li qui faction 1

Upload: angela-briggs

Post on 03-Jun-2018

228 views

Category:

Documents


0 download

TRANSCRIPT

  • 8/12/2019 Li Qui Faction 1

    1/34

    2014/3/8 1

    Discussion onsand liquefaction and

    its static approach

    Dongdong Chang

  • 8/12/2019 Li Qui Faction 1

    2/34

    2014/3/8 2

    Introduction Liquefaction of saturate sands

    Static approachsteady state approach Some divergent opinions

    Difficulties in this approach

    Suggestions

  • 8/12/2019 Li Qui Faction 1

    3/34

    2014/3/8 3

    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

  • 8/12/2019 Li Qui Faction 1

    4/34

    2014/3/8 4

    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
  • 8/12/2019 Li Qui Faction 1

    5/34

    2014/3/8 5

    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

  • 8/12/2019 Li Qui Faction 1

    6/34

    2014/3/8 6

    Related terms and concepts

  • 8/12/2019 Li Qui Faction 1

    7/342014/3/8 7

    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
  • 8/12/2019 Li Qui Faction 1

    8/342014/3/8 8

    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

  • 8/12/2019 Li Qui Faction 1

    9/342014/3/8 9

    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

  • 8/12/2019 Li Qui Faction 1

    10/342014/3/8 10

    3-D Location of SSL

  • 8/12/2019 Li Qui Faction 1

    11/342014/3/8 11

    Critical state

    = tanult

    CSL

    d

    d =

    d

    d =

    d

    d = 0

    v

    ee = e - ln0

    ln

    Straight line e

    CSL

  • 8/12/2019 Li Qui Faction 1

    12/342014/3/8 12

    Critical state

    Steady state Steady state is developed in empirical

    manner

    Critical state is on theoretical basis

    Essentially the same, can be used

    interchangeably

  • 8/12/2019 Li Qui Faction 1

    13/342014/3/8 13

    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

  • 8/12/2019 Li Qui Faction 1

    14/34

  • 8/12/2019 Li Qui Faction 1

    15/34

    2014/3/8 15

    Flow liquefaction surface FLS)

  • 8/12/2019 Li Qui Faction 1

    16/34

    2014/3/8 16

    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
  • 8/12/2019 Li Qui Faction 1

    17/34

    2014/3/8 17

    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)

  • 8/12/2019 Li Qui Faction 1

    18/34

    2014/3/8 18

    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

  • 8/12/2019 Li Qui Faction 1

    19/34

    2014/3/8 19

    Figure showing zones of flow liquefaction and cyclic mobility susceptibility

  • 8/12/2019 Li Qui Faction 1

    20/34

  • 8/12/2019 Li Qui Faction 1

    21/34

    2014/3/8 21

    Steady-State approach

    to sand-liquefaction

  • 8/12/2019 Li Qui Faction 1

    22/34

    2014/3/8 22

    Steady-State deformation

    Static triaxial test stress paths for three specimens of different

    densities (very loose, medium, and dense) (Castro 1966)

  • 8/12/2019 Li Qui Faction 1

    23/34

    2014/3/8 23

    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

  • 8/12/2019 Li Qui Faction 1

    24/34

    2014/3/8 24

    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

  • 8/12/2019 Li Qui Faction 1

    25/34

  • 8/12/2019 Li Qui Faction 1

    26/34

    2014/3/8 26

    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

  • 8/12/2019 Li Qui Faction 1

    27/34

    2014/3/8 27

    A divergent opinion

    Is the SSL or CSL unique?

  • 8/12/2019 Li Qui Faction 1

    28/34

    2014/3/8 28

    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

  • 8/12/2019 Li Qui Faction 1

    29/34

    2014/3/8 29

    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

  • 8/12/2019 Li Qui Faction 1

    30/34

    2014/3/8 30

    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

  • 8/12/2019 Li Qui Faction 1

    31/34

    2014/3/8 31

    So...

    not invalidate the steady-state approach

    but makes it inappropriate for the backanalysis of actual liquefaction failures

  • 8/12/2019 Li Qui Faction 1

    32/34

    2014/3/8 32

    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

  • 8/12/2019 Li Qui Faction 1

    33/34

  • 8/12/2019 Li Qui Faction 1

    34/34

    2014/3/8 34

    Thanks