SEISMIC GEOTECHNICALSEISMIC GEOTECHNICAL HAZARDSHAZARDS

Liquefaction Inducedque act o duced

Learning Outcomesg

Be able to describe:Be able to describe: Types and impacts of liquefaction induced

h dhazards Methods for evaluating liquefaction hazards Methods for mitigating liquefiable sites.

LIQUEFACTION INDUCED HAZARDS

Liquefaction Hazardsq

Flow Slides Lateral Spreads Lateral Spreads Reduction in Foundation Bearing

C itCapacity Ground SettlementG ou d Sett e e t Increased Pressure on Retaining

W llWalls

Liquefaction-Induced Bearing Capacity Failures

1999 M 7.4 Kocaeli Earthquake, Turkey

Liquefaction-Induced Lateral Spread q pDamage

Lateral Spread Damage to Bridges and p g gWaterfront Retaining Walls

during the 1964 Niigata earthquake

Embankment Fills – Lateral Spread1964 M 7.5 Niigata Earthquake, Japan

Showa Bridge – Collapse due to embankment lateral spread. River bed l d ( 30 f d h) li fi dloose sand (approx. 30 ft depth) liquefied.2 ft diameter steel pipe pile extracted showed 3 ft permanent displacement andshowed 3 ft permanent displacement and maximum curvature at base of liquefied layer

Bridge Approaches/ Lateral Spreadsg pp p

1987 M 6.3 Edgecumbe Earthquake, New Zealand

Bridge approach viaduct to riverBridge approach viaduct to river bank ~ 6 ft free-field lateral spread

Passive pressure on pier was resisted by piles Cracks suggest plastic hinges p gg p gdeveloped

Lateral Spreading: Impact on Foundations

1995 M 6.9 Hyogo ken Nambu Earthquake, Kobe, Japan

Lateral Loading of Piles due to Lateral Spreading

Left: Hollow cylindrical concrete pileRight: Steep pipe pilesB th it t P t I l d K b JBoth sites at Port Island, Kobe, Japan1995 Hyogo ken Nambu earthquake

Flow Slide: Sheffield Dam1925 M 6.8 Santa Barbara earthquake

Cross Section at Time of Failure

Sheffield Dam Flow Failure

Seismic Settlement Damage

1964 M 9.2 Alaska EarthquakeSettlement of approach fill 1989 M 6.9 Loma Prieta

E th kEarthquakeSettlement of approach fill

Seismic Settlement Damage

Port Island, Kobe, JapanPort Island, Kobe, Japan1995 M 6.9 Hyogo ken Nambu earthquake Bridge abutment fillBridge abutment fill

2001 M 7.6 Chi-Chi earthquake, Taiwan

EVALUATION OFEVALUATION OF POTENTIALLY LIQUEFIABLEPOTENTIALLY LIQUEFIABLE

SITES

Liquefaction: Definition & Consequencesq q

Liquefaction triggering: Pore pressure q gg g pgeneration resulting in zero (or close to zero) effective stress and loss of strengthg

• Soil saturation is a necessary condition

Liquefaction consequences include:Bearing failure• Bearing failure

• Lateral spreading• Flow failure• Settlement

Liquefaction Susceptibilityq p y

High Susceptibility• Recently deposited (Holocene), loose to

medium dense sands / silts• Poorly compacted cohesionless fills

Low susceptibilityD d / ilt• Dense sands / silts

• Medium dense to dense gravel Well compacted cohesionless fills• Well compacted cohesionless fills

Liquefaction Hazard: Initial Screeningq g

S Seismic Hazard Levels I and II : Not Required Seismic Hazard Levels III and IV : Required unless

Mean Magnitude < 6 0 Mean Magnitude < 6.0 Mean Magnitude 6.0 – 6.4 and N1,60 > 20 or N1,60 >15 and FaSa

< 0.375gand Not Required if

Soils Have Low Susceptibility (Table 3-1)Clay Content > 15% Clay Content > 15%

N1,60 > 30 for Cohesionless Soils Water Table Deeper than 50 Feetp

Liquefaction Hazard: Initial Screening Ground Motions for Liquefaction Analysis

Need peak ground acceleration andacceleration and associated earthquake

it dmagnitude Use USGS

deaggregation plotsdeaggregation plots to determine dominantdominant magnitude

Additional Criteria for SHL III and IV

Liquefaction potential is low if:

• Pleistocene age deposits or greater (> 11,000 years old)years old)

• Clay content > 15%, Liquid Limit > 35%, or w% < 90% LL90% LL

• (N1)60 > 30 or qc1 > 160W bl 60 f b l d f• Water table > 60 ft below ground surface

Liquefaction Hazard: Initial Screening

Liquefaction Hazard: Detailed Evaluations

E l ti f Li f ti P t ti l Evaluation of Liquefaction Potential Simplified procedure based on empirical observations

Used for most routine projects Used for most routine projects More rigorous numerical modeling

Sites where liquefiable soils extend to significant depths Sites where liquefiable soils extend to significant depths Sites that have significant interlayering Sites where ground remediation costs are high

Liquefaction Hazard: Detailed Evaluations

Procedures based on three primary source Procedures based on three primary source documents

P di f th 1996 NCEER W k h ( Proceedings of the 1996 NCEER Workshop (now MCEER) on evaluating liquefaction resistance (Youd and Idriss 1997; Youd et al 2001)and Idriss, 1997; Youd et al., 2001).

