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Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions (2012)

Chapter: CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions

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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
×
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
×
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Suggested Citation:"CHAPTER TWO Current State of Evaluation of Site Effects on Ground Motions." National Academies of Sciences, Engineering, and Medicine. 2012. Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions. Washington, DC: The National Academies Press. doi: 10.17226/14660.
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THE NATIONAL ACADEMIES Advisers to the Nation on Science, Engineering, and Medicine The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. On the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences. The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering. The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the ser- vices of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, on its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine. The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy's purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Acad- emy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council. The Transportation Research Board is one of six major divisions of the National Research Council. The mission of the Transportation Research Board is to provide leadership in transportation innovation and prog- ress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Board's varied activities annually engage about 7,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. www.TRB.org www.national-academies.org

NCHRP COMMITTEE FOR PROJECT 20-05 COOPERATIVE RESEARCH PROGRAMS STAFF CHRISTOPHER W. JENKS, Director, Cooperative CHAIR Research Programs CATHERINE NELSON CRAWFORD F. JENCKS, Deputy Director, Cooperative Oregon DOT Research Programs NANDA SRINIVASAN, Senior Program Officer MEMBERS EILEEN P. DELANEY, Director of Publications KATHLEEN S. AMES Maker Baker Jr., Inc. SYNTHESIS STUDIES STAFF STUART D. ANDERSON STEPHEN R. GODWIN, Director for Studies and Texas A&M University Special Programs CYNTHIA J. BURBANK JON M. WILLIAMS, Program Director, IDEA and PB Americas, Inc. Synthesis Studies BRIAN A. BLANCHARD JO ALLEN GAUSE, Senior Program Officer Florida DOT GAIL R. STABA, Senior Program Officer LISA FREESE DONNA L. VLASAK, Senior Program Officer Scott County (MN) Public Works Division TANYA M. ZWAHLEN, Consultant MALCOLM T. KERLEY DON TIPPMAN, Senior Editor Virginia DOT CHERYL KEITH, Senior Program Assistant RICHARD D. LAND DEMISHA WILLIAMS, Senior Program Assistant California DOT DEBBIE IRVIN, Program Associate JOHN M. MASON, JR. Auburn University TOPIC PANEL ROGER C. OLSON Minnesota DOT ABBAS ABGHARI, California Department of Transportation THOMAS COOLING, URS Corporation, St. Louis, MO ROBERT L. SACK TIMOTHY E. HUFF, Tennessee Department of Transportation New York State DOT G.P. JAYAPRAKASH, Transportation Research Board FRANCINE SHAW-WHITSON STEVEN L. KRAMER, University of Washington Federal Highway Administration NICOLAS LUCO, US Geological Survey, Golden, CO LARRY VELASQUEZ ELMER MARX, Alaska DOT and Public Facilities, Juneau JAVEL Engineering, LLC. JAMES STRUTHERS, Washington State Department of Transportation FHWA LIAISONS JUSTICE J.G. MASWOSWE, Federal Highway Administration, JACK JERNIGAN Baltimore, Maryland (Liaison) MARY LYNN TISCHER PHILIP YEN, Federal Highway Administration (Liaison) TRB LIAISON STEPHEN F. MAHER Cover figure: Damage to the Nimitz Freeway following the 1989 Loma Prieta earthquake (courtesy of: Dr. Neven Matasovic). ACKNOWLEDGMENTS The authors would like to thank Mr. Byungmin Kim, graduate student at the University of Illinois at Urbana­Champaign, and Mr. Spencer Marcinek, Staff Engineer with Geosyntec Consultants, for their assis- tance during development drafts of this study. The authors would also like to thank Dr. Edward Kavazanjian, Jr., of Arizona State University and Dr. Sharid Amiri of Caltrans for their continuous support of this work and to Dr. Rodolfo Sancio of Geosyntec Consultants who peer- reviewed several drafts of this study.

