ISRM Rock Engineering Practice & Design Lecture 3 RMC and Emp Design E Eberhardt

Rock Engineering Practice & Design Lecture 3: Rock Mass Classification & Empirical Design

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Erik Eberhardt – UBC Geological Engineering

ISRM Edition

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 (Vancouver, Canada). C d ) The Th course covers rock k engineering i i and d geotechnical design methodologies, building on those already taken by the students covering Introductory Rock Mechanics and Advanced Rock Mechanics. Mechanics Although the slides have been modified in part to add context, they of course are missing the detailed narrative that accompanies any l lecture. It is also l recognized d that h these h lectures l summarize, reproduce and build on the work of others for which gratitude is extended. Where possible, efforts have been made to acknowledge th vvarious the ri us ssources, urc s with ith a list of f references r f r nc s being b in provided pr vid d att the th end of each lecture. Errors, omissions, comments, etc., can be forwarded to the author at: [email protected] 2 of 40


ISRM Edition

Classification Systems in Design Even with many resources available for site investigation, there still can remain problems in applying theories in practical engineering p g g circumstances. Considering g the three main design approaches for engineering rock mechanics – analytical, observational and empirical, rock mass classifications today form an integral part of the most predominant d i d design i approach, h the h empirical i i l design d i method. h d Indeed, on many underground construction, tunnelling and mining projects projects, rock mass classifications have provided the only systematic design aid in an otherwise haphazard “trial-and-error” procedure.

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Failure Mechanisms The Stability of an underground opening is a function of: structurally-controlled f l failure

rock mass f failure

•Stress

low or high

•Structure

•Rock Mass

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Hoek (1998)

• falling f lli • sliding

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Rock Mass Classification The objectives of rock mass classifications are to: 

Identify the most important parameters influencing the rock mass.



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



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.



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 of f the th rock k mass into i t a number b of f small ll structural t t l regions. i 5 of 40


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Rock Mass Classification These objectives suggest the three main benefits of rock mass classifications:  Improving the quality of site investigations by calling for the p data as classification parameters. p minimum input  Providing quantitative information for design purposes.  E Enabling bli b better engineering i i jjudgment d and d more effective ff i communication on a project.

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Rock Mass Classification: RMR The Rock Mass Rating (RMR) system was developed in 1973 in South Africa by Prof. Z.T. Bieniawski. k The h advantage d of his system was that only a few basic parameters relating to the geometry and mechanical conditions of the rock mass were required.

Rating g adjustments j m are included to account for the adverse nature discontinuity angles may have with respect to the excavation or slope p direction. 7 of 40

Bieniawski (1989) Erik Eberhardt – UBC Geological Engineering

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

F example: For l

RMR = 6+R2+R3+R4+R5 16

RMR Rating R1 R

A mudstone outcrop contains three fracture sets. Set ‘1’ comprises bedding planes; these are highly weathered, slightly rough and continuous. The other two sets are jointing; both are slightly weathered and slightly rough 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. damp

14 12 10 8 6 4 2 0 0

50

100

150

200

250

300

Unconfined Compressive Strength, qu (MPa)

Harrison & Hudson (2000) 8 of 40


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Rock Mass Classification - RMR RMR = 6+12+R3+R4+R5 25

RMR = 6+12+10+R4+R5

20 15 10 5 0 0

10

20

30

40

50

60

70

80

90

100

Rock Quality Designation, RQD 25

RMR Rating R3

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

RMR Ratting R2

Example:

20 15 10 5 0 0.01

0.1 1 Joint Spacing (meters)

10

Harrison & Hudson (2000) 9 of 40


ISRM Edition

Rock Mass Classification - RMR RMR = 6+12+10+(15 to 20)+R5

A mudstone outcrop contains three fracture sets. Set ‘1’ comprises b ddi planes; bedding l th these are hi highly hl weathered, slightly rough and continuous. The other two sets are jointing; both are slightly weathered and d slightly li htl rough. h The Th strength t th of f 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 fractures t are observed b d to t b be d damp.

35 Rough/Unweathered

30

RMR Rating R4 R

Example:

Slightly Rough

25

Weathered

20

Set 1

15

Sets 2 & 3 Slickensided Surface or Gouge-Filled

10 5

Soft Gouge-Filled

0 0

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

6

RMR* = 53 to 58 RMR = 6+12+10+(15 to 20)+10 Harrison & Hudson (2000) 10 of 40


ISRM Edition

Rock Mass Classification: QQ-System 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 of basis f an analysis l i of f 212 tunnel t l case histories hi t i from f S Scandinavia. di i

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

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Rock Mass Classification: QQ-System The first quotient is related to the rock mass geometry. Since RQD generally increases with decreasing g number of discontinuity y sets,, the 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 p f shear zones,, squeezing q g and swelling g rocks and the in situ stress state. The quotient increases with decreasing water pressure and favourable in situ stress ratios.

