CPT Applications -Liquefaction 1

Peter K. Robertson

CPT in Geotechnical Practice

Santiago, Chile

July, 2014

What is Soil Liquefaction?

• Loss of strength and/or stiffness due toundrained loading

• Major factor in earthquakes

– Sand boils

– Ground cracks

– Slumping of embankments

– Lateral spreading

– Ground oscillations

• Flow slides

– Statically or dynamically triggered

Definitions of Liquefaction

• Cyclic (seismic)Liquefaction– Zero effective stress

(during cyclic loading)

• Flow (static)Liquefaction– Strain softening

response

– Part 2

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

Cyclic Liquefaction – Lab Evidence

Zero stiffness

Liquefaction is not ‘yes’ or ‘no’ – it’s a gradual processwith increasing pore pressures and decreasing effective

stress with reducing soil stiffness.

Liquefaction evaluationAfter Seed et al 2003

Liquefaction is a multi-step process with liquefactioninduced deformations often the primary goal.

Mitigation is warranted when consequences threaten theproject not when triggering is likely

Simplified Procedure

• Following the 1964 earthquakes in Alaska andNiigata, Japan, the “Simplified Procedure” wasdeveloped by Seed & Idriss (1971) for evaluatingseismic demand and liquefaction resistance ofsands

• CSR7.5,1 = 0.65 (amax/g) (sv/s’v) rd / (MSF*Ksigma)

• CRR7.5,1 = fn of penetration resistance (SPT & CPT)

• NCEER Workshop (1996/7) to developconsensus update paper (Youd et al, 2001)

Cyclic liquefaction – design process

Sites defined as:

Level ground, gently sloping (< 5 degrees) or levelwith nearby steep slope or free-face

Sequence to evaluate (cyclic) liquefaction:

1. Evaluate susceptibility to cyclic liquefaction

2. Evaluate triggering of cyclic liquefaction

3. Evaluate post-earthquake deformations

Sand-like and Clay-like soils

Sand-like soils

• Fine-grained soils that are essentially non-plastic and behavevery similar to sands

• Cyclic resistance within framework based on in-situ tests

Clay-like soils

• Clays and plastic silts that are more easily sampled, are lessaffected by sampling disturbance and exhibit stress-historynormalized strength properties.

• Cyclic resistance estimated based on in-situ testing, laboratorytesting and empirical correlations based on undrained shearstrength

Susceptibility to cyclic liquefaction

Seed et al, 2003

Bray & Sancio, 2006

CPT SBT

Sand-like

Clay-like

Ic = 2.6(+/-)

Behavior Characteristics

Physical Characteristics

Transition from sand to clay-like behavior

Case history field observations

• Holocene age, clean sand deposits (Ko ~ 0.5)

• Level or gently sloping ground

• Corrected to magnitude M = 7.5 earthquake

• Depth range 1 to 12m

– 95% < 10m

• Surface observations

of liquefaction

• Average CPT values

Case history range of values• CPT tip resistance: 10 < qc < 120 atm (av = 52atm, stdev = 38)

• Friction ratio: 0.1% < Fr < 3% (av = 0.86%, stdev = 0.5%)

• SBT Index: 1.4 < Ic < 2.6 (av = 2.0, stdev = 0.28)

• Fines content: 0 < FC < 85% (most < 20% & mostly NP-low PI)

• EQ magnitude: 6.0 < Mw < 9.0 (av = 7.0)

• PGA: 0.09 < a(max) < 0.84g (av = 0.21g, stdev = 0.14g)

• Depth of liquefaction: 1 < zliq < 12m (av = 4.4m, stdev = 1.2m)

• Vert. eff. stress: 0.19 < s’vo < 1.5 atm (av=0.5 atm, stdev=0.24)

• Holocene-age, uncemented, silica-based soil (~NC)

• 27% ‘Non-Liq’ case out of 253 total

Origin of CPT-based methods

All methods have similar origins:

Case histories (each summarized to 1 data point)

– CSR7.5,s’=1 = 0.65 (amax/g) (sv/s’v) rd / MSF * Ks

– Normalization (qc1N) and ‘fines’ correction to getnormalized clean sand equivalent (qc1N,cs or Qtn,cs)

Each method made different assumptions for: rd, MSF, Ks,normalization of qc & ‘fines correction’

RW98 Moss06 IB08&14

Cyc

lic

Str

ess

Rat

io,(

CS

R)

Challenges with samplingProblem with corrections based on‘fines content’:• soil behaviour not controlled onlyby fines content• random sampling every 1.5 m (~5ft) – will get a limited view of actualsoil conditions

Better to select sampling depth basedon adjacent CPT

Updated database on SBTn chart

Data after Boulanger & Idriss, 2014

Ic = 2.6

All cases have CPT SBTnIc < 2.60

Robertson, 1990

Case Histories

Modified Moss database

Clean sand equivalent, Qtn,cs

Soils with same‘clean sandequivalent’Qtn,cs have

similarbehavior

Increased resistanceto loading

Same ‘clean sand equivalent’penetration resistance

- same in-situ state

Consequences of Liquefaction

• Post-earthquake settlement caused byreconsolidation of liquefied soils, plus possibleloss of ground (ejected) and localized shearinduced movements from adjacent footings, etc.

