Rock Engineering Practice & Design - isrm.net · Rock Mass Classification: RMR The Rock Mass Rating...

Rock EngineeringRock EngineeringPractice & DesignPractice & Design

Lecture 3: Lecture 3: Rock Mass Classification Rock Mass Classification

& Empirical Design& Empirical Design

1 of 40 Erik Eberhardt – UBC Geological Engineering ISRM Edition

Author’s Note:Author’s Note:The lecture slides provided here are taken from the course “Geotechnical Engineering Practice”, which is part of the 4th year Geological Engineering program at the University of British Columbia (V C d ) Th k i i d (Vancouver, Canada). The course covers rock engineering and geotechnical design methodologies, building on those already taken by the students covering Introductory Rock Mechanics and Advanced Rock Mechanics Rock Mechanics.

Although the slides have been modified in part to add context, they of course are missing the detailed narrative that accompanies any l l d h h l lecture. It is also recognized that these lectures summarize, reproduce and build on the work of others for which gratitude is extended. Where possible, efforts have been made to acknowledge th v ri us s urc s ith list f r f r nc s b in pr vid d t th the various sources, with a list of references being provided at the end of each lecture.

Errors, omissions, comments, etc., can be forwarded to the


Errors, omissions, comments, etc., can be forwarded to the author at: [email protected]

Classification Systems in DesignClassification Systems in Design

Even with many resources available for site investigation, there still can remain problems in applying theories in practical engineering circumstances. Considering the three p g g gmain design approaches for engineering rock mechanics –analytical, observational and empirical, rock mass classifications today form an integral part of the most

d i d i h h i i l d i h d predominant design approach, the empirical design method.

Indeed, on many underground construction, tunnelling and mining projects rock mass classifications have provided mining projects, rock mass classifications have provided the only systematic design aid in an otherwise haphazard “trial-and-error” procedure.


Failure MechanismsFailure Mechanisms

structurally-controlled f l

rock mass failure

The Stability of an underground opening is a function of:

failure

•Stresslow or high

f

low or high

•Structuref lli• falling

• sliding

98)

•Rock Mass

Hoe

k(1

9


Rock Mass ClassificationRock Mass ClassificationThe objectives of rock mass classifications are to:

Identify the most important parameters influencing the rock mass.

d k f f l h Divide a rock mass formation into groups of similar behaviour.

Provide a basis for understanding the characteristics of each rock mass class.

Relate experiences of rock conditions at one site to those at another.

Derive quantitative data and guidelines for engineering design.

The boundaries of the structural regions usually coincide with a major

Provide a common basis for communication between geologists and engineers.

The boundaries of the structural regions usually coincide with a major structural feature such as a fault or with a change in rock type. In some cases, significant changes in discontinuity spacing or characteristics, within the same rock type, may necessitate the division f th k i t b f ll t t l i


of the rock mass into a number of small structural regions.

Rock Mass ClassificationRock Mass Classification

These objectives suggest the three main benefits of rock mass classifications:

Improving the quality of site investigations by calling for the minimum input data as classification parameters.p p

Providing quantitative information for design purposes.

E bli b i i j d d ff i Enabling better engineering judgment and more effective communication on a project.


Rock Mass Classification: RMRRock Mass Classification: RMRThe Rock Mass Rating The Rock Mass Rating (RMR) system was developed in 1973 in South Africa by Prof. Z.T.

k h d Bieniawski. The advantage of his system was that only a few basic parameters relating to the geometryrelating to the geometryand mechanical conditions of the rock mass were required.

Rating adjustments are g j mincluded to account for the adverse nature discontinuity angles may have with respect to the excavation or slope


pdirection.

Bieniawski (1989)

Rock Mass Classification Rock Mass Classification -- RMRRMRTh dj t d l i th fi l RMR l f th k m f hi h The adjusted value gives the final RMR value for the rock mass, for which several rock mass classes are described.

