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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 2
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 3
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 4
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 5
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 6
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 7
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 8
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 9
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 10
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 11
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
×
Page 12
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2016. State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences. Washington, DC: The National Academies Press. doi: 10.17226/23474.
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Page 13

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Summary Earthquake-induced soil liquefaction (liquefaction) is a leading cause of earthquake damage worldwide. Liquefaction is often described in the literature as the phenomena of seismic generation of excess porewater pressures and consequent softening of granular soils. Many regions in the United States have been witness to liquefaction and its consequences, not just those in the west that people associate with earthquake hazards. Earthquakes in 1811 and 1812 caused extensive liquefaction in a region along the Mississippi River, stretching for approximately 150 km from the Memphis area north (called the New Madrid zone). In 1886 an earthquake caused widespread liquefaction and associated ground displacements in the Charleston, South Carolina, area. Liquefaction may have contributed to deaths associated with tsunamis by destabilizing delta fronts during the 1964 earthquake in Alaska. In 1971, slumping due to liquefaction nearly resulted in the overtopping of a dam and discharge of the reservoir at the terminus of the Los Angeles Aqueduct above the San Fernando Valley, an event that would have flooded thousands of homes in the valley below the dam. The city of Kobe, Japan, has yet to recover economically from liquefaction-related damage at the city’s port caused by the 1995 Hyogo-ken Nanbu earthquake. Liquefaction during the 2010-2011 earthquake sequence in Christchurch, New Zealand, led to the loss of 15,000 single-family homes and hundreds of buildings in the central business district. The most damaging of those earthquakes to the built environment had a relatively modest magnitude of 6.2. These examples illustrate the importance of accurate assessments of where liquefaction is likely and of what the consequences of liquefaction may be. Such assessments are needed to protect life and safety and to mitigate economic, environmental, and societal impacts of liquefaction in a cost-effective manner. Assessment methods exist, but methods to assess the potential for liquefaction triggering are more mature than are those to predict liquefaction consequences, and the earthquake engineering community wrestles with the differences among the various assessment methods for both liquefaction triggering and consequences. This report evaluates these various methods, focusing on those developed within the past 20 years, and PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW 1

2 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES recommends strategies to minimize uncertainties in the short term and to develop improved methods to assess liquefaction and its consequences in the long term. This report represents a first attempt within the geotechnical earthquake engineering community to consider, in such a manner, the various methods to assess liquefaction consequences. Liquefaction occurs when stresses and deformation in the ground caused by earthquake shaking disturb the soil structure (i.e., the arrangement of individual soil grains—the soil fabric) of saturated, geologically unconsolidated soils. Water in the pore spaces between soil particles will resist the natural tendency of the soils to consolidate into a denser and more stable arrangement during shaking. Because the soil cannot change in volume until water is drained from the pore spaces, porewater pressure will rise, soil particles may lose contact with each other, and the soil mass may lose much of its strength. This chain of events is referred to as liquefaction triggering. In this report, the term “liquefaction” refers not only to the conditions at the onset of liquefaction triggering but also to the phenomena that occur within the soil when in that triggered state (e.g., flow liquefaction). When liquefaction triggering occurs, the soil may lose much of its stiffness and strength and it may also become easier to deform and may flow laterally. Similarly, the soil may also lose its ability to support an overlying structure or buried utility. Whether or not liquefaction is triggered largely depends on the characteristics of ground shaking and on the density and initial effective (interparticle) stress of the soil. Other factors, such as soil type, soil fabric, age, and the orientation and levels of pre-earthquake shear stresses, also influence the potential for liquefaction triggering. Consequences of liquefaction also depend on a host of factors, including soil type and stratigraphy (layering), ground surface topography, and the engineered infrastructure near where liquefaction occurs. The ground may be displaced vertically or laterally, landslides may occur, embankments may slump, foundations may fail, and mixtures of soil and water may erupt at the ground surface. These effects may lead, in turn, to settlement, distortion, and the collapse of buildings; the disruption of roadways; the failure of earth-retaining structures; the cracking, sliding, and overtopping of dams, highway embankments, and other earth structures; the rupture or severing of sewer, water, fuel, and other lifeline infrastructure; the lateral displacement and shear failure of piles and pier walls supporting bridges and waterfront structures; and the uplift of underground structures. Methods for estimating liquefaction triggering and its consequences vary for many reasons. Practitioners, clients, and regulatory agencies may differ in their technical or financial capacities. Projects vary in size, type, and acceptable consequences and risk. What may be acceptable risk for buildings and roads in sparsely populated regions may not be for dams, bridges, port and harbor facilities, and nuclear power plants in more populated regions. Project requirements may differ further with respect to relevant guidance documents, standards, and code provisions. Analysis of liquefaction and its consequences remains one of the more active areas of research and development in geotechnical engineering. In 1998 a consensus was reached within the technical community on the use of an empirical stress-based approach for liquefaction triggering assessment—called the “simplified method”—first developed in 1971. This method is still the most commonly used in practice. By 2004, however, reputable groups suggested sets of changes to this procedure, with many of the groups focused on two alternative approaches. Since then, both of those alternative approaches have been refined, and additional methods and modifications have been suggested. Practitioners now must choose among multiple methods without necessarily understanding their limitations. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

