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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Peter K. KaiserPresident/CEO Centre for Excellence in Mining Innovation
Chair for Rock Mechanics and Ground Control
Laurentian University
Practical Implication of Brittle Failure on
Hard Rock Tunnelling Construction
Tunnels in Granite - Tneles en granito
Universitat Politchnica de Catalunya
Barcelona - Spain
-
Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Acknowledgements
Collaborators: Cai, Diederichs, Hajibdolmajid, Martin, McCreath,
Contractors: MATRANS, TAT, Herrenknecht AG, ...
Mining companies: Vale INCO, Goldcorp, Rio Tinto,
Science Council: NSERC
and many more
Practical Implication of Brittle Failure on
Hard Rock Tunnelling Construction
-
Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Toronto
Experiences
from major mining and
tunnelling operations
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Mining at depthLessons learned
under stress rock is less forgiving
must learn from costly mistakes
and learn to design smart !
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Objective
Hoek/Kaiser/Bawden 1995
Review lessons learned
Interpret observed rock failure processes
Explain factors affecting constructability
to identify opportunities for improvements
support design
rock excavation techniques
ground control measures
to reduce construction problems minimize gap between designer and
contractor
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Primary rock mechanics challengewhen tunnelling in massive to moderately jointed rock
Anticipating the actual rock behaviour
Brittle or spalling failure
spalling often dominates over shear failure
Geo-engineering for constructability
fractured rock is often difficult to control
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Geological Model !
Rock Mass Model !
Rock Behaviour Model ?
Site characterization
?
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Focus on
massive to
moderately
jointed
rock
Modes of tunnel instability
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
1st Challenge - anticipate failure mode Observe
Interpret Understand
Funka-Bedretto - CH
Trondheim - No
Ltschberg - CH
El Teniente - Chile
URL-CanadaPiora - CH
Mt.Terri - CH
Spalling behaviour must be anticipated in almost rocks !
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
2nd Challenge - anticipate the extent Observe
Interpret
Stress field
Depth of Failure
Understand
Extrapolate
Stress Level ( ) (or Depth/UCS)
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Spalling
leads to
near-excavation
strength
reduction
Appropriate failure criteria to model near-wall behaviour
(Kaiser et al. 2000)
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
From field observations: micro-
seismicity to visual observations
Field rock mass strength
X
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Intact rock too! Revisited data courtesy Hoek (1961)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0 0.1 0.2 0.3 0.4 0.5
s3 /ucs
s1/u
cs
Failure criteria to model
intact rock is actually s-shaped
with full or apparent cohesion mobilization only at
high confinement
ductile
brittle
UCS(I)/10
Apparent UCS(II)
UCS(I)
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
5,15,25,35,45,50,55,65,69,70
Influence of heterogeneity on propagating path of wing crack in an unconfined sample (simulated with RFPA2D)
WHY? - Griffith crack simulation
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Why?Heterogeneity causes tensile stresses
s3 = 2.5 MPa
400
300
200
100
0
-10 0 10 20 30 40
PFC Samples:
Local tension due to
heterogeneity
Yield
Initiation
s1
s3
Courtesy Diederichs 2000
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Internal tension causes spalling
Crack
propagation
length
Propagation Spalling
Griffith, Hoek, and many others
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Stress ratio= 10 to 20
% tensile stresses spalling limit
400
300
200
100
00 10 20 30 40 50
Yield
Initiation
Area in Tension: 10% 1% 0.1%
s1
s3
Courtesy Diederichs 2000
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Spalling
leads to
near-excavation
strength
reduction
Appropriate failure criteria to model near-wall behaviour
(Kaiser et al. 2000)
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
s3 = 12MPa
Where? near excavations in low s3 range
Ko = 0.75 Ko = 1 Ko = 1.33
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Quartzite as an analogue of a rock mass
20
0
100
200
300
400
500
600
700
800
0 10 20 30 40 50 60
s1
[MP
a]
s3 [MPa]
a
~a/2
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
CoV =
15% to 45%
Probability of yield (100 0% failed elements) and deviatoric stress (s1-s3) contours
x = shear
o = tensionKaiser 2010 Eurock
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Spalling limit or s1/s3 - ratio
0
1
2
3
4
5
6
0
5
10
15
0 5 10 15 20
No
rmal
ized
cra
ck le
ngt
h
Stre
ss r
atio
1/
3
Distance along edge of yield zone [m]
Mean stress ratio+1sd-1sdNormalized crack length
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Yield actually means deep spalling
Spalling not
shear yield
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Stress issues even at shallow depth
Summary from detailed measurements
Martin 1999
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
under stress - same geology behaves differently -
When getting rock behaviour wrong numerical models and design are likely wrong and construction is often difficulties
Tender documents, tend to
emphasise description of geology, rock and rock mass, and
underemphasise description of the anticipated rock behaviour and spalling is not anticipated (?)
