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Soil Properties, Strength and Analysis. OBJECTIVE Review concerns and highlight challenges for geotechnical seismic analysis of Cascadia Subduction Zone events with a particular focus on soil properties and strength . Mt. Hood, Oregon. A few questions. - PowerPoint PPT Presentation
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CASCADIA SUBDUCTION ZONE EARTHQUAKES AND CRITICAL INFRASTRUCTURE
US Bureau of ReclamationOregon State University
July 18-19, 2012
Michael Beaty, PhD PE GE
Soil Properties, Strength, and Analysis
BEATY ENGINEERING LLCBeaverton, [email protected] (503) 746-7419
July 19, 2012
OBJECTIVE
Review concerns and highlight challenges for
geotechnical seismic analysis
of Cascadia Subduction Zone events
with a particular focus on
soil properties and strength.
Soil Properties, Strength and Analysis
2Soil Properties, Strength, and Analysis
Mt. Hood, Oregon
July 19, 2012
1. What needs to be improved for geotechnical analyses of CSZ events?
2. How does this list differ from the analysis needs for shallow, crustal events?
3. How does increasing the number of load cycles impact the reliability and conservatism of our analyses?
4. Do our current characterization methods capture all of the soil properties that we need for a CSZ analysis?
A few questions
3Soil Properties, Strength, and Analysis
4Soil Properties, Strength, and Analysis July 19, 2012
In many cases, it seems pretty simple DURATION number of load cycles & impact on soil properties
pore pressure migration and void ratio distribution during shaking
coincidence of kinematic loads from lateral spread and inertial loads from structure (e.g., deep foundations)
will conservatisms compound with each loading cycle?
will something be missed (e.g., strength loss in moderately sensitive clays? fabric disintegration in lightly cemented soils?)?
Will analyses properly capture long period effects on deformation?
Effect of non-stationary characteristics of ground motion
Impact of CSZ on geotechnical analysis?
July 19, 2012
Site response example
5Soil Properties, Strength, and Analysis
0 60 120 180 240-800
-400
0
400
800
Shea
r Str
ess (
psf) FKS014 (2011 Tohoku Earthquake M9)
0 60 120 180 240-800
-400
0
400
800
Time (s)
Shea
r Str
ess (
psf) La Union (1985 Mihoacan Earthquake M8.1)
6Soil Properties, Strength, and Analysis July 19, 2012
For critical infrastructure (focus on dams):
Nonlinear deformation analysis (NDA)[FLAC, etc.]
Typically 2D plane strain
Current state of seismic geotechnical analysis
Perlea and Beaty (2010)
7Soil Properties, Strength, and Analysis July 19, 2012
Simple -- Few properties, simple stress-strain models, residual strengths imposed. Typically undrained (total stress) analysis. Often considered conservative compared to most likely behavior, but depends.
Moderate -- Somewhat more sophisticated stress-strain, triggering analysis performed, no pore pressure dissipation. Often needs additional soil properties to describe degradation of stiffness and strength. Often undrained or simplified effective stress. Often considered conservative compared to most likely behavior, but depends.
Complex -- Effective stress analyses considering volumetric strains due to contraction and dilation as well as pore pressure migration. Requires sophisticated parameters to define soil behavior. Conservatism is less clear due to complexity.
Range of available NDA methods
July 19, 2012
Analysis requirements for dams shifting from deterministic approach to risk-based approachDeterministic
Objective is to confidently determine when a dam is “safe”safe is not defined
Risk informedObjective is to make best estimate of actual response, with an
understanding of the uncertainty in the prediction
Risk-informed evaluations raises standard on analysis quality
Deterministic vs Risk Based
8Soil Properties, Strength, and Analysis
July 19, 2012
1. Earthquake historyNeed to develop appropriate suite
Should they all look like Tohoku 2011?
What non-stationary aspects are important for CSZ?
2. Post-cyclic strengthResidual strength?
Strain-softening of clays? silts?