SUMMARY 3 The current state of knowledge is limited regarding liquefaction triggering assessment associated with, for example, the degree of saturation below the groundwater table, at large depths, beneath sloping ground, in gravelly soils, in soils with a significant component of fine- grained particles (i.e., silt and clay), in Holocene-age soils due to aging effects, or with Pleistocene-aged (and older) soils. Few data are available to validate procedures for predicting the consequences of liquefaction triggering. Large uncertainties associated with the application of methods to predict both liquefaction triggering and its consequences exist. THE STUDY CHARGE In 2010 an ad hoc committee of the Earthquake Engineering Research Institute (EERI) called for a community-based consideration of the state of the art in liquefaction assessment to be conducted under the auspices of the National Academies of Sciences, Engineering, and Medicine (the National Academies). Such a study would help the technical community rebuild consensus on issues related to liquefaction triggering assessment and foster confidence in methods used to assess liquefaction triggering and its consequences. Under the sponsorship of the U.S. Bureau of Reclamation, Nuclear Regulatory Commission, Federal Highway Administration, American Society of Civil Engineers, Port of Long Beach (California), Port of Los Angeles (California), and Los Angeles Department of Water and Power, the National Academies convened a committee of 12 engineers and scientists to solicit input from the technical community and to critically examine the state of practice in liquefaction triggering and consequence assessment (see Chapter 1, Box 1.4 for the full statement of task). The committee was tasked to evaluate the following: the sufficiency, quality, and uncertainties associated with laboratory and in situ field tests, case history data, and physical model tests for understanding liquefaction triggering and post-triggering soil behavior; methods to analyze the data from those tests; and the adequacy and accuracy of empirical and mechanistic methods to evaluate triggering and resulting deformations in the soil and the structures built in, on, and of those soils. The committee’s report focuses on developments since the 1996 and 1998 National Science Foundation/National Center for Earthquake Engineering Research (NCEER) workshops where consensus was last reached on the topic of assessing liquefaction triggering. The report considers future directions for research and practice related to collecting, reporting, and assessing the sufficiency and quality of different types of data, to assessing the spatial variability and uncertainty in the data, and to developing more accurate assessment tools. The purpose of the study was not to adjudicate disputes regarding any specific methods. The committee’s report addresses liquefaction of only naturally occurring soils and fills composed of such soils (i.e., the report does not extend to mine tailings, solid waste, or stabilized soils). Discussion of remedial measures to increase liquefaction resistance and to mitigate post-liquefaction consequences is also outside the study scope. Recommendations in the report focus on how engineering practice with respect to liquefaction triggering and consequence assessment could be improved using existing tools and on future research that could improve practice. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

