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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