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Rock Mass Classification – Examples

 massive, strong rock  low l stress t regime im  note lack of ground support pp  RMR = 90 (very good rock)  Q = 180 (extremely good rock) Courtesy - Golder Associates

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 blocky rock  low stress regime  minimal but systematic ground support  RMR = 70 (good rock)  Q = 15 (good rock)

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Courtesy - Golder Associates


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 weak/foliated rock  low stress regime  note lack of ground support  RMR = 40 (poor to fair rock)  Q = 0.9 (v.poor to poor rock)

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 massive, strong rock  extremely high stress regime  rockburst failure, 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) Courtesy - Golder Associates

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Rock Mass Classification – Examples  blocky rock  high stress regime  RMR = 40 ( (poor tto fair f i rock) k)  Q = 0.8 ((very y p poor rock))


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Application of Classification Systems Both of the classification systems described were developed for estimating the support necessary for tunnels excavated for civil engineering schemes.For example, the database for the RMR has involved over 351 case histories throughout its development. Bieniawski (1989)

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ExperienceExperience -Based 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 excavation. 19 of 40


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ExperienceExperience -Based Design: Empirical Approaches

Kaiser et al. (2000) 20 of 40


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Subjectivity in Empirical Design - JRC

Beer et al. (2002)

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

Bieniawski (1989)

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Rock Mass Characterization vs. Classification

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Rock Mass Properties - Strength

Wyllie & Mah (20 004)

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

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MohrMohr -Coulomb Failure Criterion

Hudsson & Harrisson (1997)

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

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

non-linear

Non-linear failure envelopes. envelopes 26 of 40

Scale effects. effects


ISRM Edition

HoekHoek -Brown Failure Criterion Generalized Hoek-Brown failure criterion:

Hoek et al. (11995) H

1

Intact rock strength: m = lab-determined s = 1

Rock mass strength

c

3

m & s are derived from empirical charts that

are related to rock mass quality

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m ~ Friction s ~ Cohesion


ISRM Edition

Rock Mass Properties - Strength Mohr-Coulomb

Generalized Hoek-Brown

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HoekHoek -Brown Failure Criterion

1

Intact rock strength: mi = lab-determined s = 1

c 3 Hoek & Brown (1997) 29 of 40


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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 jointing

mainly faulting

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

Not a rock mass characteristic!

Hoek et al. (1995)

Where

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For RMR 89* > 23:

GSI = RMR 89* -5

For RMR 89* < 23:

GSI = 9 LogeQ Q’ + 44

Note that the Q-system quotient terms “Jw/SRF” are dropped pp as these,, likewise,, are not rock mass characteristics! Erik Eberhardt – UBC Geological Engineering

ISRM Edition

HoekHoek -Brown Simplified Procedure A simplified procedure to determine the Hoek-Brown rock mass strength parameters:

First, calculate mb:

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HoekHoek -Brown Failure Criterion for GSI >25:

1

Intact rock strength: mi = lab-determined s = 1

for GSI <25:

c

Rock mass strength:

mb = rock mass adjusted s = <1 1 (rock ( k mass varied) i d)

3

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“s” is a rock mass constant based fractured the rock m mass is on how f (where s=1 for intact rock).


ISRM Edition

GSI Disturbance Factor

Wyllie & M Mah (2004)

A disturbance factor, “D”, may also be applied to the h Hoek-Brown k parameters to account for the degree to which a rock mass may have been subjected to blast damage and stress t relaxation. l ti

disturbance di b factor 34 of 40


ISRM Edition

GSI, HoekHoek-Brown & MohrMohr-Coulomb Where Mohr-Coulomb 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:

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GSI, HoekHoek-Brown & MohrMohr-Coulomb Where Mohr-Coulomb 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, 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/

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Rock Mass Characterization & Design

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Hoek et al. (1995 5)

Rock Mass Characterization & Design

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Lecture References Barton, N (1974). Engineering classification of rock masses for the design of tunnel support. Rock Mechanics 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. In Proceedings of the Congress of the International Society for Rock Mechanics, Denver. National Academy of Sciences: Washington, pp. 27-32. Bieniawski, ZT (1989). Bieniawski (1989) Engineering Rock Mass Classifications: A Complete Manual for Engineers and Geologists in Mining, Civil, and Petroleum Engineering. Wiley: New York. Harrison, JP & Hudson, JA (2000). Engineering Rock Mechanics – Part 2: Illustrative Worked Examples. Elsevier Science: Oxford. Hoek, E (1998). Reliability of Hoek-Brown estimates of rock mass properties and their impact on design. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 35(1): 63-68. ) Underground g Excavations in Rock. Institution of f Mining g and Hoek,, E & Brown,, ET ((1980). Metallurgy: London. Hoek, E, Kaiser, PK & Bawden, WF (1995). Support of Underground Excavations in Hard Rock. Balkema: Rotterdam. Hudson, H d JA & Harrison, H i JP (1997). (1997) Engineering E i i Rock R k Mechanics M h i – An A Introduction I t d ti to t the th Principles. P i i l Elsevier Science: Oxford. 39 of 40


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Lecture References Kaiser, PK, Diederichs, MS, Martin, D, Sharpe, J & Steiner, W (2000). Underground works in hard 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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ISRM Rock Engineering Practice & Design Lecture 3 RMC and Emp Design E Eberhardt

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