4 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES KEY FINDINGS AND CONCLUSIONS Key findings and conclusions from each chapter of the committee’s report are listed in the beginning of each chapter of the report and summarized in this section. These findings and conclusions, discussed in greater detail in the report, support the committee’s final recommendations presented in this summary and in Chapter 10. To evaluate liquefaction hazards, it is necessary to assess the susceptibility of a soil to liquefaction, the potential for liquefaction triggering, and the potential consequences of triggering given anticipated ground motions at a site. Not all soils are susceptible to liquefaction, but granular soils such as sands, silty sands, nonplastic silts, and gravels confined by lower- permeability layers are known to have liquefied in past earthquakes. Saturation is considered to be a necessary precondition for liquefaction. Laboratory testing currently cannot be used as the primary means to assess liquefaction potential in truly cohesionless soils. Instead, in situ testing of soil to assess its resistance to liquefaction is commonly used (e.g., through standard penetration testing [SPT], cone penetration testing [CPT], or shear wave velocity [Vs] testing), in spite of their own limitations. The basis of the most commonly used methods to predict liquefaction triggering in engineering practice is a simplified stress-based approach developed over 40 years ago. Whereas this approach is useful, its results are fraught with uncertainty. Methods used to predict the consequences of liquefaction triggering also include simplified approaches with high degrees of uncertainty. Additionally, the methods commonly used to predict liquefaction triggering and its consequences are generally not compatible with the current trend toward risk-based evaluation of seismic hazard (sometimes referred to as performance-based seismic design when referring to engineering design that is required to meet a certain measurable performance criteria). Uncertainties in Assessments Uncertainties are introduced into liquefaction assessments from many sources, beginning with unknowns related to site geology (e.g., the vertical and lateral extents of specific soil layers and groundwater conditions). The in situ test methods used to determine soil liquefaction resistance introduce additional uncertainties given the lack of truly standardized protocols for their use, as well as the lack of protocols to characterize relevant soil and profile properties (e.g., anisotropy in soil fabric; structure of gravelly soils). Incomplete knowledge of the influence of earthquake magnitude or duration, ground motion intensity, the in situ effective overburden stress, non-level ground conditions, and the amount of fine-grained particles in the soil introduce more uncertainties. Case histories of liquefaction are important in developing and validating liquefaction analysis procedures. Lack of protocols for documenting and rating the quality of case histories, however, introduce uncertainties. The same case history may be documented differently in databases assembled by different investigators. Even with a consistent standard for case history documentation, inconsistent information and interpretations can be found among the databases. For example, whether or not and where liquefaction has occurred or the location at which it has occurred within a soil profile; weathering and cementation characteristics; estimates of overburden stress; the layers most susceptible to liquefaction and their properties; the identification of geologic controls on soil properties that might affect liquefaction (e.g., lateral PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

SUMMARY 5 variation in the shape and soil content of a layer); the age of a soil; and estimates of prior seismic shaking have all been inconsistently interpreted or documented. These inconsistencies will contribute to uncertainties in liquefaction relationships developed from the case histories. Confidence in relationships derived from case history data (e.g., deterministic plots that relate resistance of soil to liquefaction and in situ test indices) is also affected by the lack of high-quality case history data points that plot close to and on either side of boundary lines that separate liquefaction and no-liquefaction zones. In particular, databases need case histories for critical depths (i.e., those at which liquefaction is known to have occurred) greater than about 15 meters, for earthquake magnitudes less than approximately 5.9 and greater than approximately 7.8, and for cases where liquefaction may reasonably have been expected but was not observed. The Simplified Stress-Based Approach for Assessing Liquefaction Triggering Several empirical liquefaction triggering relationships that are derived primarily from the case history data are applied to liquefaction assessments conducted with the simplified stress- based approach. Assumptions inherent with use of the simplified stress-based approach include a horizontally layered soil profile, horizontal ground surface, vertically propagating shear waves, and undrained soil response. These relationships differ in the use of the in situ parameter correlated to the soil’s cyclic resistance ratio (CRR), the various adjustment factors incorporated in the calculation of the CRR, and the earthquake-induced cyclic stress ratio (CSR) of the soil. Because each relationship was developed using an associated suite of adjustment factors, using adjustment factors developed for one relationship may result in increased uncertainty and error if applied to another relationship. Going forward, only the specific adjustment factors used for development of a particular liquefaction triggering relationship should be used with that relationship. Some adjustment factors are not well constrained over the entire range of engineering interest by the empirical data (e.g., the stress magnitude adjustment factor, K). In cases where factors are not well constrained by empirical data, adjustment factors should be developed using experimental data (including centrifuge and shaking table experiments) and engineering mechanics principles; they should not simply be based on statistical extrapolation of the data. No consensus exists regarding the appropriate adjustment factors for the presence of an initial static shear stress (i.e., K). Additional data and research are needed to allow better understanding of these effects. Other issues that require additional research include adjustment factors for the magnitude of the normal stress, the influence of geologic age on liquefaction resistance, the influence of earthquake magnitudes less than 7.0 or greater than 8.0, and how best to estimate the liquefaction potential of gravelly soils and soils with a substantial amount of silt and clay particles. Unbiased model parameters need to be employed in developing CRR curves and adjustment factors. The use of biased estimates of parameters (e.g., of the depth adjustment factor, rd) serves only to increase uncertainty regarding the results of an analysis. Significant reduction in uncertainties associated with the various adjustment factors used in the simplified approach is most dependent on expansion and improvement of the case history database, but mechanics-based methods can be used where data are insufficient. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

