soil liquefaction - presentation june 2009
TRANSCRIPT
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Seminar for the California Geoprofessionals Association
Soil Liquefaction During Earthquakes
The Cliffs Notes Version
Ross W. BoulangerIrvine, California
June 11, 2009
This seminar is based on:
Materials from the Monograph (MNO-12) published by EERI in 2008, and
Materials presented at the EERI Seminars by I. M. Idriss & R. W. Boulanger in Pasadena,
St. Louis, San Francisco & Seattle, on March 9, 11, 16 &18, 2009, respectively.
http://www.eeri.org/cds_publications/catalog/
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Plot summary
Fundamentals of liquefaction behavior Avoid confusion by being explicit with definitions.
The role of excess pore pressure diffusion.
Triggering of liquefaction New SPT and CPT curves: How they compare to others and when the
differences can be important for you.
Residual shear strength New recommendations that include consideration of void redistribution
effects.
Lateral spreading and post-liquefaction settlements Making decisions from incomplete information.
Cyclic softening of clays and plastic silts Choosing appropriate engineering procedures.
Fundamentals of
liquefaction behavior
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Figure 8. Stress paths for monotonic drained loading with constant p' and
undrained loading (constant volume shearing) of saturated loose-of-critical
and dense-of-critical sands
Figure 16. Undrained cyclic triaxial test (test from Boulanger & Truman 1996).
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Figure 17. Undrained monotonic versus cyclic-to-monotonic loading for loose-
of-critical sand (after Ishihara et al. 1991)
Figure 27. Undrained cyclic simple shear loading with an initial static shear
stress ratio of 0.31 (test from Boulanger et al. 1991).
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Figure 43. Two mechanisms by which void redistribution contributes to
instability after earthquake-induced liquefaction (NRC1985, Whitman 1985)
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Figure 44. A water film that formed beneath a silt seam in a cylindrical
column of saturated sand after liquefaction (Kokusho 1999)
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Figure 45. Localization of shear deformations along a lower-permeability
interlayer within a saturated sand slope (Malvick et al. 2008)
Take home points
"Liquefaction" means different things to different people use
more specific technical terms to avoid confusion in technical
discussions.
Critical state soil mechanics is a useful tool for appreciating
the different behaviors of various soils over a range of
densities and confining stresses.
In situ shear strengths can be affected by the diffusion of
excess pore pressures during and after shaking.
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Triggering of liquefaction
ratio0.4
0.5
0.6Curves derived by
Seed & Idriss (1982)
Seed et al (1984) & NCEER/NSF Workshops (1997)
Idriss & Boulanger (2004)
Seed (1979)
Cetin et al (2004)
1
2
3
4
5
3
4
21
5
Cyclicstres
0.1
0.2
0.3
FC 5%
Liquefaction
Figure 66 Curves relating CRR to (N1)60for clean sands with
M = 7 and 'vc= 1 atm.
Corrected standard penetration, (N1)60
0 10 20 30 40 0.0
No Liquefaction
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A primary contributor to the differences between Cetin et al, NCEER
and Idriss & Boulanger is the differences in rd.
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Other notable sources of differences are:
Figure 60 Overburden normalization factor CN: (a) dependence on
denseness, and (b) simpler approximations often used at shallower depths.
Figure 64 K
relationships derived from Rrelationships (from Boulanger and Idriss 2004).
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Figure 69 Comparison of liquefaction procedures by Idriss and Boulanger
(2006) to those from the NCEER/NSF workshop (Youd et al. 2001): (a) ratio of
CRR values, and (b) ratio of FSliq
Figure 70 Comparison of liquefaction procedures by Cetin et al. (2004) to
those from the NCEER/NSF workshop (Youd et al. 2001): (a) ratio of CRR
values, and (b) ratio of FSliq
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Figure 76 Comparison of liquefaction analysis procedures from
Idriss and Boulanger (2006), Cetin et al. (2004), and NCEER/NSF
(Youd et al. 2001) for FC=35%.
Is there a depth, like 50 ft (or 15 m) below which we dont
need to consider liquefaction as being possible?
