Geology Reference
In-Depth Information
and hand lens. Water and a container are useful for conducting index
tests such as slake tests and for making estimates of soil plasticity and
grading. Where appropriate, strength can be measured using such
tools as a hand vane, and point load testing, which can be carried
out on irregular lumps of rock. Whatever measurements are taken at
exposures, the end user needs to be aware that it may be inappropriate
to extrapolate properties because of the effects of drying out or
softening from seepage and possibly the effects of weathering.
Guidance on geological mapping and description is given in a
ve-
volume, well-illustrated handbook series by the Geological Society of
London, which deals with Basic Mapping, the Field Description of
Igneous, Sedimentary and Metamorphic Rocks (referenced in
Chapter 3)
and Mapping of Geological Structure, each with more
than 100 pages (
www.geolsoc.org.uk). M
uch of the detail that could
be recorded by a geologist, however, might prove irrelevant to an
engineering project, but what is or is not important might not be
immediately obvious. It is worth bearing in mind the observation of
Burland (2007):
'
It is vital to understand the geological processes and man-made activ-
ities that formed the ground pro
le; i.e. its genesis. I am convinced that
nine times out of ten, the major design decisions can be made on the
basis of a good ground pro
le. Similarly, nine failures out of ten result
from a lack of knowledge about the ground pro
le.
'
Despite this observation, current standards codes and textbooks deal-
ing with ground investigation tend to take a very simpli
ed, prescrip-
tive, formulaic approach in their recommendations for the description
of geological materials and structure. The reason dates back to the
1960s when Deere (1968) noted that:
'
cation
system [geological] to be inadequate or at least disappointing, in that
rocks of the same lithology may exhibit an extremely large range in
mechanical properties. The suggestion has even been made that such
geologic names be abandoned and that a new classi
Workers in rock mechanics have often found such a classi
cation system be
adopted in which only mechanical properties are used.
'
Deere went on to introduce classi
cations based on compressive
strength and elastic modulus and the Rock Quality Designation
(RQD), and theseor similar classi
cations arenowusedalmost exclusively
for logging rock core, with geological detail rarely recorded.
Deere at the same time noted, however,
the importance to consider
the distribution of the different geologic elements which occur at the
site
'
. This sentiment would have been supported by Terzhagi (1929),
some of whose insightful observations on the importance of geological
detail are revisited by Goodman (2002, 2003). Restricting geological
'