6 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES Alternative Approaches to Liquefaction Assessment Methods other than the simplified stress-based approach to assess liquefaction triggering include a cyclic strain-based alternative to the cyclic stress-based approach, energy-based approaches, laboratory tests and physical models, and computational mechanics-based methods. Further development of these methods could improve the accuracy and reliability of liquefaction assessment. Other tools, such as regional maps that relate liquefaction hazards to geologic map units, are used as screening and planning tools and in regional damage and loss assessments. Maps of probabilistic liquefaction hazards could be expanded in coverage, even to a national scale similar to the national seismic hazard maps. The complexity of phenomena subsequent to liquefaction triggering has prompted the development of various empirical and semiempirical procedures for use by earthquake engineers to assess potential consequences of liquefaction. 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. While most of these procedures are logically formulated, and some are supported by laboratory and model testing, they have undergone little field validation. The limitations of simple criteria and procedures to screen sites for severity and damage potential from liquefaction triggering are not often stated explicitly, and the applicability of many of those procedures has not been rigorously addressed. Similarly, empirical and semiempirical models to evaluate such liquefaction impacts as flow sliding, lateral spreading displacement, loss of bearing capacity, and increased lateral earth pressures and buoyancy effects, and to model loads induced on structures by liquefied soils, generally have not been validated rigorously. Lack of rigorous validation is due largely to the paucity of appropriately documented case histories of liquefaction consequences. Computational Methods Empirical and semiempirical approaches cannot account for all of the effects attributable to site-specific geology and topography, engineered structure configurations, and ground motion characteristics. When combined with appropriate generalizations of fluid flow relative to the solid phase, computational models can be used to solve boundary-value problems such as the deformation and pore-pressure response of a soil deposit with an overlying structure subjected to earthquake loading. Computational methods that use the principles of mechanics and incorporate appropriate constitutive relations (i.e., mathematical equations that describe the response of the soil to an external stress or strain) applied to a properly characterized site (i.e., a site characterized to a level of detail appropriate to the level of analysis) provide a means to describe, in detail, pore-pressure generation, redistribution, and dissipation; void (porosity) redistribution; soil and porewater flow; and soil-structure interactions. The accuracy with which a computational method describes the response of the soil to external loads (e.g., to the earthquake-induced ground motion) depends largely on the accuracy of the constitutive relations and input parameters employed. Continued development of new, more detailed, and more accurate constitutive and computational models for assessing liquefaction triggering and consequences are needed. In PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

