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  • Cyclic Behavior of Sand and Cyclic Triaxial Tests

    Hsin-yu ShanDept. of Civil Engineering

    National Chiao Tung University

  • Causes of Pore Pressure Buildup due to Cyclic Stress Application

    Stress are due to upward propagation of shear waves in a soil deposit during earthquakeStructure of the cohesionless soil tends to become more compact

    Transfer of stress to the pore waterReduction in stress on the soil grains

  • Soil grain structure rebounds to the extent required to keep the volume constantVolume reduction and soil structure rebound determines the magnitude of the increase in pore water pressure increaseAs the pore water pressure approaches a value equal to the applied confining pressure

    the sand begins to undergo deformations

  • If the sand is loose:Pore pressure increase suddenly to a value equal to the applied confining pressureThe sand will rapidly begin to undergo large deformations with shear strains exceeding around 20% or moreIf the sand will undergo unlimited deformations without mobilizing significant resistance to deformation it can be said to be liquefied

  • If the sand is dense:It may develop a residual pore water pressure (a peak cyclic pore pressure ratio of 100%)When the cyclic stress is reapplied on the next stress cycle, or if the sand is subjected to monotonic loading The soil will tend to dilatePore pressure will drop if the sand is undrainedThe soil will ultimately develop enough resistance to withstand the applied stressLarge deformation will develop during the process

  • Effect of Partial Drainage

    There will be some drainage in the fieldAdd some margin of safety against cyclic mobility or liquefactionTo ignore the effect of partial drainage is on the conservative side

  • Evaluating Liquefaction or Cyclic Mobility Potential

    Methods based on observation of performance of sand deposit in previous earthquakeMethod based on stress conditions in field and laboratory determinations of stress conditions causing cyclic mobility or liquefaction of soils

  • Observation Method

    Based on the location of the points representing the data set (N1, /0) relative to the curve representing the lower bound for sites where liquefaction occurredN1 is the corrected SPT-N value

    0

    = cyclic ratio causing liquefaction

  • dav r

    ga

    0

    0max

    0

    65.0

    amax = maximum acceleration at the ground surface0 = total overburden pressure on sand layer under consideration0 = effective overburden pressure on sand layer under considerationrd = a stress reduction factor varying from a value of one at the ground surface to a value of 0.9 at a depth of 10 m

  • Experience with the Method

    The lower bound curve is strongly supported by abundant data from Japan and ChinaWorks satisfactorily with the data from 921 earthquakeConservative for earthquakes with lesser magnitudes involving shorter duration of shaking

  • Limitations of the Method

    Need for additional reliable data points to better define the lower bound of causing cyclic mobility or liquefaction at high values of av/0Need to understand more about the significant factors affecting cyclic mobility or liquefaction

    Duration of shaking, magnitude of earthquake

  • Penetration resistance may not be an appropriate index of the cyclic mobility characteristics of soilsThe standard penetration resistance of a soil is not always determined with reliability in the field and its value may vary significantly depending on the boring and sampling conditions

  • Factors Affecting the Cyclic Mobility Characteristics of Sand

    Density or relative density Grain structure or fabric Length of time the sand subjected to sustained pressures Value of K0 Prior seismic or other shear strains

  • Factors Affecting the N Value

    The use of drilling mud vs. casing for supporting the walls of the drill holeThe use of a hollow stem auger vs. casing and waterThe size of the drill holeThe number of turns of the rope around the drum

  • The use of a small or large anvilThe length of the drive rodsThe used of nonstandard sampling tubesThe depth range over which the penetration resistance is measured

  • Evaluating Liquefaction or Cyclic Mobility Potential

    Methods based on observation of performance of sand deposit in previous earthquakeMethod based on stress conditions in field and laboratory determinations of stress conditions causing cyclic mobility or liquefaction of soils

  • Methods Based on Field/Lab Stress Conditions

    An evaluation of the cyclic stresses induced at different levels in the deposit by the earthquake shakingA laboratory investigation to determine the cyclic stresses which, at given confining pressures representative of specific depths in the deposit, will cause the soil to develop a peak cyclic pore pressure ratio of 100% or undergoes various degrees of cyclic strain

