#8 liquefaction part 2 - gregg drilling & testing, inc ... · p.k. robertson 2013 definitions...

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Evaluation of soil liquefactionusing the CPT – Part 2

P.K. Robertson

2013

Definitions of Liquefaction

• Cyclic (seismic)Liquefaction– Zero effective stress

(during cyclic loading)

• Flow (static)Liquefaction– Strain softening

response

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Cyclic (seismic) Liquefaction

• Zero effective stress dueto undrained cyclicloading

• Shear stress reversal

– Level or gently slopingground

• Controlled by size andduration of cyclicloading

• Large deformationspossible

Moss Landing

Kobe

Flow (static) Liquefaction

• Strain softening response in undrainedshear

• Trigger mechanism required

• Static shear stress greater than minimum(liquefied) undrained shear strength

• Kinematic mechanism required

– Uncontained flow

– Contained deformation

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Schematic undrained response ofsaturated, contractive sandy soil

After Olson & Stark, 2003

Flow chart toevaluate

liquefaction

After Robertson, 1994

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Flow (static) liquefactionsteeply sloping ground

Sites defined as:

Steeply sloping (> 5 degrees) or earth embankments(e.g. dams)

Sequence to evaluate flow liquefaction:

1. Evaluate susceptibility for strength loss

2. Evaluate stability using post-earthquake shearstrengths

3. Evaluate trigger for strength loss

If soils are susceptible, and instability possible, it is often prudentto assume trigger will occur

Evaluate susceptibility forstrength loss

• Soils must be strain softening in undrainedshear

• Strain softening soils are CONTRACTIVE atlarge strains

• Can we identify soils that are eitherCONTRACTIVE or DILATIVE (at largestrains) using CPT?

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State parameter in coarse-gainedsoils (sands)

State Parameter after Jefferies and Been, 1985 Relative State Parameter indexafter Boulanger, 2003

‘Loose’

(-) DenseDILATIVE

‘Dense’

(+) LooseCONTRACTIVE

y

State Parameter from CPT

• Jefferies and Been (2006) summarized ~30yrs ofresearch related to evaluation of liquefaction using aCritical State Soil Mechanics (CSSM) approach

• Problem is complex & depends mainly on: in-situstresses, shear stiffness, shear strength, compressibility& plastic hardening

• Requires combination of in-situ tests (SCPT) and labtesting (reconstituted samples to get CSL)

• Based on extensive calibration chamber test results, fieldresults (frozen undisturbed samples), lab testing &numerical simulation – estimate of state from CPT

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State Parameter

Approx.contours of soilstate for young,

uncementedsoils

(shape controlled bysoil compressibility –

i.e. slope of CSL)

Boundary for strain-softening response

y > -0.05

(After Robertson, 2009)

Same in-situ StateDifferent penetration resistance

DILATIVE

CONTRACTIVE

Seismic liq. – case histories

• Based on the early work of Seed & Idriss(1971) penetration resistance (SPT & CPT) hasbeen used to evaluate the ‘state’ of sandy soilsto evaluate the potential for liquefaction basedon extensive case histories

• Concept of “clean sand equivalent” (Qtn,cs) isused to extend liq. evaluation to wider range ofsandy soils (Robertson & Wride, 1998)

Qtn,cs = Kc Qtn [Kc = fn (Ic)]

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Seismic liq. case histories

Updated Moss database

Clean sand equivalent, Qtn,cs

Soils with same‘clean sand

equivalent’ Qtn,cs

have similarbehavior

(based on casehistories with

young,uncemented soils)

Increased resistanceto loading

same ‘clean sand equivalent’penetration resistance

- same in-situ state

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State parameter & clean sand equivalent

Robertson, 2013

Based on liq. case historiesBased on CSSM theory & CC



~ 0.56 – 0.33 log Qtn,cs

Qtn,cs = 70 at y = -0.05

DILATIVE

CONTRACTIVE

Link between Qtn,cs and y

• CPT-based clean sand equivalent, Qtn,cs curvesbased on case histories of seismic liquefaction

• CPT-based State Parameter (y) curves basedon CSSM theory, laboratory, field andnumerical studies

• Both produce very similar relationships

• Hence, CPT Qtn,cs can be used to estimate in-situ state

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Dilative/Contractive boundary

CPT-based boundary

Jefferies and Been (2006)suggested that when

Y > - 0.05soils are strain softening at

large strain

Y = - 0.05Qtn,cs ~ 70

DILATIVE

CONTRACTIVE


Case histories – flow liquefaction

• Many publications (over past 25 years) thathave studied case histories:

– e.g. Seed (1987), Seed & Harder (1990), Stark &Mesri (1992), Wride et al (1999), Olson & Stark(2002), Jefferies and Been (2006), Olson &Johnson (2008) and Robertson (2010)

– Some uncertainty with some seismically triggeredcases that involved either strength loss (flow liq.)or cyclic liq./modility (lateral spreading)?

