State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences (2021) / Chapter Skim
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4 The Simplified Stress-Based Approach to Triggering Assessment
4 The Simplified Stress-Based Approach to Triggering Assessment
Pages 75-103
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From page 75... ...
Both the seismic demand and the liquefaction resistance are characterized as cyclic stress ratios, defined as the ratio of the cyclic shear stress (τcyc) to the initial vertical effective stress (𝜎 )
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From page 76... ...
𝜎 (𝑧) where τcyc,max is the maximum cyclic shear stress at depth z, 𝜎 is the initial vertical effective stress at depth z, PGA is the horizontal peak ground acceleration at the ground surface, g is the acceleration of gravity, 𝜎 is the total vertical stress, and 𝑟 is a depth-dependent shear stress reduction coefficient that accounts for the nonrigid response of the soil deposit.
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From page 77... ...
derived their MSFs directly from the liquefaction case history database by including earthquake magnitude in the regression analysis they used to establish the triggering relationship for liquefaction resistance (described in the next section)
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From page 78... ...
The depth-dependent shear stress reduction coefficient, 𝑟 , is a function of the nonrigid response of the soil deposit (characterized in the small strain regime by the shear wave velocity [Vs] profile at the site)
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From page 79... ...
Furthermore, effective stress dynamic response analysis with pore-pressure generation should never be used to estimate the seismic loading, because the seismic loading for the simplified, stress-based procedure ignores the effects of pore-pressure generation. Estimating the shear stress from dynamic response analysis requires more information than does the simplified equation: it requires the Vs distribution for the entire soil profile and the underlying bedrock; the strain-dependent nonlinear soil properties (e.g., variation of shear modulus and damping ratio with shear strain)
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From page 80... ...
are the standard penetration test (SPT) blow count (N)
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From page 81... ...
Rathje. CRR Relationships The many relationships available that correlate CRR with measured SPT blow count, CPT tip resistance, or Vs typically predict the cyclic resistance of the soil for a set of reference conditions: an initial vertical effective stress of 1 atmosphere, an M 7.5 earthquake, clean sand (i.e., less than 5% soil grains finer than 0.075 mm)
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From page 82... ...
for an M 7.5 earthquake and a vertical effective stress of 1 atmosphere. The relationships are similar for CPT tip resistances less than about 50 atmospheres, but they deviate more substantially at larger values, with the Idriss and Boulanger (2008)
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From page 83... ...
Each variant establishes a relationship between a normalized in situ parameter (e.g., SPT blow count, CPT tip resistance, or Vs) and the CRR for a standard reference condition (e.g., M = 7.5, σ′v = 1 atmosphere, and clean sand with fines content ≤ 5%)
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From page 84... ...
. NOTE: Plotted points represent the earthquake-induced CSR for each case history corrected to an M 7.5 earthquake, and the initial vertical effective stress, σvo′, is 1 atmosphere.
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From page 85... ...
Adjustment Factors Applied to CRR Adjustments to the measured in situ resistance parameter and CRR are required for situations that do not satisfy the standard reference conditions. The situations typically adjusted in simplified stress-based liquefaction triggering assessment are summarized in Table 4.1.
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From page 86... ...
Applied to CRR? a Fines Content Yes Noa Effective Stress Yes Yes Static Shear Stress No Yes a The adjustment for fines content may be applied to either the in situ parameter (i.e., SPT N value, CPT qc, shear wave velocity)
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From page 87... ...
Until the indirect methods are fully vetted, it is good practice to measure fines content at a site directly and to develop site-specific relationships between fines content and the soil behavior type index. Adjustments for Effective Stress The stress adjustment factors applied to correct in situ penetration test parameters to a vertical effective stress of 1 atmosphere were developed from data from large-scale calibration chamber tests (see, e.g., Marcuson and Bieganousky, 1977)
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From page 88... ...
at vertical effective stresses larger than 1 atmosphere, and some investigators have also incorporated soil mechanics principles (e.g., principles from critical state soil mechanics) to constrain these adjustment factors (see, e.g., Boulanger, 2003b)
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From page 89... ...
. A parameter α is defined as the ratio of the initial static shear stress to the normal effective stress on the horizontal plane, and Kα represents the ratio of the CRR for a given value of α to the CRR for α = 0.
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From page 90... ...
