a mountain chain, or for deep mining.
Hoek (2000) reports particular problems for tunnel stability where the
in situ
stress approaches
five times the rock mass strength.
Building works throughout the world are generally covered by local
regulations, which are mandatory, together with codes of practice and
standards. Such documents cover most aspects of works, including
ground engineering, and sometimes aspects of engineering geological
practice. Some of the key documents that the engineering geologist in
the UK needs to be aware of are listed in
Table 2.2.
Similar codes and
standards exist for many other countries.