SUMMARY 7 particular, constitutive models are needed that describe the behavior of soils near and after the triggering of liquefaction, as are computational models that capture post-triggering soil behavior. These computational models need to be supplemented by laboratory and physical model testing that both elucidate key aspects of the behavior of soils near and subsequent to liquefaction triggering and can be used to validate the constitutive and computational models. Performance-Based Evaluation and Design Current application of the stress-based method for liquefaction triggering assessment focuses on computing deterministic factors of safety versus depth for a given earthquake scenario. To be compatible with current developments in other areas of earthquake engineering (e.g., performance-based design of structures), the practice of liquefaction assessment needs to move toward fully probabilistic analyses that incorporate the complete range of possible damaging earthquake ground motions (in terms of both ground motion intensity and earthquake magnitude), their probable frequency of occurrence, and the variability in the parameters and adjustment factors used to estimate the CRR. These probabilistic analyses can incorporate the epistemic uncertainty among the available empirical models by using a logic tree approach that can also be used to consider uncertainty in the site characterization. The uncertainties involved in the assessment of earthquake ground motions, system response, physical damage, and losses make probabilistic methods for liquefaction consequence assessment central to performance- based evaluation and design. Until fully probabilistic liquefaction hazard analyses (PLHAs) are developed, deaggregation data can be used in conjunction with probabilistic seismic hazard analysis results to approximate the liquefaction hazard. Fully probabilistic procedures (i.e., PLHAs), in which contributions from all potential levels of ground motion are considered, have been developed and shown to produce more complete and rational estimates of liquefaction hazards in different seismic environments than do deterministic methods. PLHAs are well developed for liquefaction triggering and recently have been extended to consequences such as lateral spreading and post-earthquake free-field settlement. They have also been used to show that estimates of actual liquefaction hazards using current procedures as applied in practice can be highly inconsistent. Consistent application of conventional procedures can result in excessive conservatism in some seismic environments and excessive hazard and risk in others; under such conditions, available funds for seismic retrofit programs, for example, cannot be allocated efficiently. Advances in performance-based procedures for liquefaction problems will require improved understanding of the potential damage to different types of structures and facilities given different levels of liquefaction-induced ground deformation and improved understanding of the costs and time requirements to repair liquefaction-associated damage. Performance-based approaches offer great opportunities to produce safer, more resilient structures and facilities and to more efficiently use available resources. Adoption of performance-based engineering concepts will require major changes in the thinking, practice, and education of engineers. It represents moving away from dependence on rules of thumb and factors of safety and toward a design process rooted in the probabilistic prediction of the behavior of engineered systems and estimation of economic losses. The adoption of performance-based liquefaction hazard evaluation in practice, however, will require computing capacity sufficient to perform the voluminous calculations involved in probabilistic performance-based frameworks. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

8 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES RECOMMENDATIONS Findings and recommendations provided in this report are based on the study committee’s assessment of past and current research. As such, recommendations in this report reinforce old ideas or are intended to expand or provide new directions for current research and practice. Given the nature and complexity of liquefaction phenomena and their potential impacts on life safety and infrastructure, improvement of liquefaction assessment methods will require multidisciplinary effort among engineers, geoscientists, infrastructure owners, and major stakeholders. Many efforts will require strong community-based collaborations that enable holistic understanding of problems associated with liquefaction. Nevertheless, it is beyond the scope of the committee’s statement of task to make recommendations regarding the levels and organization of funding necessary to implement these recommendations. Recommendation 1. Establish curated, publicly accessible databases of relevant liquefaction triggering and consequence case history data. Include case histories in which soils interact with built structures. Document the case histories with relevant field, laboratory, and physical model data. Develop the databases with strict protocols and include indicators of data quality. Field data from liquefaction case histories are important for the development, calibration, and validation of the various approaches to predicting liquefaction triggering and its consequences. Case histories are needed with parameter values beyond the ranges for which sufficient data are currently available. Incorporating into databases data collected since 1995 would enhance the value of the databases. More case histories are needed that represent behaviors of liquefiable soils at depths greater than approximately 15 meters; for soils subjected to relatively small and large magnitude events; of soils with greater than 35% fines content; and of soils with a greater than 50% fine-grained particles of low plasticity. More case histories representing soils in sloping ground or adjacent to free faces in potentially liquefiable terrain with SPT blow count values of greater than 15 blows per 30 cm or CPT tip resistance values of greater than 85 atmospheres (8.6 MPa) are also needed. Including additional case histories where there are no surface manifestations of liquefaction or its consequences will also add to the body of knowledge by increasing an understanding of where and why liquefaction does not occur. Furthermore, there is need for greater consistency of the data included in the databases, more transparency related to data quality, and greater ease of access to the databases by researchers and practitioners. Recommendation 2. Characterize locations with high probability of liquefaction and establish them as field observatories for liquefaction triggering and its consequences. Liquefaction case histories are usually developed after an area has been subject to ground motion and, in most cases, after observation of surface manifestations of liquefaction. In such cases, pre-liquefaction soil properties, changes in the soil properties, and ground displacements can only be inferred. Establishing observatories at well-characterized sites with a high probability of liquefaction-inducing earthquakes can result in better knowledge about pre- liquefaction soil properties, and fill gaps in case history databases. High-quality site characterization of surface and subsurface conditions would be conducted prior to or as part of PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

