cyclic behavior of sand and cyclic triaxial tests · cyclic behavior of sand and cyclic triaxial...

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, τ/σ’0) relative to the curve representing the lower bound for sites where liquefaction occurredN1 is the corrected SPT-N value

0στ′

= cyclic ratio causing liquefaction

dav r

ga

0

0max

0

65.0σσ

στ

′≈

′

amax = maximum acceleration at the ground surfaceσ0 = total overburden pressure on sand layer under considerationσ’0 = 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 τav/σ’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 Miner’s 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

=

′ σ

σστ 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