rock mass classification or rock mass rating of rock materials in civil and mining engineering
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Rock Mass Classification Systemsin the Design of
Underground mine openings
Siva Sankar Ulimella M.Tech
Under Manager
Project Planning, SCCL
Email : uss_7@yahoo.com
Rock Mass is an assemblage of intact rock materials separated by geological discontinuities
Rock as an Engineering material
Rock by nature is a heterogeneous, anisotropic and inelastic material and it exists in a very wide range with many geological structures built in its greater volume.
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Rock mass classification systems
Rockmass classification constitutes an integral part of empirical mine design. They are traditionally used to group areas of similar geo-mechanical characteristics, to provide guidelines for stabilit y performance and to select appropriate support .
• The first step of the application of a classification system is to characterize the rock massand in the second stepuse the advance forms of the classification systems to estimate the rock mass properties, such as modulus of elasticity, rock strength, m and s for Hoek and Brown failure criterion, etc., which are more appropriate inputs for strength parameters for any numerical analysis. Consequently, the importance of rock mass classification systems has increased over time (Milne et al., 1998)
The most widely known systems, including Deere’s RQD, Bieniawski’s RMR, and Barton’s Q, have been used extensively throughout the world
Rock mass classifications have been successful (Bieniawski, 1988) because they:
� Provide a methodology for characterizing rock mass strength using simple measurements;� Allow geologic information to be converted into quantitative engineering data;� Enable better communication between geologists and engineers, and;� Make it possible to compare ground control experiences between sites, even when the geologic conditions are very different.
In the recent years, Rock mass classification syste ms have been successfully used in tandem with analytic al and numerical tools for the design of underground openings .
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Table1. Major Engineering Rock Mass Classifications Currently in Use
Tunnels , mining openings and other openings in rock massNorwayPalmström, 1995
Rock Mass index(R Mi)
Estimation of rockmass strength properties
CanadaHoek E-1994Geological Strength
Index-GSI
General communicationInternational Society for Rock
Mechanics , 1981Basic geotechnical
description (BGD)
Tunnels and Wide openingsNorwayBarton et al., 1974Q-system
Tunnels, mines, Slopes foundationsSouth AfricaBieniawski, 1973(last modified, 1979 – USA)
RMR
TunnelingUSAWickham et al., 1972RSR concept
TunnelingUSADeere et al., 1967Rock Quality
Designation-RQD
TunnelingAustriaPacher et al., 1964NATM
TunnelingAustriaLauffer,1958Stand-up time
Tunnels with steel supportUSATerzaghi,1946Rock Load
ApplicationsCountry of
OriginOriginator and DateName of Classification
Legend:x -well defined ; 0 -very roughly defined; * -included but not defined+ -used as an additional information in RMR as adjusted value)
Classification system number:1. Terzaghi (1946); 2.Lauffer (1958); 3. NATM (1957-64); 4. Deere (1964); 5.Wickham (1972) 6. Bieniawski (1973); 7.Barton et al (1974); 8. BGD-ISRM
(1981); 9. GSI (1994)
*xxx
x++x
**
0External Features-Water condition-Rock stress condition-Blasting damage-Excavation dimensions
xxxxx
+x+
0**
**Jointing Geometry or structure-joint orientation with respect to excavation -jointing pattern-continuity-structure(fold, fault)
xx
xxXXx
xXx
xx**0Degree of jointing-Block size-joint spacing/frequency-RQD-Number of joint sets
xxxxx
*xxx
xx
0Joint conditions-joint size / length-joint separation-joint wall smoothness-joint waviness-joint filling
xx
xxx**
**
0
Rock Properties-Unit weight-porosity-rock hardness-strength-deformation-swelling
xx*0Rock-origin , name , type -weathering-anisotropy
9**87654321Classification systems
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Joint Roughness profile(Barton and Choubey, 1977)
Rockmass with 3 Joint Sets
Joint Roughness is a combination of Joint Asperities and Wavyness
RQD is the measures of discontinuity or massiveness in the rock mass and determined from drill core as given below:
1. Rock Quality Designation
where xi are the length of individual pieces of core in a drill run having lengths of 0.1 m or greater and L is the total length of drill run.
It is recommended to use standard core size of at least BMX (42 mm diameter) or NX size of 2 inch diameter.