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Implications for tunnels
Implications of behaviour misinterpretation illustrated on case example
Stand-up time issues delays $$
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Implications of gettingrock mass rock mass behaviour wrong
For example
reduction in advance rate [m/d]
Planned
Actual
Raveling rock behind the open TBM split the advance and support cycle
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Stress-driven rock mass degradationoften dominates
(b) (c)
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Stand-up time reduction
3-9
mos
6-24
hrs
3-9
mos
6-24
hrs
Bieniawski 1987
Construction tools
GSI
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Progressive failure process produces blocky, unravelling ground with rock mass bulking
Volumeincrease inside df
blocky ground = unravelling rockmass broken by stress
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Anticipating behaviour at depth
Fracture propagation from stress raisers at corners (e.g. incl. tunnel face)
Massive rock unravels
Instabilities of highly stresses tunnel face
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Anticipate TBM face behaviour
now can also anticipated spalling at tunnel face
Observe
Interpret
Understand
Extrapolate to depth
Face behaviour
Increasing stress or depth
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Spalling at tunnel face Predictions and
observations
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Unravelling tunnel face What is seen in roof is to
be expected at face !
Unravelling of face before wall ! massive
broken
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Rock support of brittle failing ground with rock mass bulking
Volume
increase
inside df
blocky ground = unravelling rockmass broken by stress
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Challenge anticipate rock mass bulking (geometric volume increase)al. 1996) showing >30% for unconfined floors and 1 to 10% depending on support type (Error!
Reference source not found..b).
0
20
40
60
80
100
120
0 0.5 1 1.5 2
Vertical displacement (mm)
Ver
tical
stre
ss (M
Pa)
-10123456789101112131415
Dila
tion
(%)
= 40s3 = 1 MPa
0
1
2
3
4
5
6
7
8
9
10
0.01 0.1 1 10
Confinement [MPa]
Dila
tion
or B
ulkin
g Fa
ctor
BF
[%]
ELFEN model
Light support
Yielding support
Strong support
(a) (b) Field measurements
Simulation
with ELFEN
al. 1996) showing >30% for unconfined floors and 1 to 10% depending on support type (Error!
Reference source not found..b).
0
20
40
60
80
100
120
0 0.5 1 1.5 2
Vertical displacement (mm)
Ver
tical
stre
ss (M
Pa)
-10123456789101112131415
Dila
tion
(%)
= 40s3 = 1 MPa
0
1
2
3
4
5
6
7
8
9
10
0.01 0.1 1 10
Confinement [MPa]
Dila
tion
or B
ulkin
g Fa
ctor
BF
[%]
ELFEN model
Light support
Yielding support
Strong support
(a) (b)
Courtesy Cai 2006
BF = 0 to 10%
Uni-directional
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Yield and s3 pattern near tunnel
Circular tunnel at
2000m
and
Ko=0.51.5m
a = 5m
R = 6.5m
R/a = 1.3
Yield
Bulking
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Distance from Springline [m]
Dilati
on
or
BF
[%
]
Plastic strain, far from faceCombined
Radial strain (%) control
with dense bolting and retention (shotcrete)
s3
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Lessons learned ...When dealing with brittle failure in tunnelling ...
observe, interpret, and understand
adjust design and construction procedures to match ground behaviour
Stressed ground is less forgiving
stress breaks even good ground
good ground becomes poor ground
massive, brittle rock disintegrates cohesionless ground
Quantum shift in constructability
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Lessons learned ...When highly stressed brittle rock fails
by spalling .. not shear
degradation cannot be prevented
conventional failure criteria mislead designers s-shaped
spalling process affects both tunnel walls roof and face
select excavation and support techniques appropriate for broken rock
No ravelling, raining rock and flying arches!
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Practical Implications of Brittle Failure in Tunnelling - Kaiser 2010
Acknowledgements
Collaborators: Cai, Diederichs, Hajibdolmajid, Martin, McCreath,
Contractors: MATRANS, TAT, Herrenknecht AG, ...
Mining companies: Vale INCO, Goldcorp, Rio Tinto,
Science Council: NSERC
Practical Implication of Brittle Failure on Hard
Rock Tunnelling Construction
www.miningexcellence.ca