FOREWORD Highway administrators, engineers, and researchers often face problems for which infor- mation already exists, either in documented form or as undocumented experience and prac- tice. This information may be fragmented, scattered, and unevaluated. As a consequence, full knowledge of what has been learned about a problem may not be brought to bear on its solution. Costly research findings may go unused, valuable experience may be overlooked, and due consideration may not be given to recommended practices for solving or alleviat- ing the problem. There is information on nearly every subject of concern to highway administrators and engineers. Much of it derives from research or from the work of practitioners faced with problems in their day-to-day work. To provide a systematic means for assembling and evaluating such useful information and to make it available to the entire highway commu- nity, the American Association of State Highway and Transportation Officials--through the mechanism of the National Cooperative Highway Research Program--authorized the Transportation Research Board to undertake a continuing study. This study, NCHRP Proj- ect 20-5, "Synthesis of Information Related to Highway Problems," searches out and syn- thesizes useful knowledge from all available sources and prepares concise, documented reports on specific topics. Reports from this endeavor constitute an NCHRP report series, Synthesis of Highway Practice. This synthesis series reports on current knowledge and practice, in a compact format, without the detailed directions usually found in handbooks or design manuals. Each report in the series provides a compendium of the best knowledge available on those measures found to be the most successful in resolving specific problems. PREFACE This study identifies and describes current practice and available methods for evaluating the influence of local ground conditions on earthquake design ground motions on a site- By Jon M. Williams specific basis. Information includes criteria used to determine when a site-specific analysis Program Director is needed, how to develop input parameters required for a site-response analysis, the nature Transportation of the site-response analysis performed (equivalent-linear, total stress nonlinear, effective- Research Board stress nonlinear), the process of model setup, and how uncertainties are dealt with in the analysis process. Information was gathered by a literature review and a survey of state departments of transportation and selected academics. Neven Matasovic, Geosyntec Consultants, Huntington Beach, California, and Youssef Hashash, University of Illinois at Urbana-Champaign, collected and synthesized the infor- mation and wrote the report. The members of the topic panel are acknowledged on the pre- ceding page. This synthesis is an immediately useful document that records the practices that were acceptable within the limitations of the knowledge available at the time of its preparation. As progress in research and practice continues, new knowledge will be added to that now at hand.

Contents 1SUMMARY 3 CHAPTER ONE INTRODUCTION 5 CHAPTER TWO CURRENT STATE OF EVALUATION OF SITE EFFECTS ON GROUND MOTIONS Introduction, 5 Site Response Evaluation Approaches, 5 Site Response Analysis Methods, 6 Information Required for Site Response Analysis, 6 Equivalent-Linear Site Response Analysis, 8 Nonlinear Total Stress Site Response Analysis, 10 Nonlinear Site Response Analysis with Pore Water Pressure Change, 13 Calibration and Benchmarking Studies, 15 Guidance Documents for Site Response Analysis, 15 Software Used in Practice, 15 17 CHAPTER THREE APPROACH TO SURVEY OF CURRENT PRACTICE Background, 17 Overview of the Survey, 17 Survey Respondents, 17 Survey Responses, 18 19 CHAPTER FOUR SURVEY RESPONSES AND RELEVANT LITERATURE General Practice, 19 Criteria and Programs Used in Site Response Analysis, 19 Dimensions, Analysis, and Model Type, 19 Seismic Hazard and Motion Input Required for Site Response Analysis, 20 Soil Profile Input Information Required for Site Response Analysis, 20 Site Response Analysis, 21 Evaluation and Use of Results, 22 General Comments on the Survey, 22 24 CHAPTER FIVE CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH Conclusions, 24 Suggestions for Further Research, 24 27REFERENCES 34BIBLIOGRAPHY 39GLOSSARY 40 APPENDIX A Survey Questionnaire 47 APPENDIX B Compiled Survey Responses

77APPENDIX C Site Response Analysis Software--Software UrlS, References, and Use in Highway Engineering Practice as Identified by Survey Respondents Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.

PRACTICEs AND PROCEDURES FOR SITE-SPECIFIC EVALUATION OF EARTHQUAKE GROUND MOTIONS SUMMARY The current AASHTO specifications for seismic design mandate site-specific evaluation of the earthquake design ground motion (i.e., the acceleration response spectrum) for ground conditions termed Site Class F. In the AASHTO specifications, Site Class F soils include soft clay sites. These AASHTO specifications also allow discretionary site-specific analy- ses for other ground conditions and a reduction in mapped design ground motions of as much as 33% if justified by a site-specific ground motion analysis. Some state departments of transportation (DOTs) are taking advantage of this site response reduction provision, particularly in cases where pore pressure generation could lead to soil liquefaction. For Site Classes C, D, and E, AASHTO's tabulated site response adjustment factors (site factors) are typically used to adjust mapped values of ground motions. However, as stipulated in AASHTO's recommendations, site-specific site response analyses have also been used in these circumstances as an alternative because the site factor approach may be inappropriate under some conditions. Of particular concern is the adequacy of the site factor approach in evaluating the response of short period structures (fundamental period of the site, To < 0.5 sec) on shallow bedrock sites (i.e., depth to bedrock less than 100 ft), and of longer period structures (To > 1.0 sec) at deep soil basin sites (e.g., depth to bedrock greater than 500 ft). For years, the equivalent-linear total stress approach, as programmed in one-dimen- sional (1-D) site response analysis codes (e.g., SHAKE) has been the primary method used to evaluate the influence of local ground conditions on earthquake design ground motions on a site-specific basis. However, this type of analysis has limitations: (1) for strong shak- ing at some sites owing to nonlinear site response effects resulting in large shear strain response, which is not properly captured in the equivalent-linear soil models; (2) at sites where soils have the potential to develop significant seismically induced excess pore water pressures, including soil liquefaction; and (3) at soft clay sites subject to moderate inten- sity/long-duration motions, because the analysis cannot take the effects of cyclic degrada- tion into consideration. This synthesis study identifies and describes current practice and available methods for evaluating the influence of local ground conditions on earthquake design ground motions on a site-specific basis. The study focuses on evaluating the response of soil deposits to strong ground shaking, and therefore does not address representation of structural response. In accordance with current practice, the study's primary focus is on one-dimensional (1-D) site response analysis (both equivalent-linear and nonlinear). Two-dimensional (2-D) and three-dimensional (3-D) analyses are discussed, but at a more limited level. The synthesis study consists of a literature review and a survey of current practice. The literature review revealed significant developments in the area of nonlinear site response over the past decade, including studies to define usage protocols of total stress nonlinear site response analyses. New generations of generic dynamic material property sets (e.g., modulus reduction and damping curves, and seismic pore water pressure gen- eration model parameters) have also been developed and are now available for use in site