RQD can also be obtained from discontinuity spacing measurements made on a core or an exposure using
RQD =100 × (0.1λ +1)× exp(− 0.1λ )
where λ = number of discontinuity per meter of drill run.Importance: 1) Quantification of rock mass2) Provide a basis for further classification of rock mass using RMR , Q - System and others3) Widely used by the mining and related industries all over the world
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Good75 - 90
Very good90 - 100
Fair50 – 75
Poor25 -50
Very Poor0 - 25
DescriptionRQD %
2. Rock Mass Rating (RMR)
The following parameters are used toclassify the rock mass using RMR system
1. Uniaxial compressive strength (UCS) of rock material (15 – 2)
2. Rock Quality Designation (RQD) (20 – 3)3. Spacing of discontinuities (20 – 5)4. Condition of discontinuities (30 – 0)5. Ground water conditions (15 – 0)6. Orientation of discontinuities
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B. Rock Mass classes determined from total rating
Rating 100-81 80-61 60-41 40-21 <20Class No. I II III IV VDescription Very Good Fair Poor Very
good Poor
C Meaning of Rock Mass Classes
Class No. Average Stand-up time Cohesion (kPa) Friction angle I 20 years for 15m span > 400 >45
II 1 year for 10 m span 300 – 400 35 – 45
III 1 week for 5 m span 200 - 300 25 – 35
IV 10 h for 2.5 m span 100 - 200 15 – 25
V 30 min for 1 m span < 100 <15
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3. NGI or Q-system of rock mass classification
SRF
J
J
J
J
RQDQ w
a
r
n
××=
nJ
rJ
aJ
= the joint set number
= the joint roughness
= the joint alteration
RQD = the Rock Quality Designation
wJ = the joint water condition
SRF = the stress reduction factor
• RQD/Jn:
Represents the structure of the rock mass. It is a crude measure of the block size. The max. value of the ratio is 200, obtained for RQD =100 and the Jn=0.5. This can be taken as the maximum size of the block which is around 200 cm.
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Jr /Ja:It represents the roughness and frictional characteristics of the joint walls and also of the filling material. This quotient is weighted in favour of rough, discontinuous unaltered joints in direct contact. When rock joints have thin clay mineral coatings and fillings, the strength is reduced significantly. This ratio is comparable to the shear strength characteristics of joint, more significantly with the frictional angle.
Jw/SRFSRF is a measure of rock stress in a competent rock = [UCS/major principal stress]. The other parameter of the ratio is Jw , which is a measure of ground water pressure. Presence of water has an adverse effect on the shear strength of jointed rock mass with the reduction in the effective normal stress across joint plane.
This Quotient is the most complicated empirical factor It should be given special attention, as it represents 4 groups of rock masses : stress influence in brittle blocky and massive ground, stress influence in deformable (ductile) rock masses, weakness zones, and swelling rock.
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In order to relate Q to behaviour and the supportrequirements of an underground excavation,Barton defined an additional quantity whichthey call the equivalent dimension De of theexcavation.This value of De is obtained dividingthe span, diameter or the height of the opening(Stope) by a quantity called the excavationsupport ratio ESR.
RatioSupportExcavation
mheightstopeordiameterspanExcavationDe
)(,, −=
ESR – indicates the length of safe unsupported span
Application
Mine openings and ESR rating
0.75Underground nuclear power stations, public facility
1.0Power stations, major road and railway tunnels, civil, defense chambers, portals etc
1.3Storage rooms, water treatment plant, access tunnels etc
1.6Permanent mine opening
3 – 5Temporary mine opening
Equivalent Support Ratio (ESR)Excavation Category
ESR is roughly analogous to inverse of Factor of Safety
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Nomogram for the max. a equivalent dimension De of an unsupportedUnderground excavation and Q system (Barton,1976)
De = 2.1927Q0.2787
1
10
100
1 10 100 1000
Rock Mass Quality Q
Eqi
vale
nt D
ime
nsio
n (D
e)
Support Required
No Support Required
Poor -Fair Good- v.Good Exeptionallygood
)(2.2 23.0QDe =
4. Modifications to Q- system based on width-height ratio of opening
The instability of underground mines is affected by many factors and ofwhich some of the important factors are:
• height of the mined-out area, • width of unsupported mine roof, • the depth of the mine from surface, • strength of the rock mass, • pillar dimensions, • hydrological conditions of the mine along with the frequency and
condition of joints, and • lastly the life time of the mine.
The modifications to the rock mass quality are suggested by KIGAMduly considering the influence of width-height factor on stress andstrength conditions of rockmass surrounding underground openings, thejoint orientation and the hydrological condition of the mine.
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The stability number N’ suggested by Potvinwhich is basically a modified Q system, includes the following parameters:
CBAQN ×××′=′
θσσ ciA = (B = joint orientation, and C = orientation
of the opening)
openingtheofnOrientationorientatioJoJ
J
J
RQDN ci
a
r
n
×××
×=′ int
θσσ
Modified Q - System
)(.
hw
J
J
J
J
RQDQ ciort
a
r
n ××
×
×=′′
θσσ
Modified Q - System
The above Eq. in fact includes stress reduction factor (SRF) value of the original Barton’s classification system which is modified to suit the mining conditions and is given as follows:
cici
hw
HEIGHT
SPANSRF
σσ
σσ θθ )(×
=
×
=
samplerockaofstrengthecompressivaxialunici −=σ
= the tangential stresses on the opening boundaryθσ
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Ratings for the joint orientation (Jort.) in terms of wetness condition
0.250.500.75Very unfavorable
0.500.750.80Unfavorable
0.600.800.85Fair
0.750.850.95Favorable
0.800.951Very Favourable
Jort.Rating
(For fully water saturated condition)
Jort.Rating
(For wet condition)
Jort.Rating
(For dry condition)
Orientation of the Joint
5. Geological Strength Index (GSI)
Hoek & Brown(1997) devised a simple chart for estimating
GSI. (matrix of 4 x 5 based on rock mass and discontinuity surface condition)
In this classification rock mass is categorized into fourmain types1. Blocky, 2. Very Blocky, 3. Folded, and 4. Crushed
And the discontinuities are classified into fivesurfaceconditions
1.Very good, 2. Good, 3. Fair, 4. Poor and 5. Very Poor
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Daesung Loc.1
Daesung Loc.2
Pyunghae Loc.1
Pyunghae Loc.2
GSI-
Characterization
of rock masses
on the basis of
interlocking and
joint Surface
condition
GSI ≈ RMR-5
CMRI – RMR(1987) India x x x x x x x x x
Rock Mass Classification for Coal Mines
(After C. Mark et. al.)
