ARMA 08-353-2008-ReprodutibilidadeClassificaçãoMaciçosRochosos-Ameli

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1. INTRODUCTION
The reproducibility of rock classification has
received little attention in literature. Rock
classification provides the basis for selection of a
support system and geotechnical analysis in mining
and civil engineering projects. It may also be used in
project contracts to determine a rate for excavation
cost. Therefore, the accuracy of geological mapping
and rock mass classification has impacts on the
design and cost of projects.
This study stems from a comparison of geological
mappings during a shaft-sinking project. Two
geotechnical engineering teams, assigned by the
project design engineer (Engineer) and the
contractor (Contractor) conducted the geological
mapping. The rock mass classification data for a
selected study zone on the shaft periphery surface, as
shown in Figure 1, are analyzed for this study.
2. TERMINOLOGY
The following terms are given for clarity as
generally they lack a universal definition and have
some philosophical meanings.
2.1. Reproducibility
Reproducibility is the ability of an experiment to
be accurately reproduced or replicated. It is the
variation in the average of rock classification
results made by different geologists mapping the
same rock exposure area.
2.2. Repeatability
Repeatability is the degree to which a geological
mapper duplicates his/her rock mass classification
for a given rock exposure area. It is an overall
measure of the credibility and quality of his/her
mapping.
2.3. Accuracy
Accuracy is defined as the closeness with which a
measurement agrees with the standard.
3. THE PROJECT
The 180 m long shaft is part of a water
conveyance project in Canada. The shaft was sunk
from ground elevation through 30 m of
overburden materials into igneous bedrock. The
overall rate of construction advancement in the
shaft was 35 mm per hour. A drill and blast
method was used for the shaft excavation in 1.7 m
advances. In each advance, the excavated
ARMA 08-353
Reproducibility in Rock Mass Classification
Ali Ameli, PhD, PEng, PE
Geo Engineering Ltd, British Columbia, Canada
Copyright 2008, ARMA, American Rock Mechanics Association
This paper was prepared for presentation at San Francisco 2008, the 42nd US Rock Mechanics Symposium and 2nd U.S.-Canada Rock Mechanics Symposium, held in San
Francisco, June 29-July 2, 2008.
ABSTRACT: Rock Mass Rating and the Q-system were employed for the classification of rock mass in the
periphery of a shaft sunk in igneous intrusive formations. The rock classification data obtained by two
geological mapping teams are compared in this experimental study. It was found that there were differences in
the rock quality assessment obtained by the two teams. The magnitude of differences was greater for Q values
when compared with those of the Q' values. The study also highlights the overall effects of the active stress
component in Q-system and the discontinuity orientation in RMR. The results presented provide factual data
for interpretation by engineers and researchers. Attention to the reproducibility of rock classification may also
be useful in contractual considerations.
12 m
180 m
periphery was scaled and mapped by two different
teams of mappers. The shaft wall was then supported
by welded wire mesh and the application of
shotcrete. Rock bolts were then installed at 1.5 m
spacing, followed by drilling for the next round of
excavation. The shaft dimensions and ground profile
are illustrated in Figure 1.
When the shaft reached its final depth, a cavern was
then excavated to launch Tunnel Boring Machines
for boring two horizontal tunnels from the bottom of
the shaft. The shaft provides access during the
construction period, and will then contain water
pipes during the service time of the project.
Figure 1. Schematic Diagram of Shaft
4. ROCK MASS CHARACTERISTICS
The bedrock consists of quartz diorite host rock with
5% to 69% pendants of fractured felsic metavolcanic
rock as described in Table 1, obtained during the
excavation period.
5. FIELD MAPPING
An outside consultant and in-house geotechnical
engineer conducted the Contractor mapping. The
quality of mapping operations was controlled in the
field. Field data was then compared with a complete
photographic profile of the mapped areas for quality
assurance.
The geotechnical engineering firm that conducted
the original site investigation for the design stage
also completed the mapping on behalf of the
Engineer. The Engineer’s mapping was also
subject to quality assurance by other consulting
firms. The construction schedule allowed 1.5
hours for mapping. This time seemed to be
sufficient for mapping intervals of exposed rock
around the shaft to the height of about 1.7 m.
For this study, the geological mapping data of the
shaft periphery along a 67 m zone is selected as a
study sample. See Figure 1. The study zone was
mapped in 40 consequent advancements, forming
an area of about 2400 m2 on the shaft periphery. A
study zone towards the bottom of the shaft was
selected to ensure that the two mapping teams had
acquired adequate familiarity with the site-specific
rock conditions by that stage. Their data are
therefore expected to be more repeatable.
6. ROCK MASS CLASSIFICATION –
COMPARISONS
The quality of the rock mass was evaluated using
both the Norwegian Geotechnical Institute (NGI)
Q-system and the Rock Mass Rating classification
system (RMR). The specific rating ranges
employed for the project are given in Table 2.
Table 2. Rock Mass Characteristics Rating Ranges
Rock
Classification
Rating Range
Q-System
Rating Range
RMR
Exceptionally to
Extremely Poor
< 0.1 < 25
Very Poor 0.1 < < 1 25 < < 40
Poor 1 < < 4 40 < < 60
Fair 4 < < 10 60 < < 65
Good to
Exceptionally
Good
> 10 > 65
The project-specific ranges on rock mass
classification for different rock qualities should
not affect the nature of this comparative study.
