rock mass characterization - srmeg

1

ROCK MASS CHARACTERIZATION

Wulf Schubert

2Short Course Singapore 11 Klima, Schubert

Rock mass properties

INTRODUCTION

■ Characterization is the process to attach physical paramters to the geological model

■ Parameters are required for modelling of ground- and system behaviour

■ In rock usually discontinuities control the behaviour, thus simplification by „smearing“ discontinuities into a continuum can lead to completely wrong results

■ Sometimes a good sketch and simple kinematical considerations are better than a sophisticated (but wrong) model

2



EXAMPLE OF EFFECT OF SIMPLIFICATION

Direct modelling of joints „ubiquitous“ joints Joints smeared into continuum



HOW TO ARRIVE AT GROUND PROPERTIES

■ When attempting to characterize ground, followingquestions should be considered already in the beginning:

□ What will I do with the parameters? □ Is it for a feasibility study or a detail design?□ Which modes of failure can be anticipated?□ Which models will I use for analysis?□ Which parameters will I need for construction?□ How much simplification is admissible in order not to loose

essential information

3

ROCK AND ROCK MASS PROPERTIESFAILURE MECHANISMS



CHARACTERIZATION

■ For an appropriate design, considering the groundcharacteristics, as well as project specific requirements and boundary conditions, a sound mechanical model is required

■ To be able to create a model, one has to determinephysical properties of the ground, assess influencingfactors, and determine potential failure modes

■ This requires some knowledge about basics of rock mechanics

4



FAILURE MODES INTACT ROCK

■ Tensile failure

□ Pure tensile failure very rare, as pure tension seldom exists□ Tensile strength≈UCS/(10-20)

fracturesource

Plumouse failure structures



BRITTLE FAILURE

■ Brittle failure

□ Common failure for brittle rocks with low confining pressure□ In brittle rocks under low

confining stress microcracksdevelop in direction of themaximum applied load

□ Microcracks develop dueto tensile stress perpendicularto load direction

□ Tensile stresses caused byinternal heterogeneity

□ With increasing load the cracks grow□ Eventually small columns

buckle

5



INTERNAL HETEROGENEITY

■ Rock is composed of variousconstituents with different properties, leading to non uniform stress field and localtension



BRITTLE FAILURE

■ If rock is foliated and loaded +/- parallel to foliation, crackwill develop on foliation planes

6



SIMULATION OF BRITTLE FAILURE

RFPA



SHEAR FAILURE

■ Shear failure

□ Failure mode for brittle rocksunder higher confining stress

□ Confining stress requiredfor brittle rocks

□ Microcracks parallel tomain stress direction are formed

□ Inclined connection betweenmicrocracks with continuing strain

□ Eventually failurealong shear plane

7



Failure localization with acoustic emmission sensors

Figure: GFZ Potsdam

Distributed crackingin initial phase

Concentration of cracksprior to shear failure



INFLUENCE OF FABRIC

8



SHEAR FAILURE, STRESS-STRAIN CHARACTERISTIC

Developmentof microcracks

Connection of microcracksDevelopmentof shear plane (zone)

Sliding alongshear plane



SIMULATION OF SHEAR FAILURE

RFPA

9



DEVELOPMENT OF SHEAR ZONE BY LATERAL DISPLACEMENT



RIEDEL SHEARS

10



QUASI DUCTILE FAILURE

■ Materials with low brittleness or brittle materials under high confining stress show „ductile“ failure

■ No or low strength drop after peak



FAILURE ENVELOPE

11



COMMON FAILURE CRITERIA

smccc

331

sin1

cos2

sin1

sin131

c

tannc Mohr-Coulomb

Hoek-Brown



COMMENTS ON FAILURE CRITERIA

■ Due to changing failure mechanisms depending on thestress conditions, a linear failure criterion is very unlikelyfor rocks

■ A huge number of empirical failure criteria can be found in literature for different rock types

■ Post failure behaviour usually not considered

12



Peak and residual strength

Cai et al 2007



INFLUENCES ON FAILURE & DEFORMATION CHARACTERISTICS

■ Rock structure (foliation, mineralogical composition, etc.)

