State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences (2021) / Chapter Skim
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7 Empirical and Semiempirical Methods for Evaluating Liquefaction Consequences
7 Empirical and Semiempirical Methods for Evaluating Liquefaction Consequences
Pages 136-160
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From page 136... ...
The complexity of the phenomena that follow liquefaction triggering has prompted the development of a variety of empirical and semiempirical procedures used by practicing engineers. These include screening procedures that employ damage indices to determine if the consequences of liquefaction are of engineering concern; procedures to evaluate the potential for lateral spreading, flow sliding, and slope instability; and procedures to evaluate the performance of deep and shallow foundations, earth retaining structures, subsurface utilities, and buried structures.
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From page 137... ...
AASHTO (2014) specifications state that a liquefaction consequence assessment is not required for bridges designed to a life safety standard if the 5% damped design spectral acceleration at one second corrected for local site conditions, SD1, is less than 0.15 g, or 0.3 g if the liquefiable soil is low-plasticity silt.
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From page 138... ...
If settlement due to liquefaction beneath a non-liquefiable capping layer is of engineering concern, the engineer may wish to evaluate the potential for such settlement using methods discussed subsequently and to consider the engineering implications of the calculated settlement. The concept described in Figure 7.1, however, is being used in New Zealand as guidance for mitigating liquefaction impacts to structures on shallow foundations by creating a uniformly nonliquefiable crust beneath the structure (van Ballegooy et al., 2015)
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From page 139... ...
As opposed to the Ishihara (1985) procedure described above for determining the influence of a non-liquefiable crust over a single liquefiable layer, the LPI applies to a profile with multiple liquefiable layers.
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From page 140... ...
Although LPIISH shows improved predictive capabilities over LPI, it nevertheless can still yield incorrect predictions of liquefaction consequences. As a result, there is still a need for further development of liquefaction damage indices if they are to be considered reliable.
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From page 141... ...
Furthermore, it has been found that damage indices based on the cumulative liquefaction response or volumetric strain potential of a soil profile are poor predictors of lateral spreading or flow sliding due to thin but continuous seams of liquefiable soil. Even for non-lateral spreading manifestations, the severity thresholds reported in the literature for any of the aforementioned severity indices need to be used with caution.
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From page 142... ...
Development of a reliability assessment framework that allows comparison of different simplified approaches to evaluating flow slide potential, and research on the parameter values required to implement such an assessment, are warranted and would provide a consistent means to incorporate uncertainties into flow sliding analyses. No documented cases exist of flow failures that resulted from liquefaction of soils with an equivalent clean-sand normalized SPT blow count, (N1)
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From page 143... ...
Nevertheless, quantifying the uncertainty associated with these models to facilitate their use (e.g., to quantify the confidence limits on anticipated lateral spreading displacements) would benefit the technical community.
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From page 144... ...
60 ≤ 15 Grain size Fines content a Denotes a regional model that includes earthquake magnitude, distance from the site to the fault, peak ground acceleration, and the duration of strong ground motion. b Denotes a site model that included the parameters above and general characteristics of the site (e.g., ground slope, height of the free face)
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From page 145... ...
This material may be found at http://ascelibrary.org/doi/abs/10.1061/ %28ASCE%291090-0241%282000%29126%3A4%28360%29. Integration of Permanent Shear Strains The permanent shear strain approach integrates the earthquake-induced permanent shear strain potential over depth to calculate a lateral spreading displacement at the ground surface.
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From page 146... ...
2004. Estimating liquefaction-induced lateral displacements using the standard penetration test or cone penetration test.
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From page 147... ...
The limited thickness of a thin seam of liquefiable soil would result in calculation of very small lateral displacements for even large permanent shear strain values, contrary to observations of lateral spreading due to thin liquefiable seams in the field. An improved level of accuracy and more information on the limitations and reliability of these models are needed to facilitate the use of these methods for detailed engineering analysis.
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From page 148... ...
of the liquefied soil; ground loss due to venting of liquefied soil (i.e., sand boils or ejecta) ; settlement associated with lateral spreading under zero volume change; and settlement due to soil-structure interaction ratcheting and bearing capacity failure.
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From page 149... ...
associated with lateral spreading movements can be evaluated using the assumption of zero volume change of the sliding mass and the estimated lateral displacement. More detailed evaluation of the vertical settlement that accompanies lateral spreading likely requires using the computational-mechanics-based methods discussed in Chapter 8.
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From page 150... ...
Idriss and Boulanger (2008) also modified these relationships to be a function of SPT blow count or CPT tip resistance.
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From page 151... ...
. While there are no quantitative analyses for total and differential settlement due to ground loss following liquefaction, the liquefaction severity indices discussed previously appeared to correlate well with observations of excessive settlement in Christchurch and may provide at least an indication of how serious a problem liquefaction-induced settlement may be.
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From page 152... ...
(2009b) relationships predicting post-liquefaction volumetric strain as a function of the clean sand SPT blow count and earthquake-induced CSR.
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From page 153... ...
There is a lack of consensus as to whether deep foundation elements in liquefiable soil are subject to downdrag, wherein settlement of a non-liquefiable crust due to reconsolidation of underlying liquefied soil pulls down on the sides of the pile. Logically, it is not necessary to consider downdrag loads imposed by reconsolidation if the liquefied soil is above the neutral plane of a driven pile.
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From page 154... ...
The reduced lateral resistance of liquefied soil in a p-y analysis is usually accounted for in one of three ways: (1) by using a reduced undrained shear strength in the liquefied zone and developing the p-y relationship by applying rules to develop p-y curves for the undrained behavior of fine-grained soil (Abdoun, 1997; Wang and Reese, 1998)
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From page 155... ...
Two kinematic loading situations often encountered in practice are depicted in Figure 7.8. In the first case, the zone of liquefaction extends to the ground surface and the liquefied soil tends to flow around the piles, applying relatively modest loads on the piles.
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From page 156... ...
, many uncertainties remain in this type of analysis, including, the validity of using the Newmark sliding block method to determine free-field displacements; the uncertainties related to residual strength of liquefied soils and related p-y curves; the uncertainties related to the thickness of the liquefied layer and the displacement distribution in the layer; the influence of potential partial pore pressure increases in adjacent soil layers; and the accuracy of the method in determining the distribution of pile bending moments. FIGURE 7.8 Idealized loading of deep foundations subjected to lateral spreading.
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From page 157... ...
model test data, field data) with which to validate this approach.
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From page 158... ...
Damage to earth retaining structures in the Port of Kobe in the 1995 Hyogo-ken Nanbu earthquake, described briefly in Chapter 1, is a notable example of this kind of damage. Lateral earth pressures due to liquefied soil are generally accounted for using an equivalent fluid pressure concept.
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From page 159... ...
There have also been some salient observations of pipeline performance during earthquakes in liquefied soil. A notable example is the observation that fusion-welded polyethylene pipe, almost without exception, did not rupture when subject to liquefaction and lateral spreading in the 2011 Christchurch, New Zealand, earthquake despite ground displacements of a meter or more (O'Rourke et al., 2014)
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From page 160... ...
Similarly, nonlinear effective-stress site-response analyses have not been well validated in the post-triggering range and, therefore, need to be used with care. Additional ground motion recordings at borehole array sites that record shaking below the liquefaction layers and at the ground surface are needed to validate nonlinear effective-stress numerical models for site response.
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