F lFor example:A mudstone outcrop contains three fracture sets. Set ‘1’ comprises bedding planes; these are highly 14

16

1

RMR = 6+R2+R3+R4+R5

bedding planes; these are highly weathered, slightly rough and continuous. The other two sets are jointing; both are slightly weathered and slightly rough The strength of 4

68

1012

RM

R R

atin

g R

and slightly rough. The strength of the intact rock is estimated to be 55 MPa with an RQD of 60% and a mean fracture spacing of 0.4 m. The fractures are observed to be damp

02

0 50 100 150 200 250 300

Unconfined Compressive Strength, qu (MPa)

R


fractures are observed to be damp.

Harrison & Hudson (2000)

Rock Mass Classification Rock Mass Classification -- RMRRMR

Example:A mudstone outcrop contains three fracture sets. Set ‘1’ comprises 15

20

25

ting

R2

RMR = 6+12+R3+R4+R5

bedding planes; these are highly weathered, slightly rough and continuous. The other two sets are jointing; both are slightly weathered 0

5

10

RM

R R

at

and slightly rough. The strength of the intact rock is estimated to be 55 MPa with an RQD of 60% and a mean fracture spacing of 0.4 m. The

0 10 20 30 40 50 60 70 80 90 100Rock Quality Designation, RQD

25


10

15

20

R R

atin

g R

3

0

5

0.01 0.1 1 10Joint Spacing (meters)

RM

RMR = 6+12+10+R4+R5


Joint Spacing (meters)


Rock Mass Classification Rock Mass Classification -- RMRRMRExample:

A mudstone outcrop contains three fracture sets. Set ‘1’ comprises b ddi l th hi hl 25

30

35

R4 Slightly

Rough/Unweathered

RMR = 6+12+10+(15 to 20)+R5

bedding planes; these are highly weathered, slightly rough and continuous. The other two sets are jointing; both are slightly weathered

d li htl h Th t th f 10

15

20

25

RM

R R

atin

g R Slightly

Rough Weathered

Slickensided Surface or Gouge-Filled

Sets 2 & 3Set 1

and slightly rough. The strength of the intact rock is estimated to be 55 MPa with an RQD of 60% and a mean fracture spacing of 0.4 m. The f t b d t b d

0

5

0 1 2 3 4 5 6Joint Separation or Gouge Thickness (mm)

R

Soft Gouge-Filled


RMR = 6+12+10+(15 to 20)+10

RMR* = 53 to 58



Rock Mass Classification: QRock Mass Classification: Q--SystemSystem

The Q-system of rock mass classification was developed in 1974 in Norway by Prof. N. Barton. The system was proposed on the b i f l i f 212 t l hi t i f S di i basis of an analysis of 212 tunnel case histories from Scandinavia.

… the motivation of presenting the Q-value in this form is to provide some method of interpretation for the 3 constituent quotients.


Rock Mass Classification: QRock Mass Classification: Q--SystemSystem

The first quotient is related to the rock mass geometry. Since RQD generally increases with decreasing number of discontinuity sets, the g y ,numerator and denominator of the quotient mutually reinforce one another.

The second quotient relates to “inter-block shear strength” with high values representing better ‘mechanical quality’ of the rock mass.

The third quotient is an ‘environment factor’ incorporating water pressures and flows, the presence of shear zones, squeezing and swelling rocks p f , q g gand the in situ stress state. The quotient increases with decreasing water pressure and favourable in situstress ratios.


Rock Mass Classification Rock Mass Classification –– ExamplesExamples

massive, strong rock

l t im low stress regime

note lack of ground supportpp

RMR = 90 (very good rock)

Q = 180 (extremely good rock)

Courtesy - Golder Associates




blocky rock

low stress regimelow stress regime

minimal but systematic ground support

RMR = 70 (good rock)

Q = 15 (good rock) Courtesy - Golder Associates



weak/foliated rock

low stress regime

note lack of ground supportsupport

RMR = 40 (poor to fair rock)

Q = 0.9 (v.poor to poor rock)



Courtesy Golder Associates


massive, strong rock

extremely high stress regime

rockburst failure complete rockburst failure, complete closure of drift, extremely heavy support, screen retains failed rock

RMR = 80 (good to v.good rock)

Q = 0.5 (very poor rock)




blocky rock

high stress regime

RMR = 40 ( t f i k)(poor to fair rock)

Q = 0.8 (very poor rock)( y p )



Application of Classification SystemsApplication of Classification Systems

Both of the classification systems described were developed for estimating developed for estimating the support necessary for tunnels excavated for civil engineering schemes.For engineering schemes.For example, the database for the RMR has involved over 351 case historiesthroughout its development.