• Lateral spreading due to ground geometry

• Loss of shear strength, leading to instability ofslopes and embankments – strain softeningresponse – flow liquefaction

Factors that can affectliquefaction-induced deformations• Soil Characteristics

– relative density (state), fines content, plasticity, age, degree ofsaturation, cementation, prior stress & strain history, in-situ stress state,depositional environment

• Earthquake Characteristics– level and duration of shaking, frequency content of motions

• Site Characteristics– topography, geometry, stratigraphy, lateral variability, static

piezometric profile, hydraulic conductivity, pore water & void re-distribution

• Other factors– 3-D effects, ground cracking effects, low permeable surface layer, near

by foundations

Post Earthquake Deformations

• Estimating deformations in sandy soils isdifficult (even under static loading)

• For low to moderate risk projects semi-empirical models are common & appropriate

– CPT-based methods provide continuous profiles ofvolumetric & shear strain

• For high risk projects numerical analyses canbe appropriate, if initial screening indicates aneed

CPT-based post earthquakedeformations

(Zhang, Robertson & Brachman, 2002 & 2004)

• Based on extensive laboratory test results

– Ishihara and Yoshimine (1992)

• Links CPT and factor of safety to volumetric& shear strains for clean sands

• Apply CPT ‘clean sand equivalent’ approachto get profiles of volumetric & shear strainsand hence, displacements

• Calibrated with case histories

Ishihara and Yoshimine (1992)

1.0

F.S.

ev

Evaluation of CPT Settlements• Zhang et al. (2000) showed:

– Good results when applied toMarina District (1998) andTreasure Island (1998) casehistories

– Importance of thin layers andtransition zones

– Importance of other factors (3-D, depth, proximity to footings,ejected soil, etc.)

• Lee et al. (2000) showed:

– Good results when applied toTaiwan (1999) case histories

Challenges estimating verticalsettlements

Liquefiedsoil

Liquefiedsoil

Liquefiedsoil

Liquefiedsoil

(a) (b) (c) (d)

Lateral Spreading

INITIAL SECTION

DEFORMED SECTION

Evaluation of Lateral SpreadApproach

• D. Chu & J. Stewart (2006) showed that Zhanget al (2004) CPT and Youd et al (2002) SPTmethods produced good results (slight over-predictions).

• Layers below base elevation of free face (z >2H) should not be included.

Schematic of howLDI vectors

influence extent oflateral spreading

After Idriss & Boulanger, 2008

Transition zoneCPT data in‘transition’when

cone is moving from one soil typeto another when there is

significant difference in soilstiffness/strength (e.g. soft clay to

sand)

CPT data within transition zonewill be misinterpreted

In interlayered depositsthis can result in

excessive conservatism

Ahmadi & Robertson, 2005

Transitionzone

detection

Based on rate ofchange of Ic near

boundary of Ic = 2.60

Very important forliquefaction analysis

“CLiq” softwarewww.geologismiki.gr

Depth of liquefaction

Ishihara, 1985

Ishihara (1985) showed thatsurface damage from

liquefaction is influenced bythickness of liquefied layer and

thickness of non-liquefiedsurface layer.

Cetin et al (2009) proposedsimple weighting of vol. strainwith depth to produce similar

results

Summary

• CPT can provide estimates of:– Potential for cyclic liquefaction & softening (via CRR)– Post earthquake settlement and lateral displacement profiles

• Updates on:– Stress normalization– Extended to include clay-like soils

• No need for an Ic cut-off– Identification of transition zones

• Seismic CPT provides two independentmethods to evaluate soil response in same soil

Summary• Recommend removing transition zones

– CLiq provides auto feature to remove

• Recommend ‘weighting’ strains with depth

– CLiq provides ‘weighting’ feature

• Recommend sensitivity analysis to evaluatesensitivity of output (deformation) to mainvariables (e.g. EQ load, etc.)

• Often no single answer – requires somejudgment