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