SUMMARY 9 establishment of the observatory. Post-event data from these sites could be used to calibrate and validate procedures for assessment of liquefaction triggering and its consequences and for providing needed information regarding liquefaction behavior at depth, in gravels and fine- grained soils, on sloping ground, and within and beneath embankments, as well as information regarding the soil-structure interaction effects in liquefied soil. Recommendation 3. Use data from the cone penetration test (CPT) for field-based estimates of liquefaction resistance where feasible. If the standard penetration test (SPT) is used for this purpose, make hammer energy measurements. Supplement field-based estimates with other methods as appropriate to characterize the site. CPT soundings offer advantages over other methods of estimating liquefaction resistance in both the detection of thin layers that may affect liquefaction triggering and subsequent pore- pressure redistribution and in the reproducibility of results. CPT results are less dependent on the equipment operator or setup than most other in situ test methods, and CPT can be performed with relative speed and economy. It may also be prudent to conduct one or more SPT borings in addition to CPT soundings to provide physical samples for soil characterization as well as to provide an additional means of liquefaction potential assessment. If SPT measurements are performed, hammer energy measures should be made and the SPT setup should minimize the need for additional correction factors. The CPT can also provide a cost-effective means to measure Vs that can enhance site characterization and liquefaction assessment. Nevertheless, CPT soundings are not feasible in very dense or gravelly soils. In such cases, other methods need to be used to estimate liquefaction resistance. In gravelly soils, instrumented Becker hammer measurements can serve the same purpose as the SPT, and it may be the best available method for soils that cannot be penetrated by the CPT. The use of multiple independent methods can provide additional data to inform the liquefaction potential assessment. Recommendation 4. When refining or developing new empirical relationships for use in liquefaction analyses, incorporate unbiased estimates for input parameters; identify and quantify when possible the uncertainty associated with those estimates; and use soil mechanics principles, seismologic principles, and experimental data to extrapolate beyond ranges in which field data constrain the empirical relationships. Identifying and, if possible, quantifying the uncertainty associated with each adjustment factor, empirical correlation, and parameter relationship used in a liquefaction assessment will help to avoid the compounding of uncertainties and to facilitate assessment of the overall uncertainty associated with the method. When developing a liquefaction triggering or consequence assessment method, using adjustment factors, empirical correlations, and parameter relationships with built-in bias will compound uncertainties and make it difficult, if not impossible, to assess the overall uncertainty associated with the method—an important consideration in liquefaction assessments. Recommendation 5. Use geology to improve the geotechnical understanding of case histories and project sites, particularly where potentially liquefiable soils vary in thickness, continuity, and engineering properties. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

10 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES The potential for liquefaction and consequent ground failures can depend greatly on geologic variability in the thickness, lateral extent, and continuity of liquefiable soils. Sedimentary stratigraphy is related to depositional setting. For example, understanding the shape and extent of a thin sand body deposited in the channel of a meandering river may provide information about the potential for lateral spreading. Geology also affects liquefaction potential through weathering and consolidation, resulting in an increase in the resistance of a soil to liquefaction with age. Understanding geology at and below the ground surface feeds into regional liquefaction hazard maps that relate surficial geology to probabilistic estimates of liquefaction potential. The geological traces of liquefaction can aid in evaluating liquefaction potential where written history contains few if any earthquakes. A database of such paleoliquefaction evidence has been compiled for the central and eastern United States. This database could be expanded nationwide or beyond, both to expand the temporal range of liquefaction case histories and to use sedimentary environment, depositional age, and tectonic setting as screening criteria in assessments of liquefaction hazards. Recommendation 6. Implement simplified stress-based methods for liquefaction triggering in a manner consistent with how they were developed. Avoid using techniques and adjustment factors from one variant of a method with other variants. Consider using more than one simplified method when making a liquefaction triggering assessment. Significant epistemic (modeling) uncertainties are associated with all variants of the simplified stress-based approach to liquefaction triggering assessment. Combining techniques or factors from different variants may serve to compound the uncertainties in unknown and unquantifiable ways. Use of multiple variants of the simplified method may be warranted if analysis results from one variant indicate a marginal factor of safety, where consequences of liquefaction are considered unacceptable, or when a method is applied beyond the bounds where method relations are constrained by the data. A more sophisticated liquefaction assessment may be warranted if results from use of multiple variants are contradictory. Recommendation 7. In developing methods to evaluate liquefaction triggering and its consequences, explicitly incorporate uncertainties from field investigations, laboratory testing, numerical modeling, and the impact of the local site conditions on the earthquake ground motions. Descriptions of procedures to evaluate liquefaction do not always state clearly where and what the uncertainties are and typically do not address how to incorporate them into the liquefaction assessment. Uncertainty in correction factors (e.g., corrections applied to SPT blow counts, magnitude scaling factors) is rarely accounted for, and in some cases, it may introduce even more uncertainty in the results of the analysis than that associated with the original measurement. Uncertainty in a liquefaction consequence or triggering assessment increases rapidly as the assessment method moves beyond the range within which it is constrained by field data. Uncertainties in field data and analytical and experimental results need to be stated in the form of error bounds, standard deviations, bias, or other statistically appropriate measures, and liquefaction assessment procedures need to clearly identify the range over which they are constrained by both field and laboratory test data. Users, in turn, need either to refrain from PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

