li qui faction 1
TRANSCRIPT
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Discussion onsand liquefaction and
its static approach
Dongdong Chang
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Introduction Liquefaction of saturate sands
Static approachsteady state approach Some divergent opinions
Difficulties in this approach
Suggestions
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Whats liquefaction
For a loose, saturate sand
Under earthquake or quick
loading Soil particles loses contact
with each other
When soil loses its strength and stiffnessand behaves like a fluid
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Where and how
Occurs in saturate sands,
commonly near rivers,
lakes, bays, and oceans
Foundation failures,
structures damages
Kobe, Japan, 1995
http://www.ce.washington.edu/~liquefaction/html/quakes/kobe/kobe.htmlhttp://www.ce.washington.edu/~liquefaction/html/quakes/kobe/kobe.htmlhttp://www.ce.washington.edu/~liquefaction/html/quakes/kobe/kobe.htmlhttp://www.ce.washington.edu/~liquefaction/selectpiclique/lakemerced/lakemer.jpg -
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Why liquefaction Rising pore water pressure
reduced effective stress
reduced shear strength
In extreme case
effective stress turns zero
loses shear strength
soil acts like fluid
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Related terms and concepts
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Critical Void Ratio
Behavior of dense and loose soils in monotonic strain
controlled triaxial tests (after Kramer, 1996)
http://www.ce.washington.edu/~liquefaction/html/references.htmlhttp://www.ce.washington.edu/~liquefaction/html/references.htmlhttp://www.ce.washington.edu/~liquefaction/html/references.htmlhttp://www.ce.washington.edu/~liquefaction/html/references.html -
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CVR line in e-logp space Critical void ratio varies with
effective confining pressure
A critical void ratio (CVR) line
in e-logp space constitute
the boundary between
dilative and contractivebehavior in drained triaxial
compression
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Steady State
2-D Projection of SSL in e-logp space
A steady state line (SSL) in
e-logp space (at large strains)
Boundary of flow liquefaction The difference between CVR
and SSL is the existence a
"flow structure", in which the
grains orient themselves sothe least amount of energy is
lost by frictional resistance
during flow
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3-D Location of SSL
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Critical state
= tanult
CSL
d
d =
d
d =
d
d = 0
v
ee = e - ln0
ln
Straight line e
CSL
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Critical state
Steady state Steady state is developed in empirical
manner
Critical state is on theoretical basis
Essentially the same, can be used
interchangeably
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Flow liquefaction A phenomenon when the
static equilibrium is destroyed
by static or dynamic loads
with low residual strength
Residual strength is the
strength of a liquefied soil
Earthquakes, blasting, and
pile driving are all example of
dynamic loads that could
trigger flow liquefaction
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Flow liquefaction surface FLS)
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A liquefaction phenomenon
Triggered by cyclic loading
Occurring with static shearstresses lower than soil strength
Deformations due to cyclic
mobility develop incrementally
Lateral spreading is a commonresult of cyclic mobility
Cyclic Mobility
1976 Guatemala earthquake
caused lateral spreading
http://www.ce.washington.edu/~liquefaction/selectpiclique/rivers/motagua.jpg -
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A key to understand
cyclic mobility is PTL
PTL:The stress path
points at which the
dense or medium
sands transform from
contractive to dilative
behavior
Phase transformation line(PTL)
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A stress path example Before PTL: contraction;
u increases, p' decreases
On PTL:no contraction ordilation; p' constant
After PTL: dilation;u decreases, p' increases Undrainded stress path
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Figure showing zones of flow liquefaction and cyclic mobility susceptibility
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Steady-State approach
to sand-liquefaction
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Steady-State deformation
Static triaxial test stress paths for three specimens of different
densities (very loose, medium, and dense) (Castro 1966)
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Limited liquefaction The stiffness of the soil depends on p',
the stiffness decreases (stress path
below the PTL) but then increases(stress path above the PTL)
This change in stiffness produces the"limited liquefaction
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Steady-State approach
Use the unique relationship
between shear strength andvoid ratio at high shear strains
Behavior of soil is dominated by
its initial state relative to SSL
Used for both loose and dense
sands
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Stress ratio (sinm) versus
Rate of dilation(sin) Rate of dilation is
function of stress ratio
(Wood 1990)
Whatever their density
or state, sands are
contractive at m < crit
dilatant at m > crit
0.3
0.4
0.5
0.6
0.7
0.8
-0.4 -0.2 0 0.2 0.4 0.6sin
sinm
Adapted from Wood 1990, after Stroud 1971
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A divergent opinion
Is the SSL or CSL unique?
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Uniqueness of SSL/CSL Specially designed simple shear tests show that
even dilatant sands reach a uniquecritical state inthe failure zone regardless of their initial density
(Cole 1967; Stroud 1971)
From series of tests, all theoretical influencingfactors of SSL (Strain rate, Sample preparationprocedures, Stress path, Consolidation stressprior to shear) do not influence true steady state(Mcroberts 1992)
True steady state or critical state is unique
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Problems of this approach Difficulties in determining the steady-state
line and the in-situ void ratio (very flat SSL for
sands is highly sensitive to its parameters)
Theoretical limitations in the validity of the
concept and the potential influence offactors that are not considered
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Limitations No claims to following aspects:
the potential for progressive failure
the magnitude of the disturbing force requiredto trigger liquefaction
the influence of in-situ stress state on
liquefaction potential effects of redistribution of void ratio in cyclic
loading
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So...
not invalidate the steady-state approach
but makes it inappropriate for the backanalysis of actual liquefaction failures
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Conclusions Useful in understanding the basic
mechanics of true liquefaction
Difficult to apply in practice highly sensitive to its parameters for sands
extremely difficult to measure requiredparameter with sufficient accuracy
might lack theoretical basis
might ignore factors that may be important
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Thanks