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Five parameters used in the classification system and their relative ratings are summarized
below:1. Layer thickness - 302. Structural features - 253. Rock weatherability - 204. Strength of roof rock - 155. Ground water seepage - 10
The five parameters should be determined individually for all the rock types in the roof upto a height of at least 2 m.
Rock Mass Rating (RMR) is the sum of five parameter ratings. If there are more than one rock type in the roof, RMR is eva luated separately for each rock type and the combined RMR is obtained as:
∑ (RMR of each bed x bed thickness)Combined RMR = ------------------------------------ ------------
∑ (Thickness of each bed)The RMR so obtained may be adjusted if necessary to take account for
some special situations in the mine like depth, stress, method of work
CMRI-ISM ROCK MASS CLASSIFICATION (1987)
CMRI-ISM ROCK MASS CLASSIFICATION
Paul Committee(1993) made guidelines on the support systems for Development workings based on the CMRI RMR
Good60 – 80
Very Good80 - 100
Fair40 – 60
Poor20 – 40
Very Poor0 - 20
DescriptionCMRI RMR
0 to 20%Gallery Span
5
+10 to – 10%Extraction Method
4
0 to 30%Induced stress
3
0 to 20%Lateral Stress
2
0 to 30%Depth1
AdjustmentParameterS.No
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CMRR USBM Classification Concept (1995)
� The Coal Mine Roof Rating (CMRR) was developed to fill the gap between geologic characterization and engineering design.
� It combines many years of geologic studies in underground coal mines with worldwide experience with rock mass classification systems.
Considers the parameters � Cohesion/roughness of weakness planes (0–35),
� Joint spacing and persistence (0–35) and
� Compressivestrength(0–30)Equations for intersection stability, bolt length and bolt density have also been given. The safe intersection span was obtained from failed and stable cases
sandstonesStrong65 – 100
Siltstones and sandstonesModerate45 to 65
Clay stones, mud rocks , shalesWeak0 to 45
Geological conditionCMRR ClassCMRR
Applications of Rockmass Rating Classifications
For Development Workings – Bord & Pillar or Longwall
Rock load In Galleries (tonnes/Sq.m)
CMRI RMR
Bieniaweski RMR
CMRI RMR
CMRR USBM
where γ is theunitweightofrock,t/m3, B is the roadway width , m, and F is the factor of safety and RMR is the average rockmas s rating of the immediate roof after adjustment. H is depth of Cover in feet and Pr in Kilo pounds/s q.ft in case of CMRR USBM
Support Load at gallery Junctions
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Applications of Rockmass Rating Classifications
CMRI RMRFor Depillaring Workings – Bord & Pillar ( After Kus hwaha, et. al. 2010)
γ is the weighted average rock densityof the immediate roof strata, t/m3, H is depth of cover m, K is the ratio of horizontal to vertical in situ stress , W is the width of split or slice , m andR is the weighted average CMRI RMR
of the immediate roof rock.
SLDjn, SLDsl, SLDsp and SLDge are the required support load density in t/m2 at the slice junction, within slice, in the split gallery and at the goaf edge respectively.
)1000(11
+−
+−
= HGE
SS vhav να
νν
Shav = Average horizontal in situ stress, MPa
V = Poisson’s ratio of coal, varied from 0.19 to 0.23
α = Co-efficient of thermal expansion of rock = 30 x 10-6/ 0C
E = Modulus of elasticity of coal, varied from 0 .84 to1.70 GPa
G = Thermal gradient, 0.03 0C/m
γγγγ = Unit rock pressure, 0.025 MPa/m
H = Depth of cover, m
Applications of Rockmass Rating Classifications
CMRI RMRIn the absence of Insitu MeasurementsHorizontal Stress Estimation
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Applications of Rockmass Rating Classifications
Q – System (Norwegian Geotechnical Institute)For Depillaring Workings – Bord & Pillar
For joint set number (Jn)> 9, the roof pressure (Proof) = 2/Jr x (5Q)-1/3
For Jn < 9, Proof = 2/3 x Jn1/2 /Jr x (5Q)-1/3
10Any value(>20)Goaf edges
5<2.5
3 - 52.5 - 5
2>5
Slices
1-21 - 10
1>10Galleries & Junctions
SRFJnLocation
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