Bieniawski (1989)Bieniawski (1989)


ExperienceExperience--Based Design: Empirical ApproachesBased Design: Empirical Approaches

… 38 different support categories have been suggested by Barton (1974) based on the relationship between the Q index and the equivalent dimension of the excavation


dimension of the excavation.

ExperienceExperience--Based Design: Empirical ApproachesBased Design: Empirical Approaches


Kaiser et al. (2000)

Subjectivity in Empirical Design Subjectivity in Empirical Design -- JRCJRC

Beer et al. (2002)


Subjectivity in Empirical Design Subjectivity in Empirical Design -- UndersamplingUndersamplingIt t b b d th h th t h id li d It must be remembered though, that such guidelines are drawn from previous experiences (i.e. case histories) and are therefore limited by the range of conditions under which these experiences were generated were generated.

Bieniawski (1989)


Bieniawski (1989)

Rock Mass Characterization Rock Mass Characterization vs.vs. ClassificationClassification


Rock Mass Properties Rock Mass Properties -- StrengthStrength

Remember!! – we’re now talking about rock mass talking about rock mass failure, not structurally

controlled failures.

004)

ie &

Mah

(20


Wyl

l

MohrMohr--Coulomb Failure CriterionCoulomb Failure CriterionThe Mohr Coulomb failure criterion expresses the relationship between The Mohr-Coulomb failure criterion expresses the relationship between the shear stress and the normal stress at failure along a shear surface.

son

(199

7)so

n &

Har

ris


Hud

s

Problems with MohrProblems with Mohr--CoulombCoulombAlth h th M h C l b f il it i i f th t Although the Mohr-Coulomb failure criterion remains one of the most commonly applied failure criterion, and is especially significant and valid for discontinuities and discontinuous rock masses, several key limitations apply to rock slope stability analyses. pp y p y y

linear

non-linear

Non-linear failure envelopes Scale effects


Non-linear failure envelopes. Scale effects.

HoekHoek--Brown Failure CriterionBrown Failure Criterion

Generalized Hoek-Brown failure criterion:

Intact rock strength:m = lab-determineds = 1

1

Rock massstrengthc19

95)

c

Hoe

k et

al.

(1

3

m & s are derived from empirical charts that are related to rock mass quality

m ~ Frictions ~ Cohesion

H


Rock Mass Properties Rock Mass Properties -- StrengthStrength

Mohr-Coulomb

Generalized Hoek-Brown



Intact rock strength:mi = lab-determined1s = 1

c

3


Hoek & Brown (1997)

Geological Strength Index (GSI)Geological Strength Index (GSI)

The GSI provides a system for estimating the reduction in rock mass strength for different geological conditions.

Values of GSI are related to both the degree of fracturing and the condition of the fracture surfaces.

mainly jointingmainly jointing

mainly faulting


GSI GSI ((for those familiar with rock mass classification)for those familiar with rock mass classification)Bieniawski (1989)

Not a rock mass characteristic!

For RMR 89* > 23: GSI = RMR 89* -5

For RMR 89* < 23: GSI = 9 Log Q’ + 44For RMR 89 < 23: GSI = 9 LogeQ + 44

WhereNote that the Q-system quotient terms “Jw/SRF” are dropped as these, likewise,

Hoek et al. (1995)


pp , ,are not rock mass characteristics!