3. Degradation of loading stiffness
Key factors in NDA
9Soil Properties, Strength, and Analysis
10Soil Properties, Strength, and Analysis July 19, 2012
Site characterization
Liquefaction (sand-like soils) triggering or initiation
residual strength
lateral spread
settlement
Cyclic mobility (clay-like soils) stiffness and strength loss with cycles
Intermediate soils (e.g., low plasticity silts)
Organic soils
Discussion on soil parameters
Foundation Inspection Trench(B.F. Sisk Dam)
July 19, 2012
Sand-like soils• CPT, SPT, Vs, Becker hammer
• Disturbed samples for classification
Clay-like soils• CPT, Vs, Vane shear, full flow
penetrometers
• Disturbed samples for classification
• Undisturbed samples for consolidation, strength, cyclic response, and post-cyclic response
Site characterization
11Soil Properties, Strength, and Analysis
(Boulanger, 2012)
July 19, 2012
Difficultiesthe in situ challenge
large particles
aged, cemented, &/or residual
organic soils
soil strata and heterogeneity
Site characterization
12Soil Properties, Strength, and Analysis
Instrumented Becker Penetration Test DeviceJoint UC Davis – CA DSOD
(Boulanger, 2012)
July 19, 2012
At range of scales (vertical and horizontal)
Impact on response and uncertainty?
How to best incorporate into analysis?
How address variability within strata?
Stratigraphy
13Soil Properties, Strength, and Analysis(Boulanger, 2012)
Pronounced layering of dam shell(photo from Upper San Fernando Dam)
Response of Lower San Fernando Dam(EQIIS, Steinbrugge collection)
14Soil Properties, Strength, and Analysis July 19, 2012
For sand-like soilssands, low plasticity silts and silty sands, gravels (etc.) with
low organic
Typical analysis approach
Estimate cyclic strength from penetration resistance difficult to obtain undisturbed samples
• freezing techniques?
• representative?
CPT and SPT most common
• Vs and BPT
Liquefaction triggering
NCEER Workshop (1997)
16Soil Properties, Strength, and Analysis July 19, 2012
Adjustments factors for CRR
Liquefaction triggering
Idriss and Boulanger (2010)
Static Overburden Stress
Static Shear Stress
Idriss and Boulanger (2010)
Earthquake Magnitude
17Soil Properties, Strength, and Analysis July 19, 2012
Epistemic uncertainty in CRR relationships
Liquefaction triggering
NCEER Workshop (1997)
Cetin et al. (2004)
Idriss and Boulanger (2010)
18Soil Properties, Strength, and Analysis July 19, 2012
Cyclic testing of frozen samples from unit 3c provides unique case for CRR comparison
Initial study: Pillai and Byrne (1994); BC HydroCRR Comparison: Idriss and Boulanger (2010)
CRR estimates for Duncan Dam
0 20 40 60 80 100Scale (m)
400
450
500
550
600
Ele
vatio
n (m
)
Ground freezing and sampling
Bedrock
Sand & gravel
Undivided: sand, silt & clay(unit 3)
Fine sand(unit 3c)
Sand & gravel
Boreholes
400
450
500
550
600
[after Pillai & Byrne 1994]
19Soil Properties, Strength, and Analysis July 19, 2012
0 0.1 0.2 0.3
CRRM=7.5,=0 usingPillai & Byrne (1994)
12
10
8
6
4
2
0
Verti
cal e
ffect
ive
stre
ss (a
tm)
0 0.1 0.2 0.3
CRRM=7.5,=0 usingIdriss & Boulanger (2008)
12
10
8
6
4
2
00 0.1 0.2 0.3
CRRM=7.5,=0 usingNCEER/NSF (Youd et al. 2001)
12
10
8
6
4
2
00 0.1 0.2 0.3
CRRM=7.5,=0 usingCetin et al. (2004)
12
10
8
6
4
2
0
Computed using different SPT-based liquefaction triggering proceduresBased on cyclic DSS and triaxial tests on frozen samples (Pillai & Stewart 1994)
(a) (b) (c) (d)
(Boulanger and Idriss 2012)
CRR estimates for Duncan Dam
July 19, 2012
Probability relationships
Also consider uncertainty in (N1)60cs and CSR
Implications to risk-based evaluations
Uncertainty in CRR relationships
20Soil Properties, Strength, and Analysis
Cetin et al. (2004)
Idriss and Boulanger (2010)
21Soil Properties, Strength, and Analysis July 19, 2012
Importance of fines and density on Cn
Bedrock
Alluvial silty & clayey sands
Boreholes at crest
at upper bench
at lower bench
at toe
0 10 20 30 40Scale (m)
420
440
460
480
500E
leva
tion
(m)
420
440
460
480
500[After CDWR 2005]
22Soil Properties, Strength, and Analysis July 19, 2012
The minimum shear strength (Sr) that can be mobilized after liquefaction
Potential factors: critical state at large strains
void ratio redistribution and pore pressure migration
mixing of soil layers at large strains
initial overburden stress
• theoretical arguments uncertain
• Sr or Sr / σvo can influence deformation pattern for dams
high overburden stress (?)
