State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences (2021) / Chapter Skim
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2 A Primer on Earthquake-Induced Soil Liquefaction
2 A Primer on Earthquake-Induced Soil Liquefaction
Pages 27-48
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From page 27... ...
These include both empirical and numerical analyses, which need to be validated and calibrated using field data before they can be considered reliable. This chapter describes, in general terms for the less technical of this report's readers, the basic behavior of liquefiable soils under cyclic loading, including the effects of material type, load amplitude and duration, soil density, initial effective stress, initial shear stress, and age of the soil deposit.
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From page 28... ...
shear loading may either decrease or increase in volume depending on its initial density and initial effective confining stress and on the levels of induced shear strain. Initially loose soil typically will tend to contract (i.e., decrease in volume and become denser)
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Behavior Under Monotonic Shear Loading Monotonic shear loading refers to load conditions under which the shear stress (or principal stress difference or deviator stress) or shear strain in the soil increases without change of direction.
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From page 30... ...
In dense soils, the tendency for dilation leads to reduced porewater pressure and increased effective stress. Saturated soils with initial states that plot well above the CVR line are highly contractive and, after reaching a peak shearing resistance at low strains, will generate high pore pressures with concurrent large reductions in effective stress.
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From page 31... ...
stress-void ratio curves for loose and dense sands at the same effective confining pressure subject to drained loading. Note that the loose and dense soils eventually converge on the same shear resistance and void ratio as shear strain increases.
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From page 32... ...
Behavior Under Cyclic Shear Stress Reversal Earthquake loading may be characterized by repeated shear stresses of fluctuating intensity, with the added characteristic that the direction of the applied shear stress reverses. Shear stress reversal is an important characteristic of the earthquake loading, as both loose and dense soils tend to contract at small induced shear strains and therefore generate positive excess porewater pressures when subject to shear stress reversal (Martin et al., 1975)
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In some cases (e.g., for sites with no initial static shear stress on the horizontal plane and for loose soil sites where cyclic loading induces stress reversal) ru can approach and may actually reach 1.0 (at which point the effective stress is zero)
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From page 34... ...
Figure 2.6 presents the results of cyclic simple shear laboratory tests performed without an initial static shear stress on the horizontal plane on soils of the same density but different effective confining pressures (Vaid and Sivathayalan, 1996)
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From page 35... ...
Figure 1 below illustrates the behavior of a soil subjected to harmonic undrained loading in cyclic simple shear without initial static shear stress on the horizontal plane. More detailed discussion of laboratory testing methods can be found in Chapter 5.
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From page 36... ...
Excess pore pressure increases with more cycles and the effective vertical stress decreases.
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From page 37... ...
Effect of Load Amplitude, Duration, and Density The excess pore pressure generated in a monotonic loading test on loose, saturated sand increases with increasing shear stress, so the excess pore pressures under cyclic loading should be expected to increase at a faster rate as the amplitude of the cyclic loading increases. An increase in the duration of strong shaking should also be expected to increase excess pore-pressure generation and the potential for triggering liquefaction.
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From page 38... ...
: shear-induced volume change is more closely related to cyclic shear strains than to cyclic shear stresses. Figure 2.8 shows the relationship between pore-pressure ratios and shear strain amplitude developed by Dobry and Ladd (1980)
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From page 39... ...
.3 Water from areas of high excess porewater pressure will often flow toward the ground surface. Sand boils, the most common surficial evidence of liquefaction, may form when water and entrained soil particles are ejected at the ground surface from a shallow liquefied soil layer (see Figure 2.9)
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From page 40... ...
Therefore, the evaluation of liquefaction triggering requires establishing consistent measures of earthquake loading and liquefaction resistance. Early investigations of liquefaction resistance were based on laboratory testing to develop cyclic strength
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From page 41... ...
This approach requires development of procedures to establish a representative earthquake-induced shear stress and the number of uniform loading cycles as a function of earthquake magnitude, as discussed in the next section. Ground Motion Intensity Measure For most liquefaction analyses, the intensity of the design ground motion is characterized by the peak cyclic shear stress and an earthquake magnitude.
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From page 42... ...
As a result, stress-corrected blow counts (N1) , energy corrected blow counts (N60)
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From page 43... ...
Alteration of Ground Motion The response of buildings, bridges, pipelines, and other elements of infrastructure underlain by liquefiable soils will be strongly influenced by how liquefaction affects the characteristics of the ground surface motions. Ground motion frequency change often occurs suddenly when liquefaction is triggered as a result of the rapid reduction in shear stiffness at high pore-pressure ratios.
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From page 44... ...
to measure porewater pressures and with geophones (seismic CPT) to measure shear wave velocities.
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From page 45... ...
When initial shear stresses are less than the residual strength of the soil (i.e., the shearing resistance of the liquefied soil at large strain) , but the seismically induced stresses exceed the residual strength, lateral displacement builds up incrementally during the earthquake and ceases when the cyclic loading stops and results in lateral spreading.
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From page 46... ...
FIGURE 2.11 Mechanism of delayed embankment failure due to redistribution of excess pore pressure following earthquake shaking: (a) high pore pressure levels are generated below center of the embankment where initial, static shear stresses are low; (b)
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From page 47... ...
deep embedment and thick crust. The magnitude of the moment induced on the pile depends on the thickness of the crust, the thickness of the liquefied layer, and the magnitude of the lateral displacement; the worst case scenario is a thick crust laterally displacing above a thin liquefied soil layer, as the thin liquefied layer localizes the bending in the pile within a narrow zone.
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From page 48... ...
(b) FIGURE 2.13 Damage to container cranes caused by liquefaction-induced lateral spreading of earth-retaining structures in the 1995 Hyogo-ken Nanbu (Kobe, Japan)
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