EERI seminar participants
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Influence of depth on liquefaction:
Mechanisms affecting:
Soil strengths
Seismic loads
Consequences
Empirical observations must have a theoretical
basis for understanding how our experiences from
one site may relate to another.
Limitations in how analysis methods handle the role
o ep .
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Figure 67 Curves relating CRR to qc1Nfor clean sands with M = 7 and = 1 atm
Adjustment for fines content (FC)
tio
0.5
0.6
35
Fines Content, FC[data points from Moss (2003)]
82
Robertson & Wride (1997)Ic= 2.59; FC = 35%
Idriss & Boulanger (2004)for clean sands
Cyclicresistancer
0.1
0.2
0.3
0.4
9275
75
35
40
5074 65
75
86
42
65
Figure 77 (a) Comparison of field case histories for cohesionless soils with
high fines content and the curves proposed by (a) Robertson & Wride (1997)
for soils with Ic= 2.59 (apparent FC = 35%)
Normalized corrected CPT tip resistance, qc1N
0 50 100 150 200 250 0.0
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atio
0.5
0.6
35
Fines Content, FC[data points from Moss (2003)]
82 66
Suzuki et al (1997)
2.25 Ic< 2.4Idriss & Boulanger (2004)
for Clean SandsSuzuki et al (1997)
2 Ic< 2.25
Cyclicresistance
0.1
0.2
0.3
0.4
9275
75
35
40
5074 65
75
86
42
65
Normalized corrected CPT tip resistance, qc1N
0 50 100 150 200 250 0.0
Figure 77 (b) Comparison of field case histories for cohesionless soils with
high fines content and the curves proposed by (b) Suzuki et al (1997) for Icvalues of 2.0 2.4
ceratio
0.4
0.5
0.6
35
Fines Content, FC[data points from Moss (2003)]
86
42
82
65
66
Derived Curvefor FC = 35%
Idriss & Boulanger (2004)for Clean Sands
Cyclicresistan
0.1
0.2
0.3
9275
75
35
40
5074 65
Normalized corrected CPT tip resistance, qc1N
0 50 100 150 200 250 0.0
Figure 79 Comparison of field case histories for cohesionless soils
with high fines content and a curve recommended for cohesionless
soils with FC = 35%
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Curves relating CRR to (N1)
60for clean sands and sands with
non-plastic fines have largely stabilized.
Curves relatin CRR to for clean sands are stabilizin but
Take home points
the effects of fines content are subject to further refinements.
Extrapolation of liquefaction correlations to depths larger than
are covered empirically requires a sound theoretical basis.
Consequences of liquefaction:
Residual Shear Strength
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Ratio,
Sr
/'v
o
0.3
0.4
Recommended Curvefor conditions where
void redistribution effectsare expected to be negligible
Group 2 &Group 3
Group 1
ResidualShearStrength
0.1
0.2
Recommended Curvefor conditions where
void redistribution effectscould be significant
Seed (1987)
Seed & Harder (1990)
Olson & Stark (2002)
Equivalent Clean Sand SPT Corrected Blowcount, (N1)60cs-Sr
0 5 10 15 20 25 30 0.0
Figure89 Normalizedresidualshearstrengthratioofliquefiedsandversusequivalent
cleansand,correctedSPTblowcountbasedoncasehistoriespublishedbySeed(1987),Seed
andHarder(1990),andOlsonandStark(2002)
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Take home points
An understanding of strength loss mechanisms is provided by
laboratory testing and physical modeling studies.
Case histories implicitly account for void redistribution.
The relationships presented in the Monograph reflect the current
understanding and capabilities for modeling this phenomenon.
More work in this area is needed.
Consequences of liquefaction:
Lateral spreading and post-liquefaction
reconsolidation settlements
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Lateral spreading analyses
Approaches Empirical
Newmark sliding block analyses
Integrate potential strains versus depth
Nonlinear dynamic analyses
None capture all
the physical
phenomena.
Figure
91.
From
Rausch
1997
Site
characterization
is a major
source of
uncertainty.