SUMMARY 11 extrapolating beyond this range or to explicitly identify when they have done so, and they need to qualify the results with, for example, laboratory test results or other information. Recommendation 8. Refine, develop, and implement performance-based approaches to evaluating liquefaction, including triggering, the geotechnical consequence of triggering, structural damage, and economic loss models to facilitate performance-based evaluation and design. Regional variations in seismicity, coupled with the significant uncertainties in earthquake loading and liquefaction resistance, warrant probabilistic characterization for the evaluation of both the hazards of liquefaction triggering and the risk of liquefaction triggering. The geotechnical community needs to develop PLHAs to meet the growing demand from the greater earthquake engineering community for risk-based liquefaction assessment as part of the trend toward performance-based design. The results of these analyses should be expressed in the form of a response curve that indicates how often different levels of response (e.g., liquefaction triggering, lateral spreading displacement, settlement) can be expected to occur. In a performance-based framework, the results of this type of analysis can be convolved with damage and loss models to estimate the risk associated with liquefaction. Risk can be expressed in terms of a risk curve that indicates how often different levels of loss can be expected to occur. Losses include loss of life, loss of functionality, and direct and indirect economic consequences. Recommendation 9. Use experimental data and fundamental principles of seismology, geology, geotechnical engineering, and engineering mechanics to develop new analytical techniques, screening tools, and models to assess liquefaction triggering and post- liquefaction consequences. New approaches to liquefaction assessment are needed that go beyond the empirical approaches commonly used in practice. These new approaches need to be based on sound science (e.g., geology, seismology, and physics) and on fundamental principles of engineering mechanics and geotechnical engineering. Such approaches need to: quantitatively relate liquefaction hazards to geologic units at appropriate depths; increase predictive capabilities in evaluations of both triggering and its consequences; be compatible with probabilistic characterization of seismic loading; be consistent with performance-based assessments of liquefaction consequences; and be readily accessible to practicing engineers. Development and implementation will require greater collaboration among geotechnical engineers and engineering geologists to characterize the depth-dependent distribution and properties of geologic units. The approaches need to be consistent with patterns of behavior identified through analysis of field case histories and laboratory and physical model test data. Strain-based and energy-based approaches are two avenues of development that merit consideration. Rigorous validation of these models, using case history data and development of implementation approaches that make the models accessible to practitioners, is needed to facilitate their adoption in engineering practice. Use of such integrated and validated models in practice would be consistent with trends in other areas of modern earthquake engineering practice. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