To use a CRR relationship in engineering practice appropriately, adjustment factors consistent with those used in the development of the CRR relationship must be employed. Engineers should not mix and match adjustment factors from different CRR relationships.
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From page 91... ...
. Ideally, any increase in liquefaction resistance with age would manifest itself in larger values of the in situ parameters used to evaluate CRR (e.g., SPT blow count, CPT tip resistance, or Vs)
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From page 92... ...
. Further research is needed to understand fully the effects of aging on liquefaction resistance and to develop rational and defensible methods for taking it into account.
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From page 93... ...
This example shows that the current probabilistic liquefaction charts based on Bayesian updating and a noninformed prior can be used to compute a posterior estimate of the probability of liquefaction that accounts for an informed prior. Reporting the likelihood ratio for the CRR curves directly, however, would make this computation easier and is preferable to relying on curves for constant values of probability based on non-informative priors.
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From page 94... ...
proposed liquefaction susceptibility criteria and liquefaction potential assessment procedures based upon the percent fines and Atterberg limits of the soil. The case history data supporting the development of these criteria are discussed in Chapter 3 in the section on liquefaction of fine-grained soils.
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From page 95... ...
FIGURE 4.5 Correlation relating soil behavior type index (Ic) and Apparent Fines Content.
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From page 96... ...
Their recommendation to sample and test all soils with FC ≥ 35% differs from the finding of Seed and colleagues (2003) -- that the liquefaction potential of soils in Zone A may be evaluated using the simplified procedure.
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From page 97... ...
This material may be found at http://ascelibrary.org/doi/abs/10.1061/%28ASCE%291090-0241%282006%29132%3A9%281165% 29. FIGURE 4.8 Liquefaction susceptibility criteria proposed by Boulanger and Idriss (2006)
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From page 98... ...
Both the BPT and the LPTs measure the number of blows to advance 30 cm and attempt to convert that number to an equivalent SPT blow count (see, e.g., Harder and Seed, 1986)
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From page 99... ...
GEOTECHNICAL FIELD DATA FOR LIQUEFACTION TRIGGERING ANALYSES Site characterization for an analysis of liquefaction triggering includes collection of information to accurately estimate the values of CRR and earthquake-induced CSR at the site. Calculation of the CRR requires in situ measurement of the profile of SPT blow count, CPT tip resistance, or Vs as a function of depth and at multiple locations across the site.
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From page 100... ...
to obtain blow counts and soil type; CPT soundings to obtain detailed profiles of in situ resistance; and Vs profiles to accurately assess the earthquake-induced CSR and to provide additional insights into the CRR. INTERPRETING THE RESULTS OF STRESS-BASED TRIGGERING ANALYSIS Assessments of liquefaction triggering may not match observed field behavior for many reasons, including the presence of unsaturated zones beneath the water table, horizontal stresses due to over-consolidation, or artesian conditions and inadequacies in the simplified methods.
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From page 101... ...
Box 4.3 presents a simple analysis that demonstrates that the uncertainty in the adjustment factors and parameters used in a liquefaction analysis can be larger than the uncertainty in the location of the CRR curve. The FS or probability of liquefaction is often computed for a single level of ground shaking and its associated earthquake magnitude.
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From page 102... ...
; CB is a correction factor for the borehole diameter; CR is a rod length correction factor; CS is a correction factor for the configuration of the SPT sampler; N is the recorded blow count; and Δ(N1) 60 is the correction factor for the fines content (percent passing the #200 sieve by weight)
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From page 103... ...
If the uncertainties associated with spatial variability in soil liquefaction resistance across a site were included in the analysis, the probabilities could increase further. TABLE 1 COVs for Parameters Used in Liquefaction Triggering Analysis Description Parameter COV Description Parameter COV Measured blow count N 0.10 CSR scale factor 0.65 factor 0.05 Each SPT borehole correction CB, CE, CN, 𝜎 0.02 Stress ratio 0.05 factor CR, and CS 𝜎 Magnitude scaling factor MSF 0.20 Peak ground acceleration PGA 0.10 Shear stress reduction CRR stress adjustment Kσ 0.07 rd 0.20 coefficient FIGURE 1 Probabilities of liquefaction due to the uncertain location of the CRR curve given certain input parameters and due to perfectly known location of the CRR curve given uncertain input parameters.
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