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
BRITTLE FAILUREBRITTLE FAILUREBRITTLE FAILURE
BRITTLE FAILUREBRITTLE FAILUREBRITTLE FAILURE
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
Biaxial tests run for Politecnico di Torinoat Dresden University
Biaxial tests run for Politecnico di Torinoat Dresden University
Field EvidenceField Evidence
Brittle FailureField Observations
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
Brittle FailureField Observations
Brittle FailureField Observations
++
++++++
++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
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)
SpallingSpallingSpalling
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
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