Geology Reference
In-Depth Information
As discussed in Appendix C, guidance on standardised terminology
is given in BS5930: 1999, in the GEO guide on rock and soil descrip-
tion (GCO, 1988) and the ISRM guidance on rock mass description
(ISRM, 1981). There are many different standards and codes of prac-
tice in use worldwide
USA practice is far removed from that in the
UK, as is that for Australia, China, New Zealand, Japan and Korea,
which leads to confusion, particularly as similar terminology is often
used to mean different things. A consequence of this fuzzy standardisa-
tion is that when projects go wrong geotechnically, as they sometimes
do, then legal arguments often hinge on incorrect or misinterpretation
of terminology. The engineering geologist needs to do his homework
before practising in any region.
Another criticism made earlier regarding
-
field mapping, but equally
applicable to logging, is that standard guides and codes to rock and soil
description tend to comprise a series of limited classi
cations that one
has sometimes to force on an unwilling rock mass. For example,
rock masses, as exposed in quarries, can seldom be simply described
as widely or closely jointed, but loggers are required to apply such
classi
is opinion, it is far better
to concentrate on recording factual data, which can then be interpreted
as the overall ground model becomes clearer. An example of over-
simpli
cations to core samples. In the author
'
cation terminology is given in
Box 4-8
with
reference to the term aperture. The problem is that by using
such terms it is implied that the feature has properly been characterised,
which is not the case. De Freitas (2009) discusses the same point and
also notes that many terms and indeed measured values such as porosity
are
ed rock classi
lumped parameters and therefore
rather
insensitive and
uninformative.
Box 4-8 De
ning aperture: an example of poor practice by geotechnical coding
committees
This example is used to illustrate the inadequacy of current geotechnical standards for soil and rock
description to convey an accurate or realistic representation of the true nature of the geological situation.
Mechanical aperture is the gap between two rock discontinuity walls (three-dimensional) and a very
important characteristic with respect to
flow and grouting. It is expressed in most codes and
standards as a one-dimensional scale of measurement, in the same way as joint spacing. The various
attempts at revising description of aperture over 25 years (leading to the current BS/Eurocode 7 require-
ments discussed later) have simply reinvented the measurement scales and terminology but have failed to
address or inform users about the fundamental dif
fluid
culties in measuring and characterising this property.
What is aperture?
It is the mechanical gap between two walls of a rock discontinuity such as a joint or a fault. An example of
a small section of joint with a gaping aperture (because the block has moved down slope and dilated over
roughness features) is shown in
Figure B4-8.1,
which is a photograph of a section of sheeting joint in
granite from Hong Kong.
