Damage Initiation in a Moderately Jointed Brittle Rock Mass

11th October, 2012

Aritra BanerjeeAritra Banerjee

M.Tech (Geotech.) (1M.Tech (Geotech.) (1stst Year)Year)

Enrolment No: 12521006Enrolment No: 12521006

ContentsContents

• Introduction

• Proposed Initiation of the Damage Process

• Approaches for Analysis of Damage Process

• Cohesion Loss due to Damage Process

• Proposed Criterion for Damage Initiation

• Conclusion

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IntroductionIntroduction

• Fracture in compressive stress field is of great concern.

• Our understanding of the damage and failure processes in

brittle rock masses in deep excavation has improved in the

last 2 decades.

• In 1996, Castro proposed that under high compression,

stress-induced damage processes begin around the opening

– by nucleation and growth of extension fractures within intact rocks.

– Also, the presence of jointing has little effect on the initiation of the

damage process as compared to low in-situ stress zone.

– So, lab test on intact specimens can be used to assess initiation of

rock damage.

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Introduction Introduction ((Contd.Contd.))

• In 2008, Kaiser and Kim, found that depending on rock

confinement, spalling (or brittle failure) dominates over

shear failure.

• For studying extension fracturing, several theoretical

micromechanical models are reviewed.

• Damage initiation (DI) criterion is developed and verified by

analysing excavation response at Neutrino Observatory in

Sudbury, Canada.

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Proposed Initiation of Proposed Initiation of Damage ProcessDamage Process

• Moderately Jointed Rock Mass – consist of rock containing

upto 4 sets of discontinuities having

– low persistence,

– not generally interconnected, but closely spaced.

• Classified as Good Quality, RMR values > 70 or Q value >20

• At greater depth (high in-situ stresses), due to lack of kinematic

freedom, movement of wedges and discontinuities, have minor

effect on damage initiation.

• Damage onset is controlled by

– inherent strength properties of intact rock and

– their relationship to the magnitude and orientation of induced

deviatoric stresses.5

Proposed Initiation of Proposed Initiation of Damage Process (Contd.)Damage Process (Contd.)• Extensive investigation and literature reviews suggest that

initiation of damage process is dominated by

– Nucleation

– Propagation of extension fractures.

• Extension Fracturing

– exhibits clean surfaces (no shear displacements)

– nucleate from stress concentrators

– forms at low angle (< 10°)

– is sensitive to changes in the magnitude and direction of major

principal stress(s1) (Castro ,1996).

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Approaches for Analysis of Approaches for Analysis of Damage ProcessDamage Process

• 2 Approaches:

– Micromechanical Approach

• considers growth of individual extension fractures from an

existing stress concentrator (existing discontinuity, pore, or grain

boundary).

• applies fracture mechanics principles to mathematically estimate

the local stress distribution around the existing crack.

– Empirical Approach

• empirically match the field observations recorded around deep

South African tunnels.

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Micromechanical ApproachMicromechanical Approach

• Several crack models have been used to explain the onset of

cracking on a microscopic scale.

Fig : Examples of crack models used to study the nucleation of extension

cracks

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Micromechanical Approach Micromechanical Approach (Contd.)(Contd.)

Fig : Examples of crack models used to study the nucleation of extension

cracks

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• Sliding and the open crack models are the ones most

commonly used.

• Both models can predict the initiation of damage due the

growth of extension cracks (or wing cracks) from local

indirect tensile stress concentrations around the tips of

stress concentrators.

Fig: Sliding and Open Crack Models


• Difference b/w these 2 models:

– Sliding crack model

• estimates the induced principal stress around an inclined crack

before the nucleation of extension cracks. So, friction is

considered.

– Open crack model

• extension crack growth occurs when local st > intrinsic cohesive

reserve or resistance of the rock material (i.e. prediction in

uncracked state).

• Open crack models were found to better represent

extension fracturing at the field scale, as shear deformation

is resisted at high in-situ stresses.

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• Considerations for DI Criterion:

– Rock is heterogeneous material

• consisting of anisotropic crystals and structural weakness at all levels.

– Under compressive loading, extension cracks nucleate when the

induced st > a critical value (depends on material property).

– Open crack model (assumption).

– Induced st at a point is driven by a linear function of (s1 - s3) at that

point.

– The driving force is linearly proportional to the local principal stress

difference, expressed as (s1 - as3), where a (> 3, generally) is a

constant for a given stress concentrator shape and orientation.

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• Considerations for DI Criterion (Contd.):

– Co-axial Stress - neglected in sliding models

• cannot simulate growth of extension cracks that do not close

under stress and/or are approx. aligned to direction of s1.

– Open Crack Model: Cohesion component of the intact rock is fully

mobilized at the onset of damage or crack initiation.

– In addition to this geometric effect on the induced stresses, the major

principal stress has to exceed the cohesion reserve of the material.

• For the nucleation of extension cracks s1 > (6 to 10) times s3.

– As per Trollope's model (1968),

• In brittle rocks, stress level (ssc) at which stable crack growth occurs in

UCS tests is proportional to 3 to 5 times the tensile strength, depending

on n.

