Geology Reference
In-Depth Information
In other words, whereas a discontinuity, as de
ned by Norbury and
lSRM, is a mechanical fracture, the European standard broadens this
to include incipient fabric (such as cleavage or bedding planes) that
might open up under stress. The implication is that a feature might be
described as a discontinuity in some logs but not in others.
C.5.3 Discussion
The distinctions might appear trivial but where it matters is when
characterising the rock mass. Rock mass classi
cations discussed
later mostly incorporate discontinuity or joint spacing as a fundamen-
tal parameter. Rock Quality Designation (RQD) is a measure in bore-
holes of the proportion of pieces of
core more than 100mm in
length between discontinuities in the sense of mechanical fractures
(natural and not drill-induced). If one were to include incipient fabric
such as intact bedding and schistosity, then the whole measure of RQD
would be changed and this has knock-on effects for rock mass classi-
'
sound
'
fications systems such as Q and RMR.
It is best to follow the Norbury/ISRM de
nition that a discontinuity
is a mechanical break (and this should be used in RQD). Other fabrics
that retain tensile strength (including bedding and cleavage) should be
described as incipient and not counted in RQD. Incipient discontinu-
ities should be logged and characterised as high tensile strength (close
to that of intact rock), intermediate or low (readily split). Geologically,
however, incipient fabric such as cleavage, where it has little effect on
intact strength, would be called faint, as in faintly de
ned cleavage.
Geometrical description and measurement of discontinuities is cov-
ered in the ISRM guidelines.
As discussed in
Chapter 3,
many rocks are weathered to great depths,
especially in tropical and sub-tropical areas of the world. Weathering
effects should be described and recorded and may be interpreted
directly from changes in colour, discontinuity spacing,
ll on
joints and intact strength. In some circumstances, weathering classifi-
in
-
cations are useful to characterise rock at the scale of an intact, uni-
form sample or at the mass scale. The classi
cation provides a
shorthand description, which is often treated synonymously with
mass strength.
Unfortunately, there are many different classi
cations, some of
which use the same terminology to describe different conditions and
pro
les using dif-
ferent terms, all of which is very confusing (Martin & Hencher, 1986;
Hencher 2008).
les; others describe the same phenomena and pro
