Rockburst hazard in tunnelling

22-02-18

Giordano Russo

A rockburst is defined as:

• damage to an excavation that occurs in a sudden and violentmanner and is associated with a seismic event (Hedley, 1992;Kaiser et al., 1996).

• a seismic event that causes violent and significant damage to atunnel or excavations of a mine (Ortlepp, 1997) .

• Explosive failures of rock which occur when very high stressconcentrations are induced around underground openings (Hoek,

2006) .

A rockburst is associated with damage to an excavation or itssupport: hence, a seismic event alone without causing damage isnot a rockburst

[extracted from Kaiser, 2017 [18], Cai and Kaiser, 2017 [1], Diederichs, 2014 [9]

Video Rockburst

1 Olmos (Perù)

2 Gotthard

3 Chile site A

4 Chile site B_1

5 Chile site B_2

6 Chile site B_3

7 Chile site C_1

8 Chile site C_2

9 Chile site C_3

10 Chile site C_4

• It is defined as the development of visible extension fracturesunder compressive loading near the boundary of an excavation.

• Spalling in hard rock excavations, while brittle in nature, can beviolent (rockburst or strain burst) or not and time dependent.Strain bursting is the violent rupture of a volume of wall rockunder high stress.

• The spalling damage (extension fractures) can happen before theactual rockburst: it is the instability created (example: buckling)by the formation of parallel and thin spall slabs that provides thekinematics for the sudden energy release.

• While even weak rocks can spall, the ability to store energy,typical of strong rocks, is required for strain bursting.

Spalling vs Rockburst

Spalling is a mode ofdamage and overbreak intunnels at depth in hardrocks (low porosity).

Extracted from Diederichs, 2005 [6]

1) Creation of boundary parallel spalling fractures compared to theprogressive shearing assuming in plasticity; (2)Transition fromnon-violent spalling (a) to bursting through buckling (b),interaction with structure (c) and dilational yield (d)

(Diederichs, 2007 [7]

Spalling vs Shear failure mode

Fault-slip rockburst refers to damage to an excavation caused byenergy released from a shear slip or shear rupture source that isremote from the excavation. Damage is caused by dynamicdisturbances from the fault-slip source and may, in part orexclusively, be related to the intensity of the related seismic event.This intensity is directly related to the source size

A pillar-rockburst refers to damage to an excavation that iscaused by excessive loading of a pillar such that the pillar wall(edge or face) or the pillar core fails.

A strainburst is a sudden and violent failure of rock near an exca-vation boundary caused by excessive straining of an un-fracturedvolume of rock (burst volume). The primary or a secondary seis-mic source is co-located at the damage location.

Different types of rockbursts

[1,3]

Ortlepp and Stacey, 1994, adjusted by Cai and Kaiser, 2017 [1]

Severity of rockburst damage

If an excavation is supported, the severity of rockburst can berelated to the support damage (minor, moderate, severe..)

The rockburstdamage severity canalso becharacterized by thedepth and lateralextent of the rockaround the openingthat is involved inthe failure process.

[3] for 3÷6m underground excavation

Potvin, 2009; modified [1]

Strainburst

“a sudden and violent failure of rock near an excavation boundarycaused by excessive straining of an un-fractured volume of rock.”Hence, strainbursts occur when the stress near an excavation reachesthe peak strength of the unsupported or supported rock mass and therock fails by a combination of extension and shear fractures.

• self-initiated

• mining-induced

• seismically triggered

• dynamically loaded

Strainburst behind support [1]

Strainburst types Features Energy

Self-initiatedGradual weakening of rock mass; relatively soft loading/mining system

Related to strainburstintensity (local stress-strength conditions)

Mining inducedInduced deformations/strainschange local stress reachingthe rock strength

Related to strainburstintensity (local stress-strength conditions)

Seismicallytriggered

Self-inititiated or Mininginduced triggered by remote seismic event

Mainly related to strainburst intensity(local stress-strengthconditions)

Dynamicallyloaded

Remote seismic eventaugments strainburstintensity:-Depth of Failure deepening-Ejection for energy transfer

Mainly from remote seismic event

Derived from [18]

Rockburst damage mechanisms (dynamic failure modes)

3. Shakedown with stand-up timereductions

1. Static stress fracturing orstrainbursting due to tangentialstraining

2. Rock ejection by momentum transferfrom remote seismic or from highbulking deformation rate duringstrainburst

[3]

This failure mode is dominated bystored strain energy, the LoadingSystem Stiffness (LSS) and the insitu stress field

It is associated with rock massbulking that causes large static anddynamic deformations near theexcavations, which are largelydefined by the depth of failure andthe mining-induced tangentialstrain.

