MR21

Tunnelling in Brittle Rock Tunnelling in Brittle Rock Mass ConditionsMass ConditionsSpalling InstabilitiesSpalling Instabilities

Department of Structural and Geotechnical Engineering

Giovanni BarlaGiovanni Barla

Lecture OutlineLecture Outline Introduction Laboratory Results Field Evidence Modelling Brittle Failure Case Study Conclusions

IntroductionIntroduction

DefinitionsDefinitionsDefinitions

BRITTLE FAILURE takes place around underground openings in massive to moderately jointed rock masses subjected to high in situ stresses. It manifests itself in the form of spalling resulting in a revised stable geometry with the onset of V-shaped notches. The extent and depth of failure is a function of the in situ stress magnitudes relative to the rock mass strength

BRITTLE FAILURE takes place around underground openings in massive to moderately jointed rock masses subjected to high in situ stresses. It manifests itself in the form of spalling resulting in a revised stable geometry with the onset of V-shaped notches. The extent and depth of failure is a function of the in situ stress magnitudes relative to the rock mass strength

BRITTLE FAILUREBRITTLE FAILUREBRITTLE FAILURE

The onset of wall yield due to boundary compression around an underground opening is one of the primary design issues in hard rock tunnelling at depth by tunnel boring machines or conventional excavation methods. In this lecture we will discuss spalling instabilities, although in cases the interest need be centred on dynamically induced tunnel failures such as rockbursts

The onset of wall yield due to boundary compression around an underground opening is one of the primary design issues in hard rock tunnelling at depth by tunnel boring machines or conventional excavation methods. In this lecture we will discuss spalling instabilities, although in cases the interest need be centred on dynamically induced tunnel failures such as rockbursts



Mine by Experiment at the URL, Canada (1990Mine by Experiment at the URL, Canada (1990--1995)1995)

Roof Tensile FailureRoofRoof TensileTensile FailureFailure

Sidewall SpallingSidewallSidewall SpallingSpalling

Laboratory ResultsLaboratory Results

It is worth to notice that a significant contribution to this

understanding has been derivedfrom recent discrete elementsimulations of Lac du Bonnet

granite (Diederichs et al.2004)

It is worth to notice that a significant contribution to this

understanding has been derivedfrom recent discrete elementsimulations of Lac du Bonnet

granite (Diederichs et al.2004)

The starting point in the understanding of this type of

behaviour is to analyselaboratory test results on

cylindrical samples

The starting point in the understanding of this type of

behaviour is to analyselaboratory test results on

cylindrical samples

Laboratory TestingLaboratory Testing

Spring Model

Spring Model

LinearLinearLinear

Non linearNon Non

linearlinear

Rock SpecimenRock Specimen

Testing Equipment

Testing Equipment

111

111333

cccccc

DDD peakpeakpeak

AAA

BBB cicici

cdcdcdCCC

IIIIIIIII

IIIIII

III

IVIV

Linear elasticdeformation

Linear elasticLinear elasticdeformationdeformation

Stable crackpropagationStable crackStable crackpropagationpropagation

Crack closureCrack closureCrack closure

PeakPeakPeak

Unstable crackpropagation

Unstable crackUnstable crackpropagationpropagation

Stages of stressStages of stress--strain and acoustic responsestrain and acoustic responsein in uniaxialuniaxial testing testing ((BieniawskiBieniawski 1967, 1967, EberhardtEberhardt et al. 1998)et al. 1998)

Crack closure, ccCrack closure, cccc

Major thresholds within a stress-strain test on rock

samples coupled with acousticemissions monitoring

Major thresholds within a stress-strain test on rock

samples coupled with acousticemissions monitoring

Crack initiation, ciCrack initiation, ciciCrack damage, cdCrack damage, cdcdPeak strength, peakPeak strength, peakpeak

Diederichs et al. 2004

Dilatingcrack

Induced hoop compression

Feedback confinement

No feedback resistance

Excavation BoundaryExcavation Boundary Small hole in plateSmall hole in plate

Laboratory TestingLaboratory Testing

Biaxial tests run for Politecnico di Torinoat Dresden University

Biaxial tests run for Politecnico di Torinoat Dresden University

Field EvidenceField Evidence

Brittle FailureField Observations


Stage I Stage I -- Damage InitiationDamage Initiation: Critically oriented flaws : Critically oriented flaws are exploited in the zone of maximum tangential are exploited in the zone of maximum tangential stress. The process initiates at the boundary of the stress. The process initiates at the boundary of the tunneltunnel

Localised damage

Stage II - Dilation: Shearing and crushing occurs in a Shearing and crushing occurs in a very narrow process zone. Extensive dilation at the very narrow process zone. Extensive dilation at the grain size scale occurs in this process zonegrain size scale occurs in this process zone