2 response analyses. The review also showed increasing availability of commercial 2-D analy- sis software for site response analysis with pore water pressure generation. However, there is a lack of agreed-upon protocols for both 1-D and 2-D nonlinear site response analysis with soils that have potential for significant pore water pressure generation. The survey identifies survey participants, survey questions, methods of processing sur- vey responses, and relevant findings of the survey. Most of the survey participants were from state DOTs and their consultants. Selected academic researchers also participated in the sur- vey. DOTs invited to participate in the survey included the AASHTO Highway Subcommit- tee on Bridges and Structures, Technical Committee T-3 states (Seismic Design) (Alaska, Arkansas, California, Illinois, Indiana, Missouri, Montana, Nevada, Oregon, South Caro- lina, Tennessee, Washington), plus DOTs of Georgia, Hawaii (T-3+), Massachusetts, New York, Rhode Island, and Utah, and their consultants. The respondents represented DOTs/ firms of various sizes; some described their own practices and others the practices of their DOTs or firms. The survey questions encompassed criteria used by the respondents to deter- mine when a site response analysis (through computer software) is required; development of input parameters required for a site response analysis; nature of site response analysis (equivalent-linear, total stress nonlinear, effective-stress nonlinear) performed; and the pro- cess of model setup and development of model input parameters. The survey also asked about how uncertainties are dealt with in the analysis process and the practice for evaluating the results of site response analyses, as well as the use of site response analyses in further engineering analyses. The survey reveals widespread use of both equivalent-linear and nonlinear site response analyses by DOTs and other participants. A significant portion of the DOTs' analyses use nonlinear site response analyses, including analyses with pore water pressure generation for liquefaction evaluation. Use of 2-D numerical models in site response evaluation, although limited, appears to be increasing. The issues associated with the use of both nonlinear effec- tive-stress and 2-D software include development of adequate input parameters and ground motions, verification and validation of the results, evaluation of uncertainty, and vertical site response. The survey respondents provided a wide range of answers on a number of key issues in site response analysis, including input motion, material properties, analysis procedures, and use of results. The responses illustrate the lack of consensus and the need to develop guidance on these important issues. The study concludes with a list of research topics suggested by the survey participants and identified from the literature review. The topics include benchmarking of site response models with pore water pressure generation, shear wave velocity correlation evaluation, benchmarking of 2-D and 3-D analysis codes, vertical site response, and calibration with recent (i.e., the 2011 Tohoku, Japan, earthquake) data. Research in these topics will facilitate future reliable use of advanced analysis techniques in highway engineering practice.