HoekHoek--Brown Simplified ProcedureBrown Simplified ProcedureA simplified procedure to determine the Hoek-Brown rock mass strength parameters:

First, calculate mb:



for GSI >25:

Intact rock strength:mi = lab-determined1s = 1

for GSI <25:

c Rock mass strength:mb = rock mass adjusted 1 ( k i d)

3

s = <1 (rock mass varied)

“s” is a rock mass constant based on how fractured the rock mass is


f m(where s=1 for intact rock).

GSI Disturbance FactorGSI Disturbance Factor

A disturbance factor, “D”, may also be applied to h k the Hoek-Brown

parameters to account for the degree to which a degree to which a rock mass may have been subjected to blast damage and t l tiM

ah(2

004)

stress relaxation.

Wyl

lie &

M

di b


disturbance factor

GSI, GSI, HoekHoek--Brown & MohrBrown & Mohr--CoulombCoulombWhere Mohr Coulomb properties are required (or preferred because we have Where Mohr-Coulomb properties are required (or preferred because we have more experience and an intuitive feel for c and ), these can be derived by fitting a linear failure envelope across the non-linear H-B envelope:


GSI, GSI, HoekHoek--Brown & MohrBrown & Mohr--CoulombCoulombWhere Mohr Coulomb properties are required (or preferred because we have Where Mohr-Coulomb properties are required (or preferred because we have more experience and an intuitive feel for c and ), these can be derived by fitting a linear failure envelope across the non-linear H-B envelope:

Note change in sig3max for increased slope height and slope height, and corresponding change in fit of linear M-C envelope.

Check out the free copy of Evert Hoek’s notes and “H-B” software available on-line:

http://www.rocscience.com/


Rock Mass Characterization & DesignRock Mass Characterization & Design


Rock Mass Characterization & DesignRock Mass Characterization & Design5)

ket

al.

(199

5


Hoe

Lecture ReferencesLecture ReferencesBarton, N (1974). Engineering classification of rock masses for the design of tunnel support. RockMechanics 6(4): 189-236.

Beer, AJ, Stead, D. & Coggan, J.S. (2002). Estimation of the Joint Roughness Coefficient (JRC)by visual comparison. Rock Mechanics & Rock Engineering 35: 65–74.

Bieniawski, ZT (1974). Geomechanics classification of rock masses and its application in tunnelling. InProceedings of the Congress of the International Society for Rock Mechanics, Denver. NationalAcademy of Sciences: Washington, pp. 27-32.

Bieniawski ZT (1989) Engineering Rock Mass Classifications: A Complete Manual for Engineers andBieniawski, ZT (1989). Engineering Rock Mass Classifications: A Complete Manual for Engineers andGeologists in Mining, Civil, and Petroleum Engineering. Wiley: New York.

Harrison, JP & Hudson, JA (2000). Engineering Rock Mechanics – Part 2: Illustrative WorkedExamples. Elsevier Science: Oxford.

Hoek, E (1998). Reliability of Hoek-Brown estimates of rock mass properties and their impact ondesign. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 35(1):63-68.

Hoek, E & Brown, ET (1980). Underground Excavations in Rock. Institution of Mining and, , ( ) g f gMetallurgy: London.

Hoek, E, Kaiser, PK & Bawden, WF (1995). Support of Underground Excavations in Hard Rock.Balkema: Rotterdam.

H d JA & H i JP (1997) E i i R k M h i A I t d ti t th P i i l


Hudson, JA & Harrison, JP (1997). Engineering Rock Mechanics – An Introduction to the Principles.Elsevier Science: Oxford.

Lecture ReferencesLecture ReferencesKaiser, PK, Diederichs, MS, Martin, D, Sharpe, J & Steiner, W (2000). Underground works inhard rock tunnelling and mining. In GeoEng2000, Melbourne. Technomic Publishing Company:Lancaster, pp. 841-926.

Marinos, P & Hoek, E (2000). GSI – A geologically friendly tool for rock mass strength estimation.In GeoEng2000, Melbourne. Technomic Publishing Company: Lancaster, CD-ROM.

Wyllie, DC & Mah, CW (2004). Rock Slope Engineering (4th edition). Spon Press: London.


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