Process zone

Process zone

BucklingProcess zone stabilised by confining stress at notch tip

Stage III Stage III -- SpallingSpalling: : Development of the process zone leads to the formation of thin slabs. These thin slabs form by: shearing, splitting and buckling. The thickness of the slabs varies. The thickest slabs form as the notch reaches its maximum size. Near the notch tip the slabs are curved

Stage IV - Stabilization: Development of the notch stops when the notch geometry provides sufficient confinement to stabilize the process zone at the notch tip. This usually means there is a slight tear-drop like curvature to the notch shape. Alternatively, if the slabs from the notch flanks are held in place by artificial support, notch development will also stop

Martin et al. 1997

The notch development is a 3D process which The notch development is a 3D process which is directly linked to the tunnel advanceis directly linked to the tunnel advance

Tunnel Longitudinal SectionTunnel Longitudinal Section

IIIIIIV

V

Stages

Tunnel AdvanceTunnel AdvanceInitiationInitiation

Martin et al. 1997



++

++++++

++xxxx

0.20.2

0.40.4

0.60.6

0.80.8

1.01.0

0.20.2 0.40.4 0.60.6 0.80.8 1.01.0

2a2a

DDff Depth of Depth of overbreakoverbreakddff/a/a

maxmax//cc

ddffaa

= 1.25= 1.25maxmaxcc

-- 0.51 0.51 0.10.1

Kaiser et al. 1995

(1) Stress induced fracturing initiates at approximately 0.3-0.5 c and the criticaldeviatoric stress for yield is essentiallyindependent of confining stress

(1) Stress induced fracturing initiates at approximately 0.3-0.5 c and the criticaldeviatoric stress for yield is essentiallyindependent of confining stress

This collection of available tunnel overbreak data shows that

This collection of available tunnel overbreak data shows that

(2) No overbreak occurs at a maximum boundary stress equivalent to 40% of the compressive strength. This is a lower boundstrength as unfailed tunnels are not plotted

(2) No overbreak occurs at a maximum boundary stress equivalent to 40% of the compressive strength. This is a lower boundstrength as unfailed tunnels are not plotted

33//cc

11//cc

Axial Splitting

Tensile Failure

In situ Strength

DamageThreshold (m=0)

SpallingFailure

Slope of Spalling Limitdepends on Heterogeneity, Surface Effects, Damage, Stress Rotation

Distributed Damageand Acoustic Emission

Damage Initiation Threshold depends onMineralogy, Grain Size, Bond Type

Shear Failure

Diederichs et al. 2004

Brittle Failure ModellingBrittle Failure Modelling

A number of possible approaches

A number of possible approaches

(2) Micromechanical Models: account formicroscopic aspects of rock fracture and/or crack propagation mechanisms

(2) Micromechanical Models: account formicroscopic aspects of rock fracture and/or crack propagation mechanisms

(1) Phenomenological Models: use appropriate constitutive equations which should describe the brittle failure processesbased on back analysis of carefullydocumented case histories

(1) Phenomenological Models: use use appropriateappropriate constitutive equationsconstitutive equations which which should describe the brittle failure processesshould describe the brittle failure processesbasedbased on back on back analysisanalysis of of carefullycarefullydocumenteddocumented case case historieshistories

(1) Phenomenological Models: use appropriate constitutive equations which should describe the brittle failure processesbased on back analysis of carefullydocumented case histories

(1) Phenomenological Models: use use appropriateappropriate constitutive equationsconstitutive equations which which should describe the brittle failure processesshould describe the brittle failure processesbasedbased on back on back analysisanalysis of of carefullycarefullydocumenteddocumented case case historieshistories

We discuss (1) onlyWe discuss (1) only

One common approach to estimate the yieldpotential and the depth of disturbance for a tunnel is the Hoek and Brown Criterion forrock mass

One common approach to estimate the yieldpotential and the depth of disturbance for a tunnel is the Hoek and Brown Criterion forrock mass

+

+= smc

'3

bc'3

'1

+

+= smc

'3

bc'3

'1

Standard uniaxialcompressivestrength

Standard uniaxialcompressivestrength

cc

11

33

Costants whichdepend upon the characteristics of the rock mass

Costants whichdepend upon the characteristics of the rock mass

s mb

s mb

11

33

determined usingthe GSI indexdetermined usingthe GSI index

It is found that this approachis of limited reliability topredict brittle failure for rock masses of good quality, typically GSI > 70-75

It is found that this approachis of limited reliability topredict brittle failure for rock masses of good quality, typically GSI > 70-75

At low confinement levels, the accumulationof significant rock damage, equivalent to lossof cohesion (i.e. mb=0), ocurs when 1- 3 = 1/3 to 1/2 c (i.e. when s= 0.11 to 0.25)

At low confinement levels, the accumulationof significant rock damage, equivalent to lossof cohesion (i.e. mb=0), ocurs when 1- 3 = 1/3 to 1/2 c (i.e. when s= 0.11 to 0.25)