silt content (?)
Residual strength
23Soil Properties, Strength, and Analysis July 19, 2012
Reliance upon empirical curves based on case histories
Difficult to obtain undisturbed samples and difficult to perform disturbance corrections
Undrained laboratory tests may not capture all field behavior in some cases
Residual strength
July 19, 2012
Residual strength
0 5 10 15 20 25 300
0.05
0.1
0.15
0.2
0.25
0.3
SPT Blowcount (N1)60cs
Sr / vo
Case history estimates from Ol-son and Stark (2002) with N
values corrected for fines con-tent
Case history estimates from Ol-son and Stark (2002) with N
values corrected for fines con-tent
32 case history estimates from Olson and Stark (2002) with N values corrected for fines con-
tent
Idriss and Boulanger (2008)
July 19, 2012
Residual strength
25Soil Properties, Strength, and Analysis
0 5 10 15 20 25 300
0.05
0.1
0.15
0.2
0.25
0.3
SPT Blowcount (N1)60cs
Sr / vo
Case history estimates from Ol-son and Stark (2002) with N
values corrected for fines con-tent
Case history estimates from Ol-son and Stark (2002) with N
values corrected for fines con-tent
32 case history estimates from Olson and Stark (2002) with N values corrected for fines con-
tent
Idriss and Boulanger (2008)
FINES CONTENT< 20%
July 19, 2012
Residual strength
26Soil Properties, Strength, and Analysis
0 5 10 15 20 25 300
0.05
0.1
0.15
0.2
0.25
0.3
SPT Blowcount (N1)60cs
Sr / vo
Case history estimates from Ol-son and Stark (2002) with N
values corrected for fines con-tent
Case history estimates from Ol-son and Stark (2002) with N
values corrected for fines con-tent
32 case history estimates from Olson and Stark (2002) with N values corrected for fines con-
tent
Idriss and Boulanger (2008)
FINES CONTENT 20%
27Soil Properties, Strength, and Analysis July 19, 2012
Rate of strength loss
Some processes may require time to develop
void ratio redistribution
Some failures have occurred after shaking
Lower San Fernando dam (1971) [inferred from limited information]
Mochikoshi Tailings Dam No. 2 (1978) [24 hour delay]
Importance?
May be more significant for denser, liquefiable
Less uncertainty for very long duration(?)
Residual strength
Lower San Fernando dam (NISEE, 2012)
28Soil Properties, Strength, and Analysis July 19, 2012
Deformations accumulate even if residual strength provides stability
Considerations:Estimating strength of all materials involved in deformations
sand-like soils with high pore pressures
liquefied soils (residual strength, or dilative behavior?)
clay soils (with localization effects?)
anisotropy (compressive versus DSS versus extension)
Progressive deformation through unload-reload cycles ratchet behavior
Lateral spreading
July 19, 2012
Example prediction of cumulative strains
Lateral spreading
29Soil Properties, Strength, and Analysis
-1000
-800
-600
-400
-200
0
200
400
600
-6% -5% -4% -3% -2% -1% 0% 1%
Shear Strain, Gamma (%)
Shea
r Stre
ss o
n X-
Y Pl
ane
(psf
)
.