2009 AASHTO Guide Specification for LRFD Seismic Bridge Design.g g

Procedures for implementing guidelines for analyzing and mitigating liquefaction in California (SCEC, 1999).g g q ( )

Liquefaction Evaluationq

L b t t ti ll t d iLaboratory testing generally not used in practice

Correlation with SPT, CPT most common• Correlation with VS can be used where CPT,

SPT not possible

Two step procedure:Determine if liquefaction is triggered1. Determine if liquefaction is triggered

2. Assess the consequences of liquefaction

In Situ Evaluation MethodsStandard Penetration Test sensitive to test procedures – need careful

tests to prevent misleading resultstests to prevent misleading results Use hammer energy measurements on

i t t j timportant projects

Cone Penetration Testf d f i t t bilit preferred for consistency, repeatability

generally more cost effectiveg y

CPT-Based Liquefaction AssessmentqCPT Advantages

Pro ides contin o s resistance profile• Provides continuous resistance profile• Good repeatability• Fast and economical

CPT LimitationsCPT Limitations• No sample is obtained

Relies upon interpreted soil type• Relies upon interpreted soil type• Hard to penetrate gravelly soils

Recommend one borehole for every 5 – 10 CPT soundings

Basic Considerations – Cyclic yResistance

Cyclic resistance of the soil described by cyclic resistance ratio (CRR) • This is the term that describes the soil

“capacity”capacity• Based upon SPT or CPT test results• Cyclic resistance is normalized by the

vertical effective stress prior to thevertical effective stress prior to the ground shaking

Basic Considerations – Cyclic yLoading

Cyclic loading described by cyclic stress ratio (CSR)( )• This term describes the cyclic load, or

“demand”demand• Directly related to the intensity of the ground

motionsmotions• The cyclic loading is also normalized by the

ti l ff ti tvertical effective stressIf CRR/CSR < 1, liquefaction is triggered

Liquefaction Hazard: Detailed Evaluations

Field ExplorationL i f Li fi bl S il Location of Liquefiable Soils

Location of Groundwater LevelDepth of Liq efaction Depth of Liquefaction

Field Exploration Methods Standard Penetration Test (SPT) Standard Penetration Test (SPT) Cone Penetration Test (CPT)

Liquefaction Hazard: Detailed Evaluations CPT Data Evaluation

Q arr

CPT-3 B-1X X'

S d t Silt S d 0

+20

4SMSM

CPT-1CPT-2 Backfill

(22)

RockRip-Rap

Quarry

n (ft

)Sand

Sand to(Hydraulic Fill)

Silty Sand

-40

-20

04(13)

7(31)

40(31)

44(>60)

34

SMSMSMSMSMSMSMSM

SP-SMSP SM

RunFill

Marine

Harbor Bottom Sediments E

leva

tion

-60

40(7)7

(13)11(13)18

(>50)17

(>50)

SP-SMCLCLMLMHMHSCSCSP

Clay / Silt(Lagoonal Clay)

S d Sil S d-80

32

>50(>50)

(>50)>50(>50)>50(>50)>50

SMSMSMSMSMSM

0 200 40048400

ance

)on o )

0 200 40048

0 200 40048

Tip

Res

ista

nce

(tsf)

Fric

tion

Rat

io(%

) Tip

Res

ista

nce

(tsf)

Fric

tion

Rat

io(%

)

Sand to Silty Sand(Lakewood-San

Pedro Formation)-120

-100

ScaleZone of Liquefaction0 20 40 feet

Key

>50SMTip

Res

ista

(tsf)

Fric

tioR

atio

(%)

SPT

Blow

coun

ts

(N)

Soil

lass

ifica

tion

(USC

S)

qBCl

Liquefaction Hazard: Detailed Evaluations CPT Data Evaluation 0 40

20 GroundEl. = +13 ft

6020

Field Measured BlowcountN (bpf)60 Description

Interpreted/Observed Soil

B-1 CPT-3

0

SMSM

MLSMML

-20

)

SM SM

-60

-40

Ele

vatio

n (ft

)

SP-SM

CL

ML

SP

CL

ML

-80

MH

SP-SCSC

SP

CLSP/SCML/SMSP-SM

Recorded SPT Blowcounts

Interpreted SPT Blowcounts from CPT Sounding

Simplified Procedure: CRR from SPT Data

Magnitude Scaling Factorg g

Simplified Procedure: CRR from CPT Data

Simplified Procedure: CSR Determination

Liquefaction Hazard: Detailed Evaluations Simplified procedure for Evaluating Liquefaction