1/21/2

= ss''33

''11-- cc == 0.330.33--0.500.50 cc

1/ c11/ / cc

3/ c33/ / cc

1.01.01.0

0.50.50.5

0.30.30.30.40.40.4

1.01.01.00.50.50.5

1.51.51.5

Kaiser et al. 2000

mb=0 Modelmb=0 Model

The cohesive strength is graduallydestroyed by tensile cracking and crack coalescence. The frictional strength can bemobilized only when the cohesive strengthis significantly reduced

The cohesive strength is graduallydestroyed by tensile cracking and crack coalescence. The frictional strength can bemobilized only when the cohesive strengthis significantly reduced

Hajiabdolmajid et al. 2002

CWFS ModelCWFS Model

aaa

aa

II

IIII

IIIIII IVIV

ccii

ccrrOnset of

microcracking

Frictional Strength

Strength componentsStrength componentsare strainare strain-- dependentdependent

Case StudyCase Study

DIAMETER: 4.75 mDIAMETER: 4.75 mTOOLS: 35 cutting TOOLS: 35 cutting disksdisksMAXIMUM THRUST: 6950 MAXIMUM THRUST: 6950 kNkNTOTAL POWER: 895 kWTOTAL POWER: 895 kW

PontPont VentouxVentoux -- Susa Susa HydropowerHydropower SystemSystem

Free Flow TunnelFree Flow Tunnel

PontPont--VentouxVentouxF2 LengthF2 Length

PontPont VentouxVentoux -- SusaSusaHydropowerHydropower SystemSystem

F2 F2 -- ClareaClareaLengthLength

Free Flow TunnelFree Flow Tunnel

0100200300400500600700800900

100011001200130014001500

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Chainage [m]

Ove

rbu

rden

[m]

0

10

20

30

40

50

60

70

80

90

100

RM

R 6

Ground Surface Gravity Spalling RMR 6

Val Clarea F2 LENGTH

Instability of rock wedges occurs with Rock Mass Quality Index RMR < 55

Spalling instability is observed to take place when the overburden is greater than 500-600 m and RMR > 65

SECTION 1 (530SECTION 1 (530--545 m)545 m)

Fault11

33

SpallingSpallingSpalling


The The overbreakoverbreak zoneszones givegive the the directionsdirections of of 11and and 33 .. From FlatFlat Jack Jack measurementsmeasurements the Stress the Stress

RatioRatio isis in thein the rangerange 0 250 25 toto 0 500 50

SECTION 2 (550SECTION 2 (550--565 m)565 m)



The The overbreakoverbreak zoneszones givegive the the directionsdirections of of 11and and 33 .. From FlatFlat Jack Jack measurementsmeasurements the Stress the Stress

RatioRatio isis in thein the rangerange 0 250 25 toto 0 500 50

100

200

300

400

500

-10-10 00 55 1010 1515 2020 2525 3030 35353 - MPa3 3 -- MPaMPa

1

-M

Pa

1

1

--M

Pa

MP

a

PeakPeak

ResidualResidual

GneissGneissIntact Rock

(Ambin Formation)Intact Rock

(Ambin Formation)

ci=135MPaci=135MPa mi=8.1mi=8.1

25

-10-10 1010 3030 5050 7070 90903 - MPa3 3 -- MPaMPa

1

-M

Pa

1

1

--M

Pa

MP

a

00

5075

100125150175200225250

Mohr-CoulombCriterion

Mohr-CoulombCriterion

Hoek-BrownCriterion

Hoek-BrownCriterion

GneissGneissRock Mass

(Ambin Formation)Rock Mass

(Ambin Formation)

mb=2.8mb=2.8 s=0.04s=0.04

Ed=36.7GPaEd=36.7GPa

GSI=70GSI=70

ElastoElasto--Plastic Plastic Ideally BrittleIdeally Brittle

Elastic Elastic Ideally PlasticIdeally Plastic

m=0 and CWFS m=0 and CWFS ModelsModels

ssbb=0.25, =0.25, cc=80MPa=80MPa

1/ c11/ / cc

3/ c33/ / cc

1.01.01.0

0.50.50.5

1.01.01.00.50.50.5

1.51.51.5

Tunnel AxisTunnel Axis

FoldingFoldingDirectionsDirections

TUNNELFACE

J1J2

J3a J3b

3D Discrete Fracture Network model created by the Fracman code with plot of joint sets. This is typical of rock conditions on the right wall

(a) (b)

YIELDED BLOCKS AND SHEAR DISPLACEMENTSYIELDED BLOCKS AND SHEAR DISPLACEMENTSAROUND THE TUNNELAROUND THE TUNNEL

Detail of deterministic model Detail of DFN model

(a)

Q =10

Q = 0.007

Q =10

Q =0,007(b)

LEFT WALL

RIGHT WALL

CH 2359 m

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MR21