■ Shape and size of samples

■ Water contents

■ Temperature

13



INFLUENCE OF LOADING ORIENTATION



INFLUENCE OF LOADING ORIENTATION

Shearing along foliation

14



DISCONTINUITIES

■ In general discontinuities have no or very low tensilestrength and no cohesion

■ Shear strength depends on:

□ Rock type□ Roughness□ Rock strength□ Filling□ Loading conditions



15



BASIC FRICTION ANGLE

■ Basic friction angle fb of joints is determined on a planarsurface

■ Shear strength of planar discontinuity:

max = n*tan fb

n ….. normal stress



ROUGHNESS

■ Roughness of joints increases initial shear strength

□ Under low normal stress the sample displaces also perpendicular to the shearing direction (dilation)

in tanmax

n

res

i

16



ROUGHNESS

■ Under high normal stress asperities are sheared off

n

res

Schubbruch im Gestein

t+i

cG

G

res

Aufgleiten Abscheren

ns n

Sliding on asperitiesshearing of asperities

shearing of intact rock



Influence of normal stiffness

■ If a block cannot dilateunrestricted, the normal stress increases duringshearing, increasing also thepeak shear strength

N= const

kn

17



BARTON´S SHEAR STRENGTH CRITERION

■ Consideration of joint roughness, rock strength and block size

r

n

n

JCSJRC

logtanmax

JRC Joint Roughness CoefficientJCS Joint Compressive Strength

002,0

0

0

JRC

n

LL

JRCJRC

003,0

0

0

JRC

n

L

LJCSJCS



NORMALIZED JOINT ROUGHNESS–SHEAR DISPLACEMENT RELATIONSHIP

Barton, Bandis, Bakhtar

18



JRC PROFILES BY BARTON & CHOUBEY

Barton & Choubey



DIFFICULTY TO ESTIMATE JRC

■ As the JRC is determined visually, there is the potential of a strong bias. Different people arrive at different values

Result of 21 persons estimating JRC on the same sample (Schieg 2006)

19



GRASSELLIS SHEAR STRENGTH CRITERION

A0 area of the joint surface which is orientatedtoward the shear direction

Ac potential contact area

C „roughness“ parameter - describes theconcavity of the fit function

Θ*max maximum apparent dip angle

φr residual friction angle

σc Uniaxial compressive strength

Decreasing cumulative distribution of thepotential contact area Ac related to surface areaversus the apparent dip angle

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DA Schieg



SURFACE CHARCTERIZATION BY STEREOPHOTOGRAPIC METHODS

Surface elements facing the shear direction arehighlighted depending on the inclination

DA Schieg

20



COMPARISON OF SHEAR STRENGTHS

Schieg, 2006

METHODS FOR THE DETERMINATION OF ROCK MASS PARAMETERS

21



PURPOSE & METHODS

■ For the analysis of engineering projects (stresses, deformation, etc.) we need material parameters

■ With the current state of the analysis tools and material laws, a direct modelling of all features of rocks and rock masses is not possible

■ Thus a „homogenization“ or „upscaling“ is required

■ Direct modelling, starting from the microscale,stepwise develop material models and upscale

■ Simplified analytical models

■ Empirical or semi-empirical methods



MULTILAYER MODEL

a5

b1

a2

a3

a4

a1

b2

b2b3

b4

l1

l2

BA

ial

1

1

ibl

1

1

Material A: EA, AMaterial B: EB, B

Volumetric proportions:

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DEFORMATION MODULUS FOR MULTILAYER MODEL

BA

BA

EEEl

bE

aE

l

l

BA

iB

iA

11

1

BA EE

E

1

Condition: no shear bond between layers A and B

AB



DEFORMATION MODULUS FOR MULTILAYER MODEL

Shear bond between layers A and B

Simplified:

BsA EE

EE

,

2

1

BB

BBBs

EE

121

1,

.EA .EB and transv,B transv,A

Es,B ... Stiffness modulus material B

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MULTILAYER, deformation parallel to layers

B A

BAII EEE

0

500

1000

1500

2000

2500

0 20 40 60 80 100

proportion of material A (%)

E-m

od

ulu

s (M

Pa)