  • Compare the results of the two evaluation:The cyclic stresses induced in the field with the stresses required to cause a peak cyclic pore pressure ratio of 100%An acceptable limit of cyclic strain in representative samples in the lab

  • 5 Basic Procedures Need to be Developed

    Suitable analytical procedures for evaluating stresses developed in an earthquakeSuitable procedure for representing the irregular stress history produced by the earthquake by an equivalent uniform cyclic stress seriesSuitable test procedure for measuring the cyclic stress conditions causing a peak pore pressure ratio of 100% or intolerable level of strain in the soil sample

  • Understanding of all the factors having a significant influence on the cyclic mobility or liquefaction characteristics of soilsUnderstanding of the effects of sample disturbance on the in-situ properties of natural deposits

  • Methods for Evaluating Stresses Induced by Earthquake Shaking

    Ground response analysis that neglects the pore pressure buildupProcedure that takes into account the pore pressure generated in the soilSimplified procedure based on a knowledge of the maximum ground surface accelerationDeconvolution of a known ground surface motion

  • Ignoring pore pressure build up during earthquake may not be particularly significantMay lead to somewhat conservative results in some cases

  • Converting Irreg. Stress His. into Equiv. Unif. Cyclic Stress Series

    Because it is usually more convenient to perform lab tests using uniform cyclic stress applications than to reproduce the actual field stress historyThere were three methods can be used and their differences have little effect on the final results

  • Three basic methods:By estimation from a visual inspection of the irregular time history involvedBy a weighting procedure for individual stress cycles use an experimentally-determined pore pressure responseA cumulative damage approach based on Miners law and involving the natural period of the deposit and the duration of earthquake shaking

  • Suitable Test Procedures

    Cyclic simple shear testsMultidirectional shaking in simple shear testsCyclic triaxial compression tests

  • Cyclic Direct Simple Shear

    DSS Roscoe-typeFour platesPure shear is applied to horizontal and vertical planeDifficulties

    Preparation of representative samplesDevelopment of uniform shear strains throughout the samplesApplication of uniform stress conditionsAvoidance of stress concentrations

  • Very long and shallow samplesStress concentrations are limited to small areas at the endsLonger samples less affected by the stiffness of the walls of the sample container

  • Cyclic Triaxial Compression Tests

    Equipment less complicated and more available than DSSDo not reproduce correct initial stress conditions for NC soils or in a simple shear test

  • Other limitationsStress concentrations at the cap and baseA 90 rotation of the direction of major principal stress during the two halves of the loading cycleNecking may develop and invalidate the test data beyond this point in the testIntermediate principal stress does not have the same relative value during the two halves of the loading cycleDifficult to achieve a high degree of accuracy for stress ratio not representative of field values

  • Cyclic triaxial stress ratio is higher than that for simple shear condition

    triaxial3shear simple2

    =

    dc

    rc

    h c

    cr = correction factor ranging from 0.5 1.0, increases with K0

  • Factors Influencing Cyclic Mobility or Liquefaction Characteristics

    Grain characteristicsRelative densityMethod of soil formation (soil structure)Period under sustained loadPeriod under sustained load

  • Previous strain historyIncrease the stress ratio

    Lateral earth pressure coefficient and overconsolidation

    Larger K0 higher stress ratioCyclic mobility and liquefaction characteristics of in-situ deposits

    Disturbance lower the stress ratio

    Cyclic Behavior of Sand and Cyclic Triaxial TestsCauses of Pore Pressure Buildup due to Cyclic Stress ApplicationEffect of Partial DrainageEvaluating Liquefaction or Cyclic Mobility PotentialObservation MethodExperience with the MethodLimitations of the MethodFactors Affecting the Cyclic Mobility Characteristics of SandFactors Affecting the N ValueEvaluating Liquefaction or Cyclic Mobility PotentialMethods Based on Field/Lab Stress Conditions5 Basic Procedures Need to be DevelopedMethods for Evaluating Stresses Induced by Earthquake ShakingConverting Irreg. Stress His. into Equiv. Unif. Cyclic Stress SeriesSuitable Test ProceduresCyclic Direct Simple ShearCyclic Triaxial Compression TestsFactors Influencing Cyclic M

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