Geology Reference
In-Depth Information
shafts
because the brick lining might have a magnetic signature and
the void is low gravity. Generally, such techniques are used as a
-
rst
pass across a site to identify any anomalies, which are then investigated
more fully using trial pits, trenches and boreholes. For such investiga-
tions, percussive holes, as used for forming holes for quarry blasting
(no coring), can be very quick and relatively cheap
the presence of
voids is indicated by lack of resistance to drilling and loss of
-
flushing
medium. The voids can later be examined using TV cameras, peri-
scopes or sonic devices to try to quantify size and shape. For many
reasons, such surveys are not always successful and therefore are not to
be relied upon to give a de
nitive answer (Clayton et al., 1995). Sewell
et al. (2000) demonstrate the usefulness of marine magnetic and
gravity surveys for identifying geological structures.
As with seismic re
ection, down-hole geophysics is used routinely in
oil and gas exploration, in mining and in sophisticated GI linked to
nuclear waste disposal studies. Tools can be used to determine minor
stratigraphic contrasts and rock properties. These tools are less used
for engineering, with the exception of rock joint orientation (using
cameras and geophysical tools) and sometimes for identifying clay-rich
layers. These tools are discussed below, together with logging and
description.
Methods and techniques for sub-surface investigation are dealt with in
many publications, including BS 5930 (BSI, 1999), Clayton et al.
(1995), GCO (1987), Hunt (2005) and Mayne et al. (2001).
There are usually four main objectives in sub-surface investigation:
1.
le
2. to determine engineering properties for the various units within the
eventual ground model
3. to establish hydrogeological conditions, and
4.
to establish the geological pro
to monitor
future changes
in ground conditions
through
instrumentation.
At many sites, it is best to use preliminary boreholes in an attempt to
establish the geological pro
le accurately. This will require sampling
over the full depth and with suf
cient boreholes to establish lateral and
vertical variability. If recovery is low, then boreholes may need to be
