Geology Reference
In-Depth Information
conditions (Items 3 and 4 in
Box 4-1).
Important questions are, how
much ground investigation is required and how should it be done?
There are no hard and fast rules, even though some authors try to
provide guidance on the basis of site area or volume for particular
types of operation (e.g.
Figure 4.1
for dredging) or on hypothetical
considerations (e.g. Jaksa et al., 2005). In reality, it depends upon the
complexity of the geology at the site, how much is already known
about the area, the nature of the project and cost. For sites with simple
geology, the plan might be for boreholes at 10m to 30m spacing, for
discrete structures like a building (BS 5939: 1999). For a linear
structure like a road or railway project, the spacing might be anywhere
between 30 and 300m spacing, depending on perceived variability
(Clayton et al., 1995). West et al. (1981) consider the particular
dif
culties in planning investigations for tunnels. So much
depends upon the depth of tunnel, the topography and variability of
geology. Often, considerable reliance is made on aerial photography
interpretation, geological mapping, a few widely spaced preliminary
boreholes and other boreholes targeted at particular perceived hazards
such as faults that might be associated with poor quality rock and
high water in
ows. For example,
Figure 4.2
shows the route of a
planned tunnel
ed,
together with a rationale for their mitigation and additional
GI. Where steeply dipping geological structures such as faults are
anticipated, inclined boreholes may be required.
Figure 4.3
shows an
in Hong Kong, with potential hazards identi
of boreholes for
dredging area (in
millions of square
metres) vs. average
thickness of
material to be
removed, based on
equation of Bates
(1981), as
presented in PIANC
(2000). Other
factors that should
be taken into
account are
variability of
ground conditions
and existing
knowledge about
the area.
60
Area to be dredged
(million square metres)
50
2.0
1.5
40
1.0
0.5
30
20
10
0
2
4
6
8
10
Thickness of layer to be dredged, metres