1. Static stress fracturing or strainbursting due to tangential straining

Failure mode is dominated byenergy transmitted from remoteseismic sources and the fracturerate due to strainbursting

2. Rock ejection by momentum transfer from remote seismic sourcesor from high bulking deformation rate during strainburst

Example of severe event with failureof support in andesitic rock. Probablycombined 1-2 mechanism (Estimatedreleased Energy 25-30kJ/m2)

3. Shakedown with stand-up time reductions

This failure mode is dominated byrock quality, span, etc., anddynamic acceleration forces from aremote seismic event or otherdynamic disturbances

Examples [1]

[3]

Strainburst Susceptibility- Rock mass quality- Intrinsic brittleness*

Strainburst Potential (SBP)+ High tangential stress

Strainburst Severity (SBS)- Burst volume- Relative brittleness* (LSS)- Consumed energy at failure (DP)- Volume increase (bulking)

- Failure (brittle)

- Stress concentration- Deconfinement

- Energy storage- Rapid release- Volume

LSS=Loading System Stiffness (mine)DP =Deformation Potential

*Tarasov and Potvin, 2013 [28][1] modified

Rockburst mechanics components [9]

18

Gray area: rupture energy dWr=post-peak rupture energyRed area: elastic energy dWe=elastic energy withdrawn during post-peakYellow area: excess energy dWa=post-peak released energy

Brittle failureIntrinsic Brittleness: Elastic/Post-peak modulus ratio (E/M)

[29]

19

Brittle failureRelative Brittleness: unloading rock (M) and system (L) modulus

L2L1

ME

(a)

(b)

The lower the LSS is or the softer the mine stiffness is, the higher isthe energy input from the surrounding rock mass and then theDeformation Potential (DP)

Guidelines for analysing rock failure as shearing or spalling based on GSI and

the ratio of compressive to tensile strength (strainburst susceptibility)

[6]

[6,9]

Modification proposed by [1]Rkb Potential and Severity(only for stiff environment)

Rockburst failure mode potential indicator

Published version Diederichs, 2017 [10]

Futher evolution of theRockburst failure modepotential indicator:

• Definition of DynamicRupture Potential (DRP)

• DRP correlated to rock blockEjection Velocity potential

Update version [10] and Proposed EjectionVelocity assessment [27]

af=increasing levels of spall damage

[7,8]

The Depth of brittle failure is related to the

Damage Index (DI) or Stress Level (SL)

expressed by ratio

max/UCS

max/CI

Crack Initiation Threshold (CI=UCS*) occurs when

max 0.4-0.6 UCS

Martin et al.,1999 [20]

Damage initiation (UCS*=CI) is taken as the first significant andsustained increase in Acoustic Emission rate after the initial flurryof events associated with crack closure

[6]

The higher the

difference, the higher the stored energy and

then the potential for

violent failure

Nicksiar and Martin, 2013; modified by Hoek and Martin, 2014 [13]

Relationship between UCS and CI for various rocks

Empirical estimation tool for spalling depthUCS* is the Crack Initiation threshold (CI)

[Moderate and Serious overbreak indicate strainburst potential]

[8]

27

Note that the Depth of Failures (DoF) reported in [20] refer mainly tono-violent events and max values for the stress levels(high DoF does not necessary mean violent event with rockejection).

28

In situ-measurements ofEDZ depths compared withMartin et al. equation forDoF.

Calculation example of damagezones for granitic rock.Note that EDZi over-predict DoF forabout max/CI>1.5.EDZi is assumed to coincide withthe Volumetric Strain Reversal.

Perras and Diederichs, 2016 [22]

DoF line should coincide with the Inner Excavation Damage ZoneEDZi (connected micro-fractures visible damage)

The same classification for spalling depth is applied in the GDE multiple graph Russo, 2014 [26]

Chart for assessment of the severity of brittle rock failure [27]

Example of application

Hoek, 2010 [12]

Another example:The classification limits are a little

different

The max resulting tangential stress variation oncircular excavation contour oscillates at each pulse

max = 4*ppvs*Vs

= rock mass densityppvs (or PGVs) = Peak particle (or Ground) Velocityof shear wavesVs = shear waves propagation velocity

As remarked, a seismic event or blast may addan increment of dynamic stress that may triggerstrainburst or increase the depth of stressfractured ground.The dynamic stress pulse of the shear wavemodifies principal stresses

1d = +*ppvs*Vs and 3

d = -*ppvs*Vs

Dynamic ground stress increase of DoF

[3] Refer also to [1, 20] for Seismic source characteristics,Ground motion velocity/acceleration, etc.

