Geology Reference
In-Depth Information
6.5 Tunnels and caverns
6.5.1 General considerations for tunnelling
Tunnels will be constructed as part of an overall project, for example,
water supply, drainage, rail, road, or in connection with power gen-
eration. As a result, there may be little
flexibility over route and,
therefore, geological and hydrogeological conditions and size and
shape of tunnel. It is up to the engineering team to come up with a
cost-effective solution.
One factor that will in
uence the chosenmethod of construction and
finish requirements for road and rail tunnels
and whether or not it might carry water under pressure in hydraulic
tunnels, as addressed at 6.6.5 below. The main issues for the engineering
geologist and design team are likely to be:
lining (or not) are the
nal
-
The geology along the route; how this will affect the selected
method of tunnelling and any particular hazards such as natural
caverns, mining or major faults.
-
Stress levels and ratio of vertical to horizontal stress. High stress at
depth and the concentrations in stress resulting fromperturbation of
the stress
field by the construction can result in failure of the rock,
which might result in spalling in brittle rocks or squeezing in gen-
erally weaker rocks (Hoek & Brown, 1980; Hoek et al ., 1995).
-
Hydrogeological conditions and the risk of unacceptable water
in
flooding; this is always a major issue for
undersea tunnels, but can also be a concern under land.
ows and possible
-
Existing structures that might be adversely affected by the tunnel
during construction, for example, by blast vibrations or undermin-
ing as the tunnel passes by. In the longer-term, lowering of ground-
water may cause settlement and/or affect water supply boreholes.
As for all geotechnical work, one needs a ground model for design.
Because tunnels are often long and may be at great depth, it may be
impractical to do more than a rather super
cial investigation, relying
largely on geological mapping and extrapolation of data, although if
a serious obstacle is anticipated, such as a major fault zone, then bore-
holes might be targeted at that feature using inclined boreholes or even
drilled along the line of the tunnel. Alternatively, a small-diameter pilot
tunnel might be constructed before the main tunnel
-
possibly for later
use as a drainage or service tunnel
-
because small diameter tunnels tend
to have fewer dif
culties (Hoek, 2000). The pilot tunnel essentially
works as a large-diameter exploratory borehole.
The ground model needs to include estimates of rock or soil quality
along the tunnel drive. For rock, this is often done using rock mass
classi
cations (RMCs) such as Q, RMR or GSI, described in Chapter 4
and Stille & Palmström (2003). This will allow some estimation of
 
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