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Case histories – flow liquefaction• Case histories involve soils that are:

– Very young

– Very loose

– Non- or low-plastic

• ~78% recent fills

• ~40% hydraulically placed

• ~16% tailings

• ~50% triggered by earthquakes

• several triggered by very minor disturbance

Failures tend to occur without warning, arerapid and can flow large distance

Stava, Italy, 1985; 268 deaths, 190,000m3

Kolontar, Hungary,2010, 700,000m3No warning

Very fast

Travel longdistances

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Case histories - flow-liquefaction

All case histories fall in‘contractive’ portion of

CPT SBT chart

DILATIVE

CONTRACTIVE


Nerlerk (sand) – 19,20,21Jamuna (sand) - 34Fraser River (silty sand) - 27Sullivan mines (silty tailings) - 35Northern Canada (silty clay) – 36L. San Fernado Dam (silt) – 15

CPT data in critical layers +/- 1 sd.

Case histories with CPT

Tennessee Valley, Kingston CoalAsh Failure 2008

April, 2008 Pre-slide

December, 2008 Post-slide

After AECOM: www.tva.gov/kingston/rca/

5 million m3 flowed ~1 km

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Kingston failure sequence



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Cross section through slide




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Laboratory test results (TVA)

Most of the Ash andSlimes are contractive andstrain softening at largestrain

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Fine-grained - Slimes• High water content: 40 to 140%

• Very high Liquidity Indices: 3 to 7

– Sensitivity, St > 30 (i.e. brittleness > 0.98)

• Low undrained shear strength at large strain

When water content > Liquid LimitSensitivity can be high

Post-failure CPT (CPT 09-104)

Fly Ash

Slimes

Data from AECOM: www.tva.gov/kingston/rca/

Clay

Alluvium

Weakest slimes

Confirmed with adjacent borehole & samples

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Post-failure CPT (CPT 09-104)

Fly Ash

Slimes

Clay

Contractive

Dilative

Weakest slimes

Peak

CPeT-IT software: www.geologismiki.gr

TVA Fly Ash Failure

CPT results(+/- 1 standard

deviation) in coarse-grained fly ash and

fine-grained (slimes)

Weakest slimes

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Ground Improvement example

After Campanella et al., 1983

Ground Improvement example

Before After

SPT, N 5 7

CPT, qt (MPa) 0.45 1.0

Peak, su/s’v 0.2 1.2

CRR (CTX) 0.15 >0.29

Silt went from softnormally consolidated

(before) to stiffoverconsolidated (afterground improvement) –Contractive to Dilative

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Contractive/Dilative boundary• Case histories suggest that when Qtn,cs < 70

soils are potentially susceptible to strainsoftening and hence, flow liquefaction

• Criteria of Qtn,cs < 70 slightly conservative sincemean + one standard deviation of CPT data fallbelow this criteria

– if using mean values, Qtn,cs < 50 to 60 maybe moreappropriate

– but very thin weak layers may not be adequatelycaptured by ‘mean’ values

Contractive/Dilative boundary

CPT-based SBT chart provides a reasonableestimate of soils that are likely eitherCONTRACTIVE or DILATIVE (at large strains)

Are all CONTRACTIVE soils at risk for flow(static) liquefaction?

– depends on brittleness (sensitivity), strain to peakstrength and static shear stress

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Generalized soil behavior(undrained shear – NC fine-grained soils)

Low PI soils tend to have smaller strain to peak strengthand tend to be more brittle

After Ladd et al, 1977

Low PIHigh PI

Both CONTRACTIVE

τ

γ

Su(p)

Su(min)

High Brittleness

Low Brittleness

Brittleness versus undrained strength ratio

Range for NC soils

Regardless of fabric and direction of loading

After Yoshimine et al, 1999

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Liquefied undrained strength ratio

Most casehistories have acalculated liq.strength ratiosu/s’v ~ 0.1 or

slightly smaller

DilativeContractive


Liquefied strength ratio relationships based on normalized CPTtip resistance

Modified from Olsen & Stark (CGJ, 2002)

Robertson, 2010Clean sand equivalent

Low Brittleness

High Brittleness

Dilative

Olsen & Stark (2002)

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Generalized CPT Soil Behaviour Type

A

B C

D

CPT Soil Behaviour

A: Drained-dilative

B: Drained-contractive

C: Undrained-dilative

D: Undrained-contractive

Regions of potential liquefaction

Cohesionless soils (A1 & A2) - Evaluatepotential behavior using CPT-based case-historyliquefaction correlations.

A1 Cyclic liquefaction possible depending onlevel and duration of cyclic loading.

A2 Cyclic liquefaction and post earthquakestrength loss possible depending on loading andground geometry.

Cohesive soils (B & C) – Evaluate potentialbehavior based on in-situ or laboratory testmeasurements or estimates of monotonic andcyclic undrained shear strengths.

B Cyclic softening possible depending on leveland duration of cyclic loading.

C Cyclic softening and post earthquake strengthloss possible depending on soil sensitivity, loadingand ground geometry.

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Summary• Strong link between ‘clean sand equivalent’

cone resistance (Qtn,cs) based on seismic casehistories and State Parameter (y)

• Contractive soil response when Qtn,cs < 70(based on mean +1 SD)

• Young, very loose, non- to low plastic soilstend to be more brittle than older, denser, moreplastic soils

Summary

• Since case histories show that failures canoccur without warning, are rapid and can flowlarge distances – “Observation approach” maynot work

If soils are susceptible, and instability possible(i.e. FS << 1.0), it is often prudent to assume

that a trigger will occur - unless you can designaway all possible triggers

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Questions?

#8 liquefaction part 2 - gregg drilling & testing, inc ... · p.k. robertson 2013 definitions...

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