a) stress-strain
Undrained Strength
-1000
-800
-600
-400
-200
0
200
400
600
0 5 10 15 20 25 30 35 40 45 50
Time (seconds)
Shea
r Stre
ss o
n X-
Y Pl
ane
(psf
)
.
b) shear stress history
Zone (99,13)Undrained Strength
30Soil Properties, Strength, and Analysis July 19, 2012
Post-liquefaction settlement
(EERI, 2011)
Settlements from Tohoku Earthquake
Common design chart(Ishihara and Yoshimine, 1992)
31Soil Properties, Strength, and Analysis July 19, 2012
Accumulation of strain with load cycles softened loading modulus
static shear stress bias
Soil properties for NDA How does stiffness and strength of clay degrade with
increasing number of cycles?
For critical structures, cyclic testing is imperative for NC and low OCR clays
Cyclic failure (clay-like)
July 19, 2012
Calibration example for NDA
Cyclic triaxial tests on cohesive soil for Tuttle Creek Dam
Cyclic failure (clay-like)
32Soil Properties, Strength, and Analysis
33Soil Properties, Strength, and Analysis July 19, 2012
Strain localization?
e.g., Tuttle Creek Dam Cyclic laboratory tests showed modest degradation of
undrained strength Vane shear tests showed significant strength degradation at
large strain Transition poorly defined Potential for strain localization requires special consideration
Cyclic failure (clay-like)
-5% 0% 5% 10% 15% 20% 25%-1
0
1
2
3
Axial Strain
Dev
iato
r Stre
ss (k
sc)
Undrained Strength
July 19, 2012
Sensitivity?
When will strains accumulate sufficiently to induce strength loss in moderately sensitive soils?
Cyclic failure (clay-like)
34Soil Properties, Strength, and Analysis
4th avenue slide, Anchorage (1964)(Hansen, 1971)Assumed strength reduction
(Idriss, 1985)
35Soil Properties, Strength, and Analysis July 19, 2012
Silts and clays with PIs of ~ 5 to 18
Intermediate Soils
0
10
20
30
0.4 0.6 0.8 1 1.2 1.4
PI
wc / LL
Not Susceptible
Moderately Susceptible
Susceptible
18
12
Bray and Sancio(ASCE , 2006)
Boulanger and Idriss(ASCE 2006)
36Soil Properties, Strength, and Analysis July 19, 2012
Transverse cracking (dams)
Analysis tools are very limited and empirical
Effect of duration uncertain
Assumption on depth can be significant to risk analysis due to erosion potential
Cracking of dams and levees
(EERI, 2011)(EERI, 2011)
July 19, 2012
Jet grout columns(John Dillon 2009)
Stone columns(Hayward Baker, 2012)
Tuttle Creek Dam, USACE
Ground improvement
Piles(Sardis Dam)
Analytical research(e.g., Goughnour & Pestana
1998)
38Soil Properties, Strength, and Analysis July 19, 2012
Need best estimate of responseConservative bias may cloud usefulness of analysis result
Should develop and consider probability distribution of predictions
Questions for CSZ How good are the analyses?
How does increasing the number of load cycles affect conservatism
Risk analysis
Earthquake Intensity MeasureDi
spla
cem
ent
Possible fragilitycurve for liquefiable site
39Soil Properties, Strength, and Analysis July 19, 2012
Improve analysis tools (constitutive models, etc.)?A range of analysis tools are available, but there is plenty of room for continued advancement (complex load path, void ratio redistribution, 3D, etc.).
Improve analysis approach?Recognize importance of sensitivity studies, transparent documentation, and model validation for NDA. Include in budget planning and analysis scope.
Improve laboratory test and in situ methods?Continued adoption of cyclic DSS for clay-like and transition soils.
Improve collection, documentation, and use of case histories?
May be most significant current need.
Concluding remarks