Potential C li R i t R ti (CRR)Potential Factor of Safety =

Cyclic Resistance Ratio (CRR)Earthquake Cyclic Stress Ratio (CSR)

CRR determined from empirical charts based on SPT or CPT data

CSR determined from design peak ground CSR determined from design peak ground accelerations

depth

CSRCRR

Liquefiable zonedepth

Liquefaction Hazard: Detailed Evaluations

Example of a Liquefaction Triggering Analysis for a Single SPT Boring (Idriss andBoulanger 2008)Boulanger, 2008)

Liquefaction Hazard: Detailed EvaluationsNumerical Modeling Equivalent linear or non-linear site-specific, one Equivalent linear or non linear site specific, one

dimensional ground response analyses Require representative acceleration time histories to q p

define input ground motions Common approach is equivalent linear total stress

computer program “SHAKE” for CSR (apply 0.65 factor)

Alternative is non-linear effective stress methods to determine pore water pressure developmentLi l i l li bl f d ti Linear analysis less reliable for ground motions > 0.4g in softer soils or where max shear strain amplitude > 1 – 2%amplitude > 1 2%

Liquefaction Hazard: Detailed Evaluations

Liquefaction results used to evaluate potential severity of the following hazards:severity of the following hazards: Flow Failures Limited Lateral Spreads Limited Lateral Spreads Ground Settlement

If liquefaction safety factor < 1.3, liquefaction q y , qinduced hazard should be evaluated based on: Vulnerability of Structure Acceptable Level of Risk Damage Potential Design Earthquake Magnitude Design Earthquake Magnitude Uncertainty in SPT/CPT derived liquefaction strengths

Liquefaction Induced Hazard Evaluation

Flow Failures Flow Failures Potential massive

translational failuretranslational failure when static factor of safety <1 where

t li f tipost-liquefaction undrained residual strength mobilized.g

Liquefaction Induced Hazard Evaluation

L t l S di Lateral Spreading Progressive down slope deformation under cyclic

inertial loading during time intervals when F S <1inertial loading during time intervals when F.S.<1 Four approaches to assess the magnitude of lateral

spread displacement:p p1. Youd et. al empirical approach2. Newmark time history analyses3. Simplified Newmark Charts4. Numerical Modeling

Liquefaction Induced Lateral Spreading Youd et. al empirical approach

Liquefaction Induced Lateral Spreading

Newmark time history analyses

Liquefaction Induced Lateral Spreading Newmark time history analysis – Simplified Charts

Volumetric Strain in Liquefied Soilsq

Notes for application

•Based on SPT data•Similar in form to the Simplified Liquefaction Procedure•The chart applies for M•The chart applies for M 7.5

Evaluation of Soil Settlement Hazard

Tokimatsu and Seed (1987) Methodology:M t th d f t t d d Most common method for saturated and dry/unsaturated sands

Estimates valid only for level ground sites w/o potential for lateral spreading

Cyclic strength adjustments required for fines content

Caution required for stratified subsurface conditionsconditions

Multiply settlement estimate by 2 to account for multidirectional shakingmultidirectional shaking

Soil Settlement – Liquefied Soilsq

Differential settlements evaluated based upon variability between boringsvariability between borings

• Use 50% of total in absence of borings• Smaller where soil strata relatively horizontalSmaller where soil strata relatively horizontal• Bridging effects of non-liquefied layers could

also reduce differential settlementalso reduce differential settlement

Volumetric Strain in Liquefied Soils

Ishihara and Yoshimine (1992)

MITIGATION OF LIQUEFIABLE SITES

Bridges on Liquefiable Soils:Bridges on Liquefiable Soils:

Identify Potential Liquefiable Soil Strata Identify Potential Liquefiable Soil Strata Check for Flow Slide Potential (FOS < 1) Determine Lateral Spread Displacements

(FOS > 1) using Newmark Method Evaluate Pile/Soil Interaction Mechanism Evaluate Pile Pinning Effects Evaluate Pile Pinning Effects Evaluate Mitigation Options if Needed

Ground Improvement Ground Improvement Pin Piles

Summary of Ground Improvement Methods (cont.)

Summary of Ground Improvement Methods (cont.)

Summary of Ground Improvement Methods (cont.)

Summary of Ground Improvement Methods (cont.)

Site Remediation Using Ground Improvement Vibro-Replacement Technique – most widely used

densification method

Site Remediation Using Ground Improvement (cont.) Representative Applications

Site Remediation Using Ground Improvement (cont.) Compaction Grouting

Learning Outcomesg

Be able to describe:Be able to describe: Types and impacts of liquefaction induced

h dhazards Methods for evaluating liquefaction hazards Methods for mitigating liquefiable sites.

Q ti ?Questions?