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5 Alternative Approaches to Liquefaction Triggering Assessment
Pages 104-125

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From page 104...
... One is a screening tool -- regional maps that delineate areas susceptible to liquefaction triggering largely on geologic grounds. Other methods described include a cyclic strain-based alternative to the cyclic stressbased approach, energy-based approaches, laboratory and physical model tests, field measurement of pore-pressure generation under dynamic loading, and computational mechanics approaches.
From page 105...
... and cyclic shear strain (γ) for a given number of constant amplitude strain cycles in strain-controlled tests falls in a relatively narrow band, particularly at low strains and pore-pressure ratios, across different sands prepared at different relative densities using different techniques and tested at different effective confining pressures.
From page 106...
... This method ignores the effect of pore-pressure generation on the shear modulus of soil. Given the computed earthquake-induced shear strain and the number of strain cycles, the pore-pressure ratio can be estimated from a curve such as shown in Figure 5.2.
From page 107...
... Correlations developed for the number of equivalent stress cycles could be used (see, e.g., Liu et al., 2001) to evaluate the average shear strain based on an average shear modulus, but this approach assumes that the equivalency of stress cycles (which is based on a cyclic strength curve)
From page 108...
... . These investigators coupled this modified seismic wave energy with an empirical relationship that relates dissipated energy per unit volume of soil to excess pore-pressure generation to develop an expression for seismic demand.
From page 109...
... . Nevertheless, the use of Arias intensity as the sole intensity measure for quantifying seismic demand for evaluating liquefaction triggering potential has been questioned (see, e.g., Green and Mitchell, 2003)
From page 110...
... Nevertheless, the efficacy of centrifuge model tests for liquefaction studies has been questioned given sample preparation issues, grain size and permeability effects, and the influence of boundary conditions unique to centrifuge models. There have been few advances in laboratory testing for liquefaction triggering assessment since the 1985 report (NRC, 1985)
From page 111...
... Both approaches are intended to enforce a zero lateral strain boundary condition representative of vertically propagating shear waves in a horizontally layered soil profile. Cyclic loading in a CyDSS test is usually applied via application of a shear force or shear displacement on a horizontal plane to either the top or the bottom of the specimen (with the other end fixed)
From page 112...
... Annular brass rings (inside diameter equal to the diameter of the top cap) enforce a zero lateral strain condition representative of vertically propagating shear waves.
From page 113...
... CyDSS, CTX, and cyclic hollow cylinder torsional shear tests can all be used to study patterns of behavior that affect the liquefaction triggering potential of soils in situ (e.g., the effect of the initial effective stress level and the effect of an initial static shear stress) on reconstituted samples.
From page 114...
... Due to the expense and difficulty in obtaining relatively undisturbed samples through ground freezing or other special techniques, laboratory testing for site-specific evaluation of liquefaction triggering in sandy and gravelly soils is generally limited to projects of high value, where there are critical consequences, and for research purposes. A technique that allows costeffective recovery and testing of cohesionless soil samples such that in situ cyclic resistance was retained would facilitate laboratory testing for liquefaction triggering evaluation of such soils in situ for more routine projects.
From page 115...
... If enough specimens are available, laboratory testing can be used to generate the complete cyclic strength curve. The cyclic strength curve still must be related to the specific value of the CRR, however, to use in the liquefaction potential assessment based on Neq.
From page 116...
... FIGURE 5.8 Relationship among laboratory cyclic strength curve, Neq (assumed to equal Nliq)
From page 117...
... Considering the importance of Neq to both laboratory assessment of liquefaction triggering potential and to the MSF used in the simplified method for liquefaction triggering assessment and their interrelationship, this issue deserves further attention. Approaches for site-specific liquefaction assessment that employ laboratory testing also include a hybrid approach in which the laboratory test results are used to calibrate a numerical pore-pressure generation model and the numerical model is then used to assess the liquefaction triggering potential of the soil.
From page 118...
... The rigid walls may also create multiple reflections of the simulated earthquake waves off the sides of the soil container, resulting in complex stress states that do not represent field stress states. Therefore, both types of shake table tests (1-g and centrifuge)
From page 119...
... The use of large-scale 1-g shake table tests for site-specific evaluation of liquefaction triggering is also problematic given issues associated with preparation of a large-scale soil deposit in the model that is representative of the soil in situ (as discussed above with respect to preparation of specimens for laboratory testing)
From page 120...
... . If centrifuge testing can be shown to be reliable for the study of liquefaction triggering, it can provide a means to evaluate many factors that influence triggering, including the same factors investigated with other types of laboratory testing (e.g., the influence of density or relative density, grain size distribution [including fines content]
From page 121...
... . Unlike laboratory tests, neither the cyclic shear strain nor the cyclic shear stress amplitudes induced by vibroseis loading are constant after porewater pressures are generated.
From page 122...
... Other models that can be used in the loosely coupled approach relate pore-pressure generation directly with the induced shear strain and number of cycles of loading (see, e.g., Dobry et al., 1982; Vucetic, 1986; and Matasovic and Vucetic, 1993)
From page 123...
... Without dilatancy and the associated reduction in excess pore pressure, the loosely coupled approach may shield upper layers from the earthquake ground motions when deeper layers liquefy. This "base-isolation" effect may appear beneficial, but it may not be realistic in the field due to lateral variability, shaking due to non-vertically propagating shear waves or surface waves, and the transmission of shaking during instances of dilation-induced stiffening (Anderson et al., 2011)
From page 124...
... empirical estimates of CRR-SPT blow count relationship and (b) overburden stress adjustment factor Kσ.
From page 125...
... As discussed above, however, questions about the interpretation of the results and concerns about the adequacy of these models as the soil approaches liquefaction limit their applicability for predicting liquefaction triggering. Further, although studies have been conducted to evaluate the ability of nonlinear, total stress site response analysis to predict site response (see, e.g., Stewart and Kwok, 2008)


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