12 STATE OF THE ART AND PRACTICE IN THE ASSESSMENT OF EARTHQUAKE- INDUCED SOIL LIQUEFACTION AND ITS CONSEQUENCES Recommendation 10. Develop and validate computational models for liquefaction analyses. Use laboratory and physical model tests at different spatial scales and case histories to provide insight into fundamental soil behavior and to validate the application of constitutive models to boundary-value problems. Computational models are valuable tools when empirical models give contradictory or inconclusive results, or when a more detailed assessment of site and structure response is needed. However, computational models do not always accurately reproduce behavior across laboratory- test, physical-test, and field scales. They are not always able to capture complex material behavior under cyclic loading, including the intimately coupled deformation and flow patterns before and after triggering. Although models based on discrete element mechanics offer a promising avenue for modeling mechanisms at the grain level, at present they are computationally burdensome. Further developments are needed to enhance the accuracy and computational efficiency of discrete methods and to couple the discrete methods with simulations across scales. Meshless methods offer the potential to predict flow behavior of soil subsequent to liquefaction. To realize this potential, however, further insight from grain-scale models (e.g., through the use of multiscale methods) is needed. Rigorous validation procedures that distinguish between the accuracy of the computational method and the complexity of the problems for which the predictive model has been validated are required for all computational procedures, regardless of the computational approach and method of analysis used. Recommendation 11. Conduct fundamental research on the stress, strain, and strength behaviors of soils prior to and after liquefaction triggering; devise new laboratory and physical model experimental techniques to aid development of constitutive models of those behaviors. Advancing the state of the art and practice in liquefaction assessment requires fundamental research on liquefaction phenomena, both theoretical and experimental, because the stress-strain behavior of soil prior to and following liquefaction triggering is complex and not fully understood. No available constitutive model captures all of the relevant soil behavior within a rigorous physics-based framework. Research is needed on soil dilation prior to and after liquefaction triggering, on the solid-to-viscous-liquid behavior transformation that can accompany triggering, and on the mechanisms that control the post-triggering fabric degradation that influences stress-strain behavior of soils. Mechanisms of post-triggering particle rearrangement and reconsolidation, as well as the effects of relative density and consolidation stress on these mechanisms, are not well understood and are in particular need of further research. IMPROVING FUTURE RESEARCH AND PRACTICE Liquefaction assessment will be improved if informed by more and better data, by quantification of uncertainties, and by fundamental scientific and engineering principles. Engineering practice will advance when new methods are developed that take into account the needs and resources of practicing engineers and are designed to be easily implementable. The fundamental basis and applicability of new assessments methods need to be understandable, and PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

SUMMARY 13 outputs of models need to be translatable to problems of interest. Limitations of models need to be clearly defined so that model reliability can be understood under given circumstances. Liquefaction assessment approaches also need to keep pace with other aspects of earthquake engineering practice—for example, by being compatible with performance-based design. Advancing the state of knowledge and practice related to liquefaction assessment will require concerted efforts not only by researchers and engineering practitioners but also by facility owners and stakeholders at large. Researchers can provide more high-quality field case history data supplemented with laboratory and model test data as well as more accurate models to assess earthquake-induced soil liquefaction. Practitioners need to better understand the limitations of characterization and assessment approaches and the geologic and tectonic controls on liquefaction hazards at their sites. Facility owners can support the establishment of public case history databases and of well-characterized and instrumented field observatories for the study of liquefaction effects. Stakeholders such as regional planners could support the development of region-wide liquefaction hazard mapping. It is only through collaborative and interdisciplinary efforts that liquefaction triggering and consequence assessment can undergo the fundamental changes necessary to substantially improve practice. PREPUBLICATION VERSION – SUBJECT TO FURTHER EDITORIAL REVIEW

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Earthquake-induced soil liquefaction (liquefaction) is a leading cause of earthquake damage worldwide. Liquefaction is often described in the literature as the phenomena of seismic generation of excess porewater pressures and consequent softening of granular soils. Many regions in the United States have been witness to liquefaction and its consequences, not just those in the west that people associate with earthquake hazards.

Past damage and destruction caused by liquefaction underline the importance of accurate assessments of where liquefaction is likely and of what the consequences of liquefaction may be. Such assessments are needed to protect life and safety and to mitigate economic, environmental, and societal impacts of liquefaction in a cost-effective manner. Assessment methods exist, but methods to assess the potential for liquefaction triggering are more mature than are those to predict liquefaction consequences, and the earthquake engineering community wrestles with the differences among the various assessment methods for both liquefaction triggering and consequences.

State of the Art and Practice in the Assessment of Earthquake-Induced Soil Liquefaction and Its Consequences evaluates these various methods, focusing on those developed within the past 20 years, and recommends strategies to minimize uncertainties in the short term and to develop improved methods to assess liquefaction and its consequences in the long term. This report represents a first attempt within the geotechnical earthquake engineering community to consider, in such a manner, the various methods to assess liquefaction consequences.

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