6.1. Q-System
The Q value is defined as developed by Barton, et
al [1]:
Q = (RQD/Jn)
. (Jr/Ja)
. (Jw/SRF) (1)
Cavern
67 m
Overburden
Granitic
Metavolcanic
Rock
Study
Zone
Table 1. General Rock Mass Characteristics
Quartz Diorite
Metavolcanic
(Felsic)
Texture Medium grained, gray with hornblende Fine grained, pale orange gray
Weathering Fresh to highly weathered Fresh to highly weathered
Alteration Slight to intense
Slight to intense, penetrating to
20 mm
Joints Widely to very closely jointed
Moderately to extremely close
jointed
Strength Very strong to weak
Very strong to weak
(UCS <160 MPa)
Tensile Strength 10 MPa 5 MPa
RQD 40% to 90% 0% to 40%
Joint Set 3 plus a random 3 to 4 plus a random
Joint Spacing 40 mm to 800 mm 10 mm to 450 mm
Persistence 1 m to 7 m 1 m to 7 m
Joint Separation 0.1 mm to 10 mm 0.1 mm to 0.7 mm
Joint Surface Planar to undulating Planar
Roughness Slickensided, smooth, rough Smooth
Infilling
None, or with biotite, chloride, epitode,
zeolite, clay, sand, …
None, or with sand, silt, calcite,
…
Joint Surface
Weathering
Slightly to moderately weathered
Slightly to moderately
weathered
Others With or without rust stains With or without rust stains
Water Moist to flowing Moist to flowing
Shear Zones Some up to 500 mm width and wedges Often at interfaces
Contact Zones Highly altered, curvilinear, often highly fractured or brecciated
Horizontal stress Stress-induced damage in the rock not detected by velocity anisotropy tests
where:
RQD = Rock Quality Designation
Jn = Number of main joint sets
Jr = Joint roughness
Ja = Joint alteration
Jw = Joint water reduction factor
SRF = Stress reduction factor
Figure 2 compares the percentages of rock classified
by Q values obtained by the Contractor’s and the
Engineer’s teams.
0
10
20
30
40
Extremely to
Exceptionally Poor
Very Poor Poor Fair Good to
Exceptionally
Good
R
o
ck
,
%
Engineer Contractor
Figure 2. Q Values Reproducibility
A modified Q value, Q', was also calculated. This is
based on an assumption that the active stress factor
(Jw/SRF) is equal to one, i.e.:
Q’ = (RQD/Jn)
. (Jr/Ja) (2)
Figure 3 depicts the variationsin rock mass
classification for Q' values by the two geological
mapping teams for the same zone.
0
10
20
30
40
Extremely to
Exceptionally
Poor
Very Poor Poor Fair Good to
Exceptionally
Good
R
o
ck
,
%
Engineer Contractor
Figure 3. Q' Values Reproducibility
6.2. Rock Mass Rating (RMR)
The following six parameters are used to classify a
rock mass using the RMR system, as developed by
Bieniawski [2].
i) Uniaxial compressive strength of rock materials
ii) RQD
iii) Discontinuity spacing
iv) Discontinuity conditions
v) Groundwater conditions
vi) Discontinuity orientation
The applicability of the discontinuity orientation in
the shaft was a debatable issue. In this
comparative study, the RMR values that exclude
the above rating adjustment are expressed as
RMR'.
0
10
20
30
40
Extremely to
Exceptionally
Poor
Very Poor Poor Fair Good to
Exceptionally
Good
R
o
ck
,
%
Engineer Contractor
Figure 4. RMR' Values Reproducibility
Figure 4 compares the percentage of rock mass
classified by RMR' values obtained by the
Contractor’s and the Engineer’s mapping teams.
The effect of the active stress component (Jw/SRF)
on Q-System values is shown in Figure 5, which
compares the percentage of rock mass classified
by Q and Q' values. As one mapping group
obtained these values, the results are not
influenced by reproducibility.
0
10
20
30
40
Extremely to
Exceptionally
Poor
Very Poor Poor Fair Good to
Exceptionally
Good
R
oc
k,
%
Q Q'
Figure 5. Comparison of Q vs Q' Values
The effect of the discontinuity orientation
component on rock mass classification can be
understood by comparing the percentage of rock
mass classified by RMR and RMR' in Figure 6. One
mapping team obtained both RMR and RMR’
values.
0
10
20
30
40
Extremely to
Exceptionally
Poor
Very Poor Poor Fair Good to
Exceptionally
Good
R
o
ck
,
%
RMR' RMR
Figure 6. Comparison of RMR vs RMR' Values
7. SUMMARY
The findings in this comparative study are subject to
interpretation by researchers and practitioners. The
design of tunnels, shafts, mines, foundations and
slopes in rock are often based on rock mass
classification results. Thus, the efforts made to
standardize the mapping activities are not wasted.
The evaluation of reproducibility in rock mass
classification is an initial stage for such efforts.
The assignment of a single independent consultant
for rock mass classification may assist in avoiding
discrepancies of this nature, and facilitating the
contract administration. However, the assessment
of reproducibility would still be valid when more
than one consultant carries out geological
mapping.
8. REMARKS
The thorough practice of professionals in the
project and considerable quality assurance efforts
by independent consultants were the incentives to
publish these data. The use of the term
reproducibility in measurements carried out by
two independent teams with conflicting interests in
a contract may be debatable.
9. FUTURE STUDIES
Further research may target parameters that could
minimize the discrepancies in the mapping data
for different rock types. These parameters include
the:
 division of rock mass into structural zones
for mapping purposes
 capability of each of the RMR and Q-
System methods for providing
reproducible and repeatable results on
rock classification.
 contractual responsibilities.
An analytical evaluation of the mapping data is
planned for the next stage, subject to a research
sponsorship.
10. REFERENCES
1. Barton, N., R. Lien and J. Lunde, 1974.
Engineering classification of rock masses for
design of tunnel support. In Rock Mechanics
6(4):189-236.
2. Bieniawski, Z.T. 1989. Engineering Rock Mass
Classifications. Wiley & Sons, Chichester.