Geology Reference
In-Depth Information
nightmare
if a borehole hits a conductive section, then high permeabilities will be measured and an
installed instrument will be responsive to changes in water pressure, but this is literally a hit or miss
business, as evidenced by many examples in investigations associated with nuclear waste (e.g. Thomas &
La Pointe, 1995). The author has the experience of working in a deep tunnel 150m below the sea, where
over one section, the rock was highly jointed but dry, but elsewhere, at the same level, there was a steady
in
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ow through what was apparently intact rock. Clearly, it is not just local aperture that matters, but the
characteristics of the full fracture network and its connectivity leading to the point of observation. It is an
important area for research and for observation linked to geochemical and structural studies together with
an appreciation of coupled mechanisms (e.g. Olsson & Barton, 2001; Sausse & Genter, 2005). Without
getting to grips with the concept of channelised
flow on rock joints and through joint networks, it may be
impossible to ever make a safety case for nuclear waste disposal, with all the corollaries, i.e. no nuclear
power, global warming and the end of civilisation. Well, perhaps slightly overstated, but not that much.
Apart from the natural variability of fracture networks, are there any other considerations?
Yes. Most rock joints are sampled in boreholes where aperture simply cannot be measured. Furthermore,
it is very unlikely that any borehole sample would be representative of the discontinuity at any great
distance. Down-hole examination with cameras and periscopes can be used to examine borehole walls,
but again there is a problem with sampling and representativeness. In exposures such as quarries or
tunnels, exposure is better but there is a question of disturbance
blasting, stress relief and block
movement and whether observations at one location are relevant to the rock mass as a whole.
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So what advice is given in recommended methods and standards?
1978 ISRM. The discussion on aperture is very useful. Its importance is recognised and many of the
dif
culties in measurement and interpretation are highlighted.
For description purpose and where appropriate, apertures are split into closed, gapped and open
features, each subdivided into three. It is advised that:
a. modal (most common) apertures should be recorded for each discontinuity set
b. individual discontinuities having apertures noticeably wider or larger than the modal value should be
carefully described, together with location and orientation data, and
c. photographs of extremely wide (10
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100cm) or cavernous (>1m) apertures should be appended.
1999 UK BS5930 (BSI, 1999). Says little about aperture other than noting that it cannot be described in
core. Five classes are introduced, which use some of the same terms as ISRMbut with different de
nitions.
2003 INTERNATIONAL STANDARD ISO 14689
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1 (BSI, 2003) (for Eurocode 7 users). Provides a
new mandatory terminology for one-dimensional measurement that differs from that of BS5930: 1999
and ISRM (1978), as illustrated in
Table 4 B8.1
(see below).
Aperture size term
BS5930 1999
ISO 14689
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1: 2003
<0.1mm
Very tight
Very tight
Very tight
0.1
-
0.25mm
Tight
Tight
Tight
0.25
-
0.5mm
Partly open
Partly open
0.5
-
2.5mm
Open
Moderately open
Open
2.5
-
10mm
Moderately wide
Open
Moderately wide
10
-
100mm
Very wide
Very open
Wide
100
-
1,000mm
Extremely wide
Very wide
>1,000mm
Cavernous
Extremely wide
1
nes a termwide for gapped features >10mm; the
other terms above, also for apertures >10mm are for open features but the difference is not fully obvious.
In detail, there is further confusion in that ISRM also de