E parallel

E normal

EA=2.000 MPaEB=200 MPa

Example for influence of loading direction on stiffness



JOINTED ROCK MASS

■ Representing joint properties with a normal stiffness:

knsEi

EiEm

*1

Amadei & Goodman, 1981

Ei ….E-modulus intact rock (MPa)Em …..E-modulus rock mass (MPa)s………spacing (m)Kn ….. joint normal stiffness (MPa/m)

Amadei & Goodman

0

100

200

300

400

500

600

700

800

900

0 0,5 1 1,5 2 2,5

s

Em

Example for Ei =1000 MPa, kn=3000 MPa/m

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CONSIDERATION OF JOINT CLOSING

0

50

100

150

200

250

300

0 0,1 0,2 0,3 0,4 0,5

Normal displacement

No

rma

l str

es

s

Measured

System

Joint

System + Joint

Pötsch, 2007



CLASSIFICATION

■ Classification is the procedure to group rock massesaccording to some attributes or quality. Selection of parameters and weighting is purely empirical

■ Example:

□ fracture frequency: 3-4 per m rating: 16□ UCS: 50-70 MPa rating: 7□ joint spacing: 0,5 m rating: 12□ joint condition: slightly rough rating: 20□ ground water: none rating: 15

total rating: 70

Class: good rock

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CHARACTERIZATION

■ Characterization of rock masses involves the description and quantifiaction of properties. Information can be directly usedfor modelling

■ Example

□ Rock type: limestone, not karstified□ Bedding thickness: 20-40 cm□ UCS: 50 – 70 MPa□ Deformation modulus: 15-20 GPa□ Number of joint sets: 2□ Joint spacing: set 1: 40-60 cm; set 2: 50-70cm□ Relative orientation between joint sets: perpendicular□ Roughness: rough□ Basic friction angle: 30° -32°□ etc.



CLASSIFICATION

■ Basic idea of classification systems was, to allow assessingrock mass quality and „design“ excavation and support also by people with poor engineering background, usingstandardized parameters and „look-up tables“ for theweighting of the parameters

■ Rating systems used for classification are based on specificexperience, thus the use in other conditions may lead to a misjudgment

■ Parameters used are always the same, some may beirrelevant for certain ground conditions

■ Different combinations of parameters can produce thesame rating

■ By reduction of a number of properties to a single numberinformation is lost

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RMR – ROCK MASS RATING

rock strength (0 - 15)

RQD (0 - 20)

joint spacing (0 - 20)

joint condidtion (0 - 30)

ground water (0 - 15)

joint orientation (0 – (-12))

RMR

Rock Mass Rating

(0 - 100)



DISCUSSION ON RMR

■ RQD is a measure for the fracturing of a rock mass. In addition joint spacing is considered, which basically shouldshow in the RQD.

■ RQD is a measure for the core recovery and should indicatethe degree of fracturing. Measured is the length of the corepieces longer than 100 mm.

■ Orientation of boreholes in relation to discontinuityorientation can lead to a strong sampling bias

■ Joint orientation and ground water may vary locally. RMR forthe same rock mass can be different in different locations

27





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Q – ROCK MASS QUALITY

RQD (0 - 100)

Number of joint sets (0,5 - 20)

Jr joint roughness (0,5 - 4)

Ja - joint condition (0,75 - 20)

Jw – ground water (0,05 - 1)

SRF – stress red. factor (0,5 - 400)

(0,001 - 1000)

SRF

Jwx

Ja

Jrx

Jn

RQDQ



PARAMTERS FOR Q

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PARAMTERS FOR Q



PARAMTERS FOR Q

30



PARAMTERS FOR Q



PARAMTERS FOR Q

31



PARAMTERS FOR Q

Update 1994



DISCUSSION ON SRF

■ SRF contains ratings for weakness zones intersecting thetunnel, for rock stress problems like low stress levels close to the surface or rock burst, as well as squeezing and swelling

■ This means, one should know the behaviour to be able to assess rock mass quality.

