Geology Reference
In-Depth Information
Figure B4-6.2 Borehole periscope in use .
There we were, perched on a precarious and extremely expensive platform. A drilling rig had been
brought in and lowered down the cliff to drill an inclined borehole of perhaps 73mm diameter, at great
cost, and we had been brought to the site from London to log the hole using a periscope. Meanwhile,
the full rock exposure was available to be mapped and interpreted at very little cost, which would have
allowed a much better and more reliable interpretation of the geological structure than was possible
from a single borehole.
Lesson: Use the freely available information
first (desk study and walk-over/mapping) before deciding
on what ground investigation is necessary at a site.
Mapping can be done in the traditional geological manner, using
base maps and plans, or on air photographs, which may need to be
recti
ed for scale. Observations such as spring lines ( Figure 4.8) are not
only important in delineating probable geological boundaries but also
in their own right for hydrogeological modelling. Observation points
can be marked in the
field, to be picked up accurately later by surveyors.
Alternatively, locations can be recorded by GPS and input directly into
a computer, as illustrated in Figure 4.9. The success of preliminary
mapping can be enhanced by letting an early contract to clear vegeta-
tion, allow safe access and to put down trial pits and trenches on the
instruction of the mapping geologist ( Figure 4.10).
Soils and rock can be examined, described and characterised in
natural exposures and in trial pits and trenches, and full descriptions
should be provided, as discussed later. Samples can be cut by hand for
transfer to the laboratory, with relatively slight disturbance
(Figures 4.11 and 4.12).
Access can be facilitated by using hydraulic platforms or by tempor-
ary scaffolding ( Figure 4.13). Trial pits and trenches should not be
entered unless properly supported, and care must be taken in examining
any steep exposure; as a general rule, for safety reasons,
field work
should be conducted by teams of at least two people.
 
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