Use of The Mining Rock Mass Rating (MRMR) Classification: Industry Experience
Use of The Mining Rock Mass Rating (MRMR) Classification: Industry Experience
Use of The Mining Rock Mass Rating (MRMR) Classification: Industry Experience
INDUSTRY EXPERIENCE
By Jarek Jakubec, C.Eng.,1 and Gabriel S. Esterhuizen, Ph.D.2
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Table 1.—Comparison of main classification systems
used in the mining industry
Laubscher 2000
Beniawski RMR
Laubscher 90
Category Parameters
Barton Q
Intact rock strength UCS x x x x
Open joint frequency RQD x x x -
FF/m x - x x
Joint set (Jn) x x x x
Open Joint strength Roughness (Jr) x x x x
Alteration (Ja) x x x x
Infill (Ja) x x x x
Cemented joints CJ/m - - - x
quantity and strength CJ strength - - - x
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Figure 5.—Cemented joints in the core (left) could
significantly influence rock block strength and frag-
mentation in a caving environment (right).
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When the effects of these features are included, the mass conditions has a significant impact on the choice of
assigned rock block strength is reduced, which in turn mine layout, operating procedures, and financial invest-
dramatically reduces the predicted fragmentation. The ment. In this case, the IRMR calculated from the FF/m was
expected fragmentation has a significant impact on the considered to be more representative of the actual rock
likely production rates, mine layout, and operational cost mass conditions than the values based on the RQD. Third-
of a block-caving operation. party review of the outcomes, inspection of exposures in
the current open-pit mine, and comparison to values esti-
100 mated from the GSI rating confirmed this conclusion.
90
80
Percent passing (%)
70
60
50
40
30
Veinlets included
20
Veinlets ignored
10
0
0.01 0.1 1 10 100 1000
3
Block size (m )
Figure 6.—Effect of calcite-filled veinlets on predicted
fragmentation in block caving.
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However, if only drill core is used for rock mass assess- • Accurate assessment of weak joint infill that is
ment, we are exposed to a whole range of biases, and the washed out in most drilling processes. Triple tube
resulting description of the rock mass could be signifi- techniques can help to alleviate this problem.
cantly skewed. The potential problems and pitfalls were • Rock strength assessment in weathered/altered sen-
described by Laubscher and Jakubec [2001] and Murphy sitive rock types such as kimberlites and mudstones.
and Campbell [in press]. It is important to realize that rock Using specialized drilling fluids, very careful sam-
mass assessment based on drill core only can easily be off ple collection/preservation programs, and speedy
by 50%. delivery to the laboratory can partly mitigate these
The main challenges in rock mass assessment based on problems.
core logging, regardless of the classification system used, • Material anisotropy. Assessment of both intact rock
are: strength and discontinuity strength anisotropy from
the drill core could be a problem. The core cross-
• Differentiation between artificially induced breaks section is simply too small to capture joint geom-
and natural defects. In situ borehole scanners can etry. (See the example shown in Figure 10.)
help to assess in situ conditions.
• Assessment of discontinuities in foliated or highly Any of the points mentioned above can have a signifi-
laminated rocks. In such rock masses, the borehole cant impact on the rock mass assessment, and it is neces-
scanner may not be effective. sary that data be scrutinized in that respect.
• Differentiation between continuous joints and
discontinuous fractures. This problem cannot be
successfully resolved without rock mass exposures
(see Figure 9).
• Drilling orientation bias. Missing or under-
estimating discontinuity sets subparallel to the drill-
hole. Different orientation of the drillholes can
mitigate the problem.
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we should strictly follow the letter of the classification REFERENCES
systems or whether we should treat classification systems
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ACKNOWLEDGMENT
Littleton, CO: Society for Mining, Metallurgy, and
The authors would like to thank Dr. Dennis Laubscher Exploration, Inc., pp. 475–481.
for reviewing this paper and for his valuable comments. Laubscher DH, Taylor HW [1976]. The importance of
geomechanics of jointed rock masses in mining operations.
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