□ In all engineering problems, it is understood that behaviour canonly be assessed after the material characteristics and theinfluencing factors have been determined

32



EXAMPLE FOR INCONSISTENCY OF SRF

Palmstroem & Broch, 2007



(MIS)USE OF RATING SYSTEMS

■ Rock mass parameters are derived from rating

□ First various information is collected, then condensed to a singlenumber. From that single number again a number of independent parameters is derived (UCSm, Em,…)

■ Recommendations on support are given withoutconsideration of project specific requirements and groundbehaviour

■ Some even attempt to correlate deformations, advancerates, permeability, etc. with ratings

33

RMR – Rock Mass Rating

Q – Support chart

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SUMMARY RATING SYSTEMS

■ Such systems may be applied in very early stages of a project when information is very limited, to get an overall„feeling“ of support requirements

■ They cannot reasonably be used to design excavation and support, as different failure mechanisms, time dependentbehaviour, deformations, and project specific boundaryconditions cannot be considered

■ Unfortunately rating systems are widely (mis)used due to theapparent simplicity of application



Correllation ?

35



Correllation ?

Sapigni et al, 2002. Int. J.RM&MSCi



CONCLUSION CLASSIFICATION

■ Schematic classification systems on the first glance appearattractive, as they are easy to use – no specific knowledgerequired

■ But: rock masses are very complex, and can exhibit a number of different behaviours

■ The behaviour and project specific boundary conditions and requirements need to be considered to arrive at a reasonabledesign

■ Applying prefabricated systems, which were developed forspecific conditions in most cases will lead to a non-optimaldesign

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SOME EMPIRICAL MODELS



E-MODULUS BASED ON RMR

100*2 RMREM

10 40

10

RMR

ME

BieniawskiBieniawski

SerafimSerafim & Pereira& Pereira

Em (GPa) Bieniawski ; Serafim&Pereira

0,00

20,00

40,00

60,00

80,00

100,00

120,00

140,00

160,00

180,00

200,00

0 20 40 60 80 100 120

RMR

Em

(G

Pa)

Bieniawski

Serafim & Peirera

37



E-MODULUS BASED ON EI AND RMR

100cos15,0

RMREiEm

Mitri et al 1994 Em/Ei nach Mitri

0,00

0,20

0,40

0,60

0,80

1,00

1,20

0 20 40 60 80 100 120

RMR

Em

/Ei



RMi – Rock Mass Index

■ Palmstrøm combines the UCS of the intact rock with a jointing parameter to arrive at the index RMi, which represents the rock mass strength ( ) JPRMi ccm *

38



DETERMINATION OF JP

jAjRjLjC /*



GEOLOGICAL STRENGTH INDEX (GSI)

■ The Geological Strength Index has similarities with the classification systems discussed above, however it is not a rock mass classification system

■ It is a simple index value used to quantify the influence of the discontinuities on the ground strength (Hoek-Brown failure criterion)

■ It combines only two parameters, namely the block volume (representing the overall degree of fracturing of the rock mass) and joint state (representing the frictional characteristics of the discontinuities) into one value

39



GSI

E-MODULUS BASED ON GSI AND Q

QEM log25

cM QE 3

1

10

Hoek et al.Hoek et al.20022002

ffüür Q > 1r Q > 1

100

cc QQ

BartonBarton

ffüür Q < 1r Q < 1

40

10

101002

1

GSIcDEm

40

10

102

1

GSIDEm

sc<100MPa sc>100MPa

Hoek & DiederichsHoek & Diederichs20062006

)1

2/102.0(

)11/)1560(( GSIDim e

DEE

40



EVALUATION OF GSI

■ Besides estimating the GSI from the chart, Chai et al proposed an evaluation as follows:

with Vb block volume (cm3)Jc joint condition (-)Ja joint alterationJs small scale roughnessJw waviness

bC

bCCb VJ

VJJVGSI

ln0253,0ln0151,01

ln9,0ln79,85,26,

A

SWC J

JJJ



EVALUATION OF GSI

Qualitative description Waviness rating Jw

Stepped 2,75 Undulating 1,75

Planar 1,00

Qualitative description Waviness rating Js

Rough 2,50 Smooth 1,50

Slickenslided 0,75

Joint contact Filling / Alteration Description Ja

Healed joints Impermeable filling (quartz, epidote, etc) 0,75

Fresh rock walls No coating of filling of the joint surface, except for (possible) staining

