different rock mass classifications or rating used in civil and mining engineering

1. Rock Mass Classification Systems
in the Design of
Underground mine openings
Siva Sankar Ulimella M.Tech
Under Manager
Project Planning, SCCL
Email : uss_7@yahoo.com
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.
Rock Mass is an assemblage of intact rock materials separated
by geological discontinuities
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2. 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 stability
performance and to select appropriate support.
• The first step of the application of a classification system is
to characterize the rock mass and in the second step use
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)
In the recent years, Rock mass classification systems
have been successfully used in tandem with analytical
and numerical tools for the design of underground
openings.
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.
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3. Table1. Major Engineering Rock Mass Classifications
Currently in Use
Country of
Name of Classification Originator and Date Applications
Origin
Rock Load Terzaghi,1946 USA Tunnels with steel support
Stand-up time Lauffer,1958 Austria Tunneling
NATM Pacher et al., 1964 Austria Tunneling
Rock Quality
Deere et al., 1967 USA Tunneling
Designation-RQD
RSR concept Wickham et al., 1972 USA Tunneling
Bieniawski, 1973
RMR South Africa Tunnels, mines, Slopes foundations
(last modified, 1979 – USA)
Q-system Barton et al., 1974 Norway Tunnels and Wide openings
Basic geotechnical International Society for Rock
General communication
description (BGD) Mechanics , 1981
Geological Strength Estimation of rockmass strength
Hoek E-1994 Canada
Index-GSI properties
Rock Mass index Tunnels , mining openings and other
Palmström, 1995 Norway
(R Mi) openings in rock mass
Classification systems 1 2 3 4 5 6 7 8 9**
Rock 0 * x x
-origin , name , type
-weathering
-anisotropy
Rock Properties * * * x x x x
-Unit weight * x
-porosity 0
-rock hardness
-strength
-deformation
-swelling
Joint conditions 0 x x * x
-joint size / length x x x
-joint separation x x
-joint wall smoothness x
-joint waviness x
-joint filling
Degree of jointing 0 * * x x x x x x
-Block size X X x
-joint spacing/frequency x X
-RQD x
-Number of joint sets
Jointing Geometry or structure * * * 0 x + x
-joint orientation with respect to excavation * + x
-jointing pattern x
-continuity x
-structure(fold, fault) x
External Features 0 * x x *
-Water condition * + x
-Rock stress condition + x
-Blasting damage x
-Excavation dimensions
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)
Legend:
x -well defined ; 0 -very roughly defined; * -included but not defined
+ -used as an additional information in RMR as adjusted value)
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4. Rockmass with 3 Joint Sets
Joint Roughness is a combination of
Joint Roughness profile(Barton and
Joint Asperities and Wavyness
Choubey, 1977)
1. Rock Quality Designation
RQD is the measures of discontinuity or massiveness in the rock mass and
determined from drill core as given below:
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 mass
2) Provide a basis for further classification of rock mass using RMR , Q - System
and others
3) Widely used by the mining and related industries all over the world
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5. RQD % Description
0 - 25 Very Poor
25 -50 Poor
50 – 75 Fair
75 - 90 Good
90 - 100 Very good
2. Rock Mass Rating (RMR)
The following parameters are used to
classify 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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6. B. Rock Mass classes determined from total rating
Rating 100-81 80-61 60-41 40-21 <20
Class No. I II III IV V
Description 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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7. 3. NGI or Q-system of rock
mass classification
RQD J r J
Q= × × w
Jn J a SRF
RQD = the Rock Quality Designation
J n = the joint set number
J r = the joint roughness
Ja = the joint alteration
Jw = 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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8. 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/SRF
SRF 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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9. Application
In order to relate Q to behaviour and the support
requirements of an underground excavation,
Barton defined an additional quantity which
they call the equivalent dimension De of the
excavation. This value of De is obtained dividing
the span, diameter or the height of the opening
(Stope) by a quantity called the excavation
support ratio ESR.
Excavation span, diameter , or stope − height ( m)
De =
Excavation Support Ratio
ESR – indicates the length of safe unsupported span
Mine openings and ESR rating
Excavation Category Equivalent Support Ratio (ESR)
Temporary mine opening 3–5
Permanent mine opening 1.6
Storage rooms, water 1.3
treatment plant, access
tunnels etc
Power stations, major road 1.0
and railway tunnels, civil,
defense chambers, portals etc
Underground nuclear power 0.75
stations, public facility
ESR is roughly analogous to inverse of Factor of Safety
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10. Poor -Fair Good- v.Good Exeptionally
good
100
Eqivalent Dimension (De) De = D2=2(Q 0.23 )
. 2.1927Q
e
0.2787
Support Required
10
No Support Required
1
1 10 100 1000
Rock Mass Quality Q
Nomogram for the max. a equivalent dimension De of an unsupported
Underground excavation and Q system (Barton,1976)
4. Modifications to Q- system based on
width-height ratio of opening
The instability of underground mines is affected by many factors and of
which 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 KIGAM
duly considering the influence of width-height factor on stress and
strength conditions of rockmass surrounding underground openings, the
joint orientation and the hydrological condition of the mine.