Chart for assessment of tangential stress variation fordifferent ppvs and Vs [27]

Depth of Failure (df=DOF) increase for Dynamic ground stress

df/a= C1(max/c)-C2=C1*SL-C2

For static condition on average C1=1.37 and C2=0.57 [15]

df/a= 1.37(max/c)-0.57(SL=Stress Level=Damage Index)

Constitutive models for brittle yield simulation Mohr-Coulomb “error”

Martin, 1997 [19]

In brittle rock the mobilization of the cohesive and frictionalcomponent is strain dependent. Cohesion mobilizes before thanfriction angle [b and i are residual and interlocking (dilation)component] ”m=0 approach”

The composite strength envelopeillustrating in principal stressspace the zones of behaviour asbounded by the damage initiationthreshold, the upper bound shearthreshold (damage interaction),and the transitional spalling limit

CI= Crack Damage Initiation threshold

(usually 0.4÷0.6UCS)

CD= Crack Damage Interaction threshold

(usually 0.7÷0.9UCS)

[5, 10]

transitionSpalling Limit RatioSLR = 1/3 1015 or more

Numerical modelling: Elastic analysis: “m=0” approach

Simulation of brittle spalling behaviour by Hoek and Brown failure criterion

Hoek et al., 2002 [11]

[20]

Hoek-Brown constants:m=0a=0.5

s0.5=0.33 s=0.11(s0.5=0.41 s=0.17 for Kaiser, 2016 [18]

Examples of 2D-3D elastic (m=0 type) analyses by Rs2 and Examine3D (Rocscience):

Differential stress and Spalling Criterion

Numerical modelling: Elastic-plastic analysis

Simulation of brittle spalling behaviour by Hoek and Brown failure criterion [11]

[7, 22]p=peakr=residual

DISL (Damage Initiation Spalling Limit)

3critic

Example of DISL application with distinction of damaged and yielding zoneas a function of Maximum Shear Strain (PHASE 2=Rs2 Rocscience)

[6]

Example of DISL application in severe rockburst environment withindications of Yielding zone and Volumetric Strain Reversal (referable toDepth of Failure of Martin et al., 1999).

Case 1: equivalent-continuum modelling by Rs2 (Rocscience)

Contraction

Extension

Example of DISL application in severerockburst environment

Case 2: joint network modelling by Rs2

Burst potential based on energystorage and release according tostress path [9]

Strain Energy

In terms of principal stresses the Strain Energy Density (SED) iscalculated by the formula:

SED = [(12+22+32)-2(12+23+31)]/(2Ey)

1,2,2= Principal Stresses; =Poisson Ratio; Ey=Young Modulus

The stored strain energy can beconsumed by process as rockfracturing or to be released inform of kinetic energy.

The severity of rockburst isessentially related to the amountof the energy in excess

Example of relationship between Damage Index and estimatedReleased Energy of severe rkb events in Andesitic rocks

Kinetic Energy release

Ejection velocity

Ejection velocity and Energy demand

Based on laboratory tests,Villaescusa et al. 2016

[31] relate ejection velocityto intact rock strength(UCS).The Energy demand isderived as a function of thepotential Unstable mass.

[n.d.r.: Caution in estimating theejection velocity is required for thereduced scale of tests!]Chart derived according the cited Authors

Proposed correlation between the Overbreak classification, the Depth of Failure (DOF=r-a) and the expected released Energy [26]

[3]

48

Rock mass bulking in brittle rocks

Extension fracture initiation/propagation and shear along joints lead torock mass disintegration and rock mass bulking.Bulking process is result of geometric block incompatibilities, leading tolarge volume increase. If the rock mass is supported, bulking can berestrained to smaller value.

WD (Inelastic Wall Displacement)= DoF*BF [3]

49

[7,8,9]

Additional equations for

Bulking Factor

Support in burst prone ground[1,3,18]

All support functions are needed, with different

contributions..

[16]

Optimal support system combinesadequately the deformationproperties and capacity of eachbasic component:Reinforcement [bolts, cables,..]and Surface Support [(fiber-reinforced) Shotcrete , mesh,…]

Load-displacement curves

Static energy absorbing capacity

Reinforcing/Holding elements

[30]Energy Capacity of Reinforcement elements

Potvin et al. 2010, [24]

Energy Capacity of Surface support

[FRS=Fiber-Reinforced Shotcrete]

Steel mesh dynamic test results[30]

Example of suggestedSupport system

(and Safety factor..)[3]

Rock support capacity

In general, three Factor of Safety should be satisfied in Design

Key factor: Capacity of the combined Support System

According to Potvin et al., 2010-2013 [24,25], the surface support (FRS and chain-

link) could guarantee an additional safety margin or represent a potential

“weakest link” depending if the support system, in function also of the bolt

spacing, will be able to work on “serial” or “parallel”, respectively.

