evaluation of geotechnical hazards -...

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Evaluation of Geotechnical Hazards

byGeoffrey R. Martin

Appendix B:Evaluation of Geotechnical Hazards

Describes Evaluation ProceduresSoil LiquefactionSoil SettlementSurface Fault RuptureFlooding

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Evaluation of Soil Liquefaction Hazard

Procedures based on three primary source documents

Evaluation of Soil Liquefaction Hazard (cont.)

Field ExplorationLocation of Liquefiable SoilsLocation of Groundwater LevelDepth of LiquefactionField Exploration Methods

Standard Penetration Test (SPT)Cone Penetration Test (CPT)

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CPT Data Evaluation

Scale

RockRip-Rap

Quarry

Zone of Liquefaction

Harbor Bottom Sediments

CPT-3 B-1X X'

Elev

atio

n (ft

)Sand

Sand to

0 20 40 feet

(Hydraulic Fill)Silty Sand

-60

-40

-20

0

+20

Key

4(13)

7(31)

40(31)

44(>60)

34(7)7

(13)11(13)18

(>50)17

(>50)

32

>50(>50)

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

>50

SMSMSMSMSMSMSMSMSM

SP-SMSP-SM

CLCLMLMHMHSCSCSP

SMSMSMSMSMSM

SM

0 200 40048400

Tip

Res

ista

nce

(tsf)

Fric

tion

Rat

io(%

)

CPT-1CPT-2

0 200 40048

0 200 40048

Tip

Res

ista

nce

(tsf)

Fric

tion

Rat

io(%

) Tip

Res

ista

nce

(tsf)

Fric

tion

Rat

io(%

)

SPT

Blo

wco

unts

(N

)

Soil

Cla

ssifi

catio

n (U

SCS)

Backfill

RunFill

(22)

Clay / Silt(Lagoonal Clay)

Marine

Sand to Silty Sand(Lakewood-San

Pedro Formation)-120

-100

-80


CPT Data Evaluation 0 40

-80

-60

-40

-20

0

20

Recorded SPT Blowcounts

Interpreted SPT Blowcounts from CPT Sounding

GroundEl. = +13 ft

6020

Field Measured BlowcountN (bpf)60

Elev

atio

n (ft

)

DescriptionInterpreted/Observed Soil

B-1 CPT-3

SM

SM

SP-SM

CL

ML

MH

SP-SCSC

SP

SM

MLSM

SM

SP

CL

ML

CLSP/SCML/SMSP-SM

ML

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Evaluation of Liquefaction PotentialSimplified procedure based on empirical observations

Used for most routine projects

More rigorous numerical modelingSites where liquefiable soils extend to significant depthsSites that have significant interlayeringSites where ground remediation costs are high


Simplified procedure for Evaluating Liquefaction Potential

Factor of Safety =

CRR determined from empirical charts based on SPT or CPT dataCSR determined from design peak ground accelerations

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

depth

CSRCRR

Liquefiable zone

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Simplified Procedure: CRR from SPT Data


Simplified Procedure: CRR from CPT Data

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Simplified Procedure: CSR Determination


Liquefaction Hazard AssessmentFlow FailuresLimited Lateral SpreadsGround Settlement

Choice of Factor of Safety – Depends on:Vulnerability of StructureAcceptable Level of RiskDamage PotentialDesign Earthquake MagnitudeValues to Warrant Hazard Assessment could range from 1.0-1.3

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Flow FailuresPotential massive translational failure when static factor of safety <1 where post-liquefaction undrained residual strength mobilized.


Lateral SpreadingProgressive down slope deformation under cyclic inertial loading during time intervals when F.S.<1Four approaches to assess the magnitude of lateral spread displacement:1. Youd et. al empirical approach2. Newmark time history analyses3. Simplified Newmark Charts4. Numerical Modeling

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Youd et. al empirical approach


Newmark time history analyses

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Newmark time history analyses

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.00 0.05 0.10 0.15 0.20 0.25 0.30

ky (g)

Gro

und

Dis

plac

emen

t (ft)

HM HM-

LP LP-

IMP IMP-

DUZ DUZ-

ERZ ERZ-

IMPV IMPV-

KOBE KOBE-

upperbound

lowerbound


Newmark time history analysis – Simplified Charts

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Evaluation of Soil Settlement HazardTokimatsu and Seed (1987) Methodology

Evaluation of Surface Fault Rupture Hazard

Studies to Refine Definition of Hazard

Location of fault tracesTiming of most recent slip activelyGround rupture characteristics

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Evaluation of Surface Fault Rupture Hazard (cont.)

Fault Displacement Evaluation

Observation from past earthquakesEmpirical relationship to earthquake magnitudeValues based on fault slip rate

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