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11. Modified Q - System
The stability number N’ suggested by Potvin
which is basically a modified Q system,
includes the following parameters:
N ′ = Q′ × A × B × C
σ ci
A= (B = joint orientation, and C = orientation
σθ of the opening)
 RQD J r  σ ci
N′ =  × × × Joint orientation × Orientation of the opening
 Jn Ja  σθ
Modified Q - System
 RQD J r  J ort . × σ ci
Q ′′ =  × ×
 Jn J a  σ θ × ( w h)
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:
 SPAN  σ θ  σ θ × (w h)
SRF =  ×  =
 HEIGHT 
  σ ci 
 σ ci
σ ci = uni − axial compressive strength of a rock sample
σθ = the tangential stresses on the opening boundary
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12. Ratings for the joint orientation (Jort.) in terms of wetness condition
Jort. Jort. Jort.
Orientation of the
Rating Rating Rating
Joint
(For dry condition) (For wet condition) (For fully water saturated condition)
Very Favourable 1 0.95 0.80
Favorable 0.95 0.85 0.75
Fair 0.85 0.80 0.60
Unfavorable 0.80 0.75 0.50
Very unfavorable 0.75 0.50 0.25
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 four
main types
1. Blocky, 2. Very Blocky, 3. Folded, and 4. Crushed
And the discontinuities are classified into five surface
conditions
1.Very good, 2. Good, 3. Fair, 4. Poor and 5. Very Poor
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13. GSI-
Characterization
of rock masses
on the basis of
interlocking and
Daesung Loc.2
joint Surface
Daesung Loc.1
condition Pyunghae Loc.1
Pyunghae Loc.2
GSI ≈ RMR-5
Rock Mass Classification for Coal Mines
(After C. Mark et. al.)
CMRI – RMR(1987) India x x x x x x x x x
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14. CMRI-ISM ROCK MASS CLASSIFICATION (1987)
Five parameters used in the classification system and their relative ratings
are summarized
below:
1. Layer thickness - 30
2. Structural features - 25
3. Rock weatherability - 20
4. Strength of roof rock - 15
5. 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 evaluated
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
S.No Parameter Adjustment CMRI RMR Description
1 Depth 0 to 30% 0 - 20 Very Poor
2 Lateral 0 to 20%
Stress 20 – 40 Poor
3 Induced 0 to 30% 40 – 60 Fair
stress
60 – 80 Good
4 Extraction +10 to – 10%
Method 80 - 100 Very Good
5 Gallery 0 to 20%
Span
Paul Committee(1993) made guidelines on the support systems for
Development workings based on the CMRI RMR
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15. 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
CMRR CMRR Class Geological condition
0 to 45 Weak Clay stones, mud rocks , shales
45 to 65 Moderate Siltstones and sandstones
65 – 100 Strong sandstones
Applications of Rockmass Rating Classifications
For Development Workings – Bord & Pillar or Longwall
Rock load In Galleries (tonnes/Sq.m)
CMRI RMR
Bieniaweski RMR
CMRR USBM
Support Load at gallery Junctions
CMRI RMR
where γ is theunitweightofrock,t/m3, B is the roadway width, m, and F is
the factor of safety and RMR is the average rockmass rating of the
immediate roof after adjustment.
H is depth of Cover in feet and Pr in Kilo pounds/sq.ft in case of CMRR
USBM
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16. Applications of Rockmass Rating Classifications
CMRI RMR
For Depillaring Workings – Bord & Pillar ( After Kushwaha, et. al. 2010)
γ is the weighted average rock density
of 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 and
R 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.
Applications of Rockmass Rating Classifications
CMRI RMR
Horizontal Stress Estimation In the absence of Insitu Measurements
ν α EG
S hav = Sv + ( H + 1000 )
1 −ν 1 −ν
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, 0.030C/m
γ = Unit rock pressure, 0.025 MPa/m
H = Depth of cover, m
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17. 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
Location Jn SRF
>10 1
Galleries & Junctions
1 - 10 1-2
>5 2
Slices 2.5 - 5 3-5
<2.5 5
Goaf edges Any value(>20) 10
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