Cala et al. ,2013 [2] remarked a redistribution of the energy in the different support

elements variable in function of the stiffness of the surface component: typically

for stiff conditions the reinforcement and surface support adsorbed 75% and 25%

of the released energy, respectively.

In other tests [30] the reinforcement adsorbed 72 to 93% of the released energy.

GDE design approach applied in Rockburst environment

Rockburst Energy

Demand

Reinforcement

Capacity

Surface Support

Capacity

ED ≥2ED ≥ED

Example High Energy

(Severe Event)

n.2 orders of high

capacity grouted

elements

n.2 FRS+high

capacity chain

link mesh layers

Basic Safety statement: Automatized support installation without any workers exposure

Outline of Microseismic Monitoring

Seismic monitoring enables the quantification of exposure toseismicity and provides a logistical tool in prevention, control andprediction of rockburst (from Mendecki et al., 1999).

[14]

Cai et al.,2007,reported in [1]

Examples for typical time domainwaveforms of microseismic signals

in tunnels ISRM, 2016 [14]

Rock fracturing<1s

10-3000Hz10-2-10-7 m/s

Blasting>1s

100-500Hz10-2-10-3 m/s

Dotted lines remark some hypothesis of preferential alignment ofevents along main shear zones, while square symbols are tunnelfaces in advancement.

An insight on a presumably Seismically triggered strainburstoccurrence is shown in the following.

Example of seismicmonitoring weekly report fortunnels of Hydroelectricproject in Andean regionshowing the concentration ofevents (Mw=-31)

The analysis* focus on the time lapse between two blastings (B2 and B3) intunnels #2 and #3 (distance 100m)

B2=h7:20 (day 1) * Elaborated by Carlo Chiesa

B3=h19:00 (day 2)

[Note: previous blasting in #3= h2:05 (day1): Some related seismicity still measured in#2 and #3 at B2 time]

Tunnel #3

Tunnel #2

Note (sky-blue points) some new seismicity between tunnels and increase around Tunnel #3 (previous blast was in #2)

#3

#2

Severe rockburst occurrence in tunnel #3

(after 19h from B2 and 24h from previous blast in #3)

#3

#2

Tunnel #3 estimated conditions:

DRPModerate to High

Damage Index Moderate to Serious overbreak

Seismic rate (events per hour) and max Moment Magnitude (Mw)event after B2.

[Mw=(2/3)log10(Mo)-6 and Mo=Seismic Moment=Au

=rock mass rigidity, A=fault area; u=slip displacement]

Blasting B2

(15/10 – 7:20 –

tunnel #2)

Blasting B3

(16/10 – 19:00 –

tunnel #3)

RKB (16/10 – 2:10

– tunnel #3)Previous Blasting

B1 (15/10 – 2:05 –

tunnel #3)

B2 (15/10 –

7:20 –

tunnel #2)

RKB (16/10 – 2:10

– tunnel #3)

B3 (16/10 –

19:00 –

tunnel #3)

B2

Intense seismicity

Reducedseismicity

Rockburst

Previous Blasting

B1 (15/10 – 2:05 –

tunnel #3)

Distance from tunnel #2

Local increment after B2 and then minor variation Very high after B2

Reduce before Rkb

>Events close #2tB2=1h

(Time after B2)

>Eventsclose #3tB2=2h

>Eventsclose #2[& #3]tB2=5h

>Eventsclose #2tB2=6h

>Eventsclose #3

& #2tB2=19h

Rockburstin #3

Seismic event rate after B2 and comparative Omori decay laws

Intense seismicity

Reducedseismicity

Seismicitymoves from

#2 to #3

Main #2

Seismicitystarts againaround #3

Rockburst

Other example with induced main seismic event, successiveseismicity reduction (E and Mo) and final rockburst occurrence

Conclusive Remarks

• Blasting induces seismicity in thesurrounding rock mass and local geo-conditions may increase the radiatedenergy, eventually resulting in strainburstin other tunnel.

• Seismicity from successive blastings, in thesame or different tunnels, mayoverlap/interfere, so increasing theprobability of triggering strainburst

• Some drop of energy after main seismicevent is frequently observed beforestrainburst (see also [13, 20])

Thank you for your attention!

71

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