Figure 98. How LDI vectors may relate to the extent of lateral spreading
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Figure 98. How LDI vectors may relate to the extent of lateral spreading
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Take home points
Appropriate site characterization is essential for identifyingand quantifying liquefaction hazards.
Simplified procedures for estimating liquefaction-induced
ground deformations are inherently limited in their accuracy by
the fact they cannot account for all the physical mechanisms
or initial conditions.
The insights from various types of analyses, even if their
accuracy is limited, can still guide effective decision making.
Cyclic softening in
clays and plastic silts
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What is liquefaction & what is cyclic softening?
Using "liquefaction" to describe ground failure in both sands andlow-plasticity clays implies:
a common behavior, and
An interpretation problem
a common se o eng neer ng proce ures.
If a silt/clay is deemed "liquefiable", it is common to use SPT- andCPT-based liquefaction correlations
E.g., NCEER/NSF workshop (e.g., Youd et al. 2001)
Recommendations to sample and test "potentially liquefiable"silts/clays are often not heeded.
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Question: What is the best way to estimate the potential for strength
loss & large strains in different types of fine-grained soils?
Reposing the question
r, w a ypes o ne-gra ne so s are es eva ua e us ngprocedures modified from those for sands, versusprocedures modified from those for clays?
Terminology:
"Sand-like" (or cohesionless) refers to soils that behave likesands in monotonic and cyclic undrained loading. Onset ofstrength loss and large strains is "liquefaction."
" "-in monotonic and cyclic undrained loading. Onset of strength
loss and large strains is "cyclic softening."
Atterberg limits of fine-grained soils exhibiting
sand-like versus clay-like behavior
Distinguishes between soils whose seismic behaviors are bestevaluated using different engineering procedures.
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Figure 135. Schematic of transition from sand-like to clay-likebehavior for fine-grained soils
Figure 136. Relationship among sensitivity, LI, and effectiveconsolidation stress (after Mitchell and Soga 2005)
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"Liquefaction" procedures for cohesionless soils
Semi-empirical correlations based on in situ penetration tests.
Consequences depend on relative density (e.g., bad if loose, notso bad if dense).
"Cyclic softening" procedures for cohesive soils
Procedures based on estimation of undrained shear strength(e.g., may include correlations, in situ tests, lab tests).
Consequences depend on sensitivity (e.g., bad for quick clays,. ., n .
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20
30
40
Index,
PI
Soils reported by Bray et al. (2004b) tohave liquefied at Adapazari in 1999.
CHorOH
Criteria by Bray et al. (2004a):
(1) PI12 & wc>0.85LL: susceptible to liquefaction.
(2) 120.8LL: systematically more resistant
to liquefaction but still susceptible to cyclic mobility.
Comparing criteria The common message
Boulanger & Idriss (2004, 2006) andBray et al. (2004, 2006).
PI < 4 no issue Anal ze usin
0 10 20 30 40 50 60
Liquid Limit, LL
0
10Plasici
MLorOL
CL or OL MHor
OH
CL-ML
PI20
PI12
40
50
I
Clay-like behavior
Intermediate
Sand-like behavior CH
liquefaction correlations.
PI > 20, no issue; Analyze usingprocedures for clays.
4 PI 20
Agree soil may develop high
0 10 20 30 40 50 60
Liquid Limit, LL
0
10
20
30
PlasticityIndex,
P
Transition inbehaviors
CL-ML
CLorOL
ML or OL
7
4
MHorOH
orOHu, , .
Call it liquefaction, cyclicsoftening, or XYZ?
Issue: How best to evaluateXYZ behavior?
Do not use the Chinese Criteria.
Potential for cyclic softening of clay-like or cohesive fine-grainedsoils is best evaluated usin rocedures that are similar to or
Take home points
build upon, established procedures for evaluating the monotonicundrained shear strength of such soils (e.g., Boulanger & Idriss2004).
Fine-grained soils transition from behavior that is best analyzedas "clay-like" versus "sand-like" over a narrow range of PI values.
Fine-grained soils with PI
7 are best analyzed as clay-like. These
criteria may be refined on the basis of site specific testing.
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Life is the art of drawing sufficient conclusions from
insufficient premises.
Samuel Butler (1612 1680)