Soil Properties, Strength and Analysis

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


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


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)


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)


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


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



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



(Boulanger, 2012)

July 19, 2012

Difficultiesthe in situ challenge

large particles

aged, cemented, &/or residual

organic soils

soil strata and heterogeneity



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)


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)


Adjustments factors for CRR


Idriss and Boulanger (2010)

Static Overburden Stress

Static Shear Stress


Earthquake Magnitude


Epistemic uncertainty in CRR relationships


NCEER Workshop (1997)

Cetin et al. (2004)



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]


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


Cetin et al. (2004)



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]


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


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



32 case history estimates from Olson and Stark (2002) with N values corrected for fines con-

tent


July 19, 2012

Residual strength


0 5 10 15 20 25 300

0.05

0.1

0.15

0.2

0.25

0.3


Sr / vo






tent


FINES CONTENT< 20%

July 19, 2012

Residual strength


0 5 10 15 20 25 300

0.05

0.1

0.15

0.2

0.25

0.3


Sr / vo






tent


FINES CONTENT 20%


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)


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


-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


Post-liquefaction settlement

(EERI, 2011)

Settlements from Tohoku Earthquake

Common design chart(Ishihara and Yoshimine, 1992)


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




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


-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?



4th avenue slide, Anchorage (1964)(Hansen, 1971)Assumed strength reduction

(Idriss, 1985)


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)


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)


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


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

Soil Properties, Strength and Analysis

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