Geology Reference
In-Depth Information
Data from ground investigations are generally presented in a
report comprising factual data as well as an interpretation of
conditions (if the GI contractor is requested to do so). One of
the important jobs for an engineering geologist is to examine and
record the nature of samples retrieved from boreholes. The data
from individual boreholes is usually presented in a borehole log,
which provides a record not only of the ground pro
le but many
details of how the borehole was carried out. In the oil industry,
where the hole is advanced by a rock-roller bit or similar destruc-
tive method, logging is done by examining small chips of rock
carried in the
flushing mud (well logging); in civil engineering, we
generally have rather better samples to examine.
Logging is generally conducted using a checklist approach and
employing standard terminology to allow good communication, for
example, on the apparent strength of a sample. Such standardisation
can, however, result in over-simpli
cation and lack of attention to
geological detail. The task might be delegated to junior staff who
might not have the experience and training to fully understand what
they are examining. In addition, GI contractors will not routinely
describe all features of samples recovered, partly because they want
to avoid disturbing the samples before the client/design engineer has
made a decision on which samples he wishes to select for laboratory
testing. Several examples of borehole and trial pit logs are provided
in Appendix D. The examples prepared by GI contractors in the UK
and Hong Kong demonstrate good practice, whereby the whole
process of drilling a hole, testing down the hole and sampling are
recorded. The materials encountered are described following stan-
dard codes and normal practice. Given the limitations discussed
above, designers and investigators may need to examine samples
and core boxes themselves and not rely on those produced by the
contractor. In Appendix D, examples are given of logs prepared by
engineering geologists who have the responsibility for the overall
site investigation. These are supplementary to the logs produced by
the GI contractors. The Australian example is from an intensive
investigation of a failing slope that was threatening a road. There
is considerable attention to detail, especially regarding the nature of
discontinuities and far more so than in the contractor
slogs.In
practice, even this level of logging may be inadequate to interpret
the correct ground model, and selected samples and sections of core
will need to be described in even more detail by specialists, perhaps
employing techniques such as thin-section microscopy, radiometric
dating and chemical analysis. In all cases and at all levels, logs
should be accompanied by high-quality photographs with scales
included.
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