1,00

Slightly weathered joint The joint surface exhibits one class higher weathering then the rock

2,00

Highly weathered joint The joint surface exhibits two classes higher weathering then the rock

4,00

Sand, silt, calcite filling etc. Coating of friction surfaces without clay 3,00Roc

k w

all c

onta

ct

(„cl

osed

fea

ture

s“

acco

rdin

g to

IS

RM

)

Clay, chlorite, talg etc. Coating of friction surfaces with cohesive, “lubricating” minerals

4,00

Sand, silt, calcite filling etc. Filling with frictional material without clay

4,00

Compacted clay materials “Hard” filling with softening and cohesive materials

6,00

Soft clay materials Low over-consollidation of the filling material, loose filling with clay

8,00

Fil

led

join

ts w

ith

part

ial o

r no

con

tact

be

twee

n ro

ck w

all

surf

aces

(“g

appe

d an

d op

en f

eatu

res”

ac

cord

ing

to I

SR

M)

Swelling clay minerals Filling material exhibits swelling properties

10,00

41

Rock mass strength - empirical

a

cbc sm

3

31

28

100exp

GSI

m

m

i

b

9100

expGSI

s

5,0a

Für GSI > 25

Für GSI < 25 0s

20065,0

GSIa

Hoek et al.



MOHR COULOMB PARAMETERS FROM GSI

■ Radoncic used closed for solutions for a parametric study, and proposes reduction factors for c and :

Reduction factor for cohesion fc Reduction factor for friction f

Radoncic 2008

42



ANISOTROPIC ROCK MASSES

■ Foliated rocks have a strong anistropic behaviour. Homogenization thus should be done with care. Strengthand deformation properties have to be assigned fordifferent directions.

■ In particular shear strength may be very low parallel to thefoliation.

■ Strength usually is higher perpendicular to the foliation

■ When simplification is done, care has to be taken not to loose characteristic behaviour

■ When tunnel axis is +/- parallel to foliation, influence of foliation shall be always considered



FAULTED ROCK MASSES

■ Fault zones very rarely are homogeneous, but can have a wide variety of compositions

■ We should distinguish between:

□ Fine grained cohesive□ Sandy, non cohesive□ Corse, non cohesive□ Block in matrix

■ Faults in general anisotropic, with low shear strength in direction of main movement, in particular when fault gougedeveloped during shearing

43



CHARACTERISTICS OF FAULT ZONES

■ Fault and shear zones may have blocksmillimeters to 100s of meters large

■ Block size distributions in general scaleindependent

■ Block ratio important for strength and deformability

5 cm, 5 m, 5 km

Medley, 2006



TYPICAL BRITTLE FAULT ZONE

44



Blocks in the North Anatolian Fault Zone



DIFFERENT APPEARANCES OF A FAULT ZONE

45



BLOCK IN MATRIX



46



CHARACTERIZATION OF BIM ROCKS

■ Important is the Block-Matrix ratio, as it has an influenceon the strength

-5

0

5

10

15

20

25

30

0 20 40 60 80 100

Volumetric Block Proportion (%)

Incr.. F

riction

An

gle, d

egrees

Scott Dam melange

Physical modelsIrfan and Tang, 1993

conservative trend (Lindquist 1994a)

Scott Dam melange

© Dr. E. Medley http://bimrocks.geoengineer.org



CHARACTERIZATION OF BIM ROCKS

■ A significant increase of stiffness can be expected with a block content above 25%. Example of possible influence:

E‐Modulus vs Block/Matrix ratio

0

500

1000

1500

2000

2500

0 10 20 30 40 50 60 70 80 90 100

Percentage blocks

E‐Modulus

E block 2.000 Mpa

E matrix 200 MPa

47



STRENGTH OF TECTONIZED ROCKS

■ Habimana (2002) proposes a modified Hoek Brown criterium in relation to the degree of tectonization

asm *)*( 331

Sandstones

Phyllitic shists



STRESS DEPENDENT DEFORMABILITY

n

papakEi

3*

Habimana, 2002Example of stress dependent E Modulus according to Habimana

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30

Sigma3 (MPa)

E-M

od

ulu

s (M

Pa)

rock mass characterization - srmeg

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