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Empirical ApproachEmpirical Approach

• Observations from 3 - 4 m wide square or D-shaped tunnels

and ore pass system in gold mines of S.A., shows that

– extension fracturing is controlled by major principal far-field stress.

– Onset of rock mass damage starts when:

• s1 field > 0.2 sc (Hoek, Brown, 1980) or,

• For D-shaped tunnels, max stngt > 0.25 to 0.4 sc

• By Back-analysing 20 km of tunnels (Wiseman, 1979),

Degree of fracturing,

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1 3(3 )field field

c

fs s

s

Cohesion Loss due to Damage Cohesion Loss due to Damage ProcessProcess

• Loss of cohesion

– overall integrity is maintained, but locally the continuity is gradually

lost

• because the bonding between grains or particles of the rock is

gradually broken.

• Fracture initiation & damage accumulation processes cause

a reduction of the intrinsic cohesion of the intact, hard rock.

• Macrofracture formation changes the internal structure of the

rock mass and degrades its cohesion reserve.

• Verified by lab. compression tests on coarse-grained

Wombeyan marble, by applying temperature stresses.

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Cohesion Loss due to Damage Cohesion Loss due to Damage Process (Contd.)Process (Contd.)

Fig :Triaxial test results on intact and granulated Wombeyan marble

(Gerogiannopoulous, Brown 1978) and Hoek-Brown (1988, 2002)

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LossLoss ofof CohesionCohesion duedue toto CyclicCyclic LoadingLoading

• By applying a compressive cyclic loading in the Lac-du-

Bonnet granite specimens, a gradual loss of cohesive

strength occurred by

– decreasing the cohesion reserve from the intact rock to a residual

value without mobilizing the frictional component.

• Damage initiation occurred when the load first exceeded

about 30 to 40% of the Lac-du-Bonnet granite peak strength.

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LossLoss ofof CohesionCohesion duedue toto BrazilianBrazilian teststests

• A stress rotation research project was started at the

Geomechanics Research Centre at Laurentian University in

Canada.

• Hypothesis : with stress rotation, reopening of existing cracks

and the nucleation and propagation of new cracks in different

directions will cause a decrease in Brazilian tensile strength

(Gramberg, 1989).

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Cohesion Loss due to Damage Cohesion Loss due to Damage Process Process (Contd.)(Contd.)

• Types of stress rotation (both, diametral loading every 22.5°):

1. Stress rotation in 1 direction – 0° to 90° diametral loading, again

stressed to failure at 0° (Granite & Norite) .

2. Stress rotation in 2 directions – preloading from 0° to 90° and 90° to

0° (Norite).

• Brazilian Tensile strength reduced by 20% for Granite

samples and 15 – 20% for Norite samples.

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Loss of Cohesion due to Loss of Cohesion due to Brazilian tests (Contd.)Brazilian tests (Contd.)

Fig: Brazilian tensile strengths for granite and norite samples with

and without pre-loading.

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Proposed Criterion for Proposed Criterion for Damage InitiationDamage Initiation

• Onset of damage in high in-situ stressed and low

confinement condition for jointed rock mass is dominated by

development and propagation of extension fractures.

• Salient Features:

a. Lab tests on intact specimens can give valid information

b. Cohesive reserve and tensile strength decreases with damage

initiation.

c. Cohesive reserve is directly proportional to ssc.

d. Extension crack nucleate when st > critical stress (comp. loading)

e. st is driven by a linear function of (s1 – s3).

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Proposed Criterion for Proposed Criterion for Damage InitiationDamage Initiation (Contd.)(Contd.)

• Kaiser (2008) and Martin & Christiansson (2009)

– a tri-linear or S-shaped (linear) failure envelope best represents the

rock mass behaviour.

Fig: Tri-linear behaviour of rock mass

Proposed Criterion for Proposed Criterion for Damage InitiationDamage Initiation (Contd.)(Contd.)

• Proposed zones of rock mass damage initiation can be

estimated by ,

where, ssc is in the range of 25 % to 40 % of sc (Bruce, 1966).

• Successfully predicted depth of DI zones in 2070 m deep

Sudbury Neutrino Observatory cavern area, S.A.

– average ssc = 76 MPa (34 % of sc) ( Castro, McCreath, 1996).

• ssc can be estimated using UCS with strain gauges.

• Criterion of the form (s1 – s3) = a critical constant

– has been used earlier by Tresca (1864), Wiseman(1979); Martin &

Read (1996) for analysing rock mass around Mine-by Test Tunnel at

AECL, Canada.

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1 3 sc( – ) s s s

ConclusionConclusion

• Field experience showed that

– extension fracturing is dominant in damage initiation within the intact

rock in a moderately jointed rock mass.

– Also, at SNO, presence of jointing has little effect on initiation of

damage initiation process.

• Lab tests on intact specimens can give valid information

• DI criterion can be applied to elastic numerical analysis to

identify potential DI zones around deep openings.

• Estimation of areal extent and depth of potential DI zones

assists the design of openings excavated in a moderately

jointed, highly stressed brittle rock mass.

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Damage Initiation in a Moderately Jointed Brittle Rock Mass

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