3 CHAPTER ONE INTRODUCTION AASHTO specifications for seismic design, including both being used in practice, including methods that account for the 2009 Interim AASHTO Load and Resistance Factor deep soil basin effects and for pore water pressure genera- Design (LRFD) Bridge Design Specifications and the 2009 tion. Significant expertise is required to conduct and inter- Guide Specifications for LRFD Seismic Bridge Design, pret the results from these newer methods, often leading to mandate site-specific evaluation of the earthquake design questions about the validity of results. For instance, experi- ground motions (i.e., the acceleration response spectrum) ence with the newer nonlinear analysis methods show that for ground conditions termed Site Class F. In the AASHTO strains (and hence stiffness reduction) may become more specifications (AASHTO 2010a), Site Class F soils are soft localized than in an equivalent-linear total stress analysis. clay sites. These AASHTO specifications also allow discre- As a result, details of the soil profile, particularly soft lay- tionary site-specific analyses for other ground conditions ers and impedance contrasts, can have a larger effect on the and a reduction in mapped ground motions by as much as results of a nonlinear analysis than they do on the results 33% if justified by a site-specific ground motion analysis. of an equivalent-linear analysis. Furthermore, all available Some state departments of transportation (DOTs) are tak- methods of site response analysis (including equivalent- ing advantage of this site response reduction provision, linear total stress analysis) require significant expertise particularly in cases where pore pressure generation could and numerous discretionary decisions. For example, the lead to liquefaction. Furthermore, there is some evidence, analysis requires selection of an appropriate suite of time part of which is based on the authors' experiences, that histories and a determination as to whether the small strain the AASHTO site factors, used to adjust mapped values of modulus and other soil properties should be measured in the design ground motions for local ground conditions, may be field and/or laboratory or obtained using correlations. The inappropriate under some conditions. For example, they may analysis also requires decisions on the extent of sensitivity not be appropriate for short period structures (fundamental analyses and what modulus reduction and damping curves period of the structure, To < 0.5 sec) at shallow bedrock sites to use. More expertise and discretionary decision making is (that is, depth to bedrock less than 100 ft), and for structures required with nonlinear methods than with equivalent-lin- with a relatively long predominant period (To > 1.0 sec) at ear analysis and is greatest with analyses that consider pore deep soil basin sites [e.g., depth to bedrock greater than 500 pressure generation and dissipation. Commentary within ft (Park and Hashash 2005a, 2005b)]. Site-specific analyses the AASHTO specifications cautions the reader of potential are also being used in these circumstances as an alternative issues when conducting site-specific ground motion studies, to the use of AASHTO site factors. but the commentary does not provide guidance on the nature of these issues, and how or when to consider these potential For years, the equivalent-linear total stress approach, as issues. This lack of guidance raises concerns as to whether programmed in one-dimensional (1-D) site response analy- appropriate estimates of site-specific ground motions are sis codes, has been the primary method used to evaluate being made for design, potentially resulting in either exces- the influence of local ground conditions on earthquake sive project construction costs when ground motion response design ground motions on a site-specific basis. However, is overestimated or unacceptable risk to the public when this type of analysis has limitations: (1) for strong shak- ground motion response is underestimated. ing at some sites owing to nonlinear site response effects resulting in large shear strain response; (2) at sites where This synthesis study identifies and describes current prac- there is a potential for significant seismically induced tice and available methods for site-specific analysis of earth- pore water pressure buildup, including soil liquefaction, quake ground motions. The study is primarily concerned because it cannot consider the effects of pore pressure gen- with the response of soil deposits to strong ground shaking eration; and (3) at soft clay sites subject to moderate inten- and, as such, does not address representation of structural sity/long-duration motions as it cannot consider the effects response. The study's primary focus is on one-dimensional of cyclic degradation. (1-D) analyses as this represents both the majority of site response analysis work to date and current state of practice. A number of nonlinear site response analysis methods Two-dimensional (2-D) and three-dimensional (3-D) analy- have become available over the past decade and are now ses are discussed, but at a limited level.

4 This study starts with a discussion of current knowledge pated in the survey. DOT's invited to participate in the survey based on a review of technical literature and contacts with included T-3 states (Alaska, Arkansas, California, Illinois, select publishers and software authors (researchers) for clar- Indiana, Missouri, Montana, Nevada, Oregon, South Caro- ification. As a part of the documentation of this literature lina, Tennessee, Washington), plus DOTs of Georgia, Hawaii search, an attempt was made to identify and explain key (T-3+), Massachusetts, New York, Rhode Island, and Utah, concepts involved in current site response analysis practice. and their consultants. The respondents represented DOTs/ The study also summarizes experience gained in developing firms of various sizes; some were describing their own prac- and employing these methods, including challenges in their tices and others the practices of their DOTs or firms. application and perceived advantages and disadvantages of the different methods. As appropriate, the synthesized survey results are related to findings from a review of current knowledge. The research The literature search is followed by a survey of current and development needs identified through the work and sur- practice. Most of the survey participants were from state vey responses documented here are provided at the end of DOTs and their consultants. Selected researchers also partici- this study.

5 CHAPTER TWO CURRENT STATE OF EVALUATION OF SITE EFFECTS ON GROUND MOTIONS INTRODUCTION SITE RESPONSE EVALUATION APPROACHES Site-specific evaluation of earthquake ground motions Three general approaches can be used to evaluate soil (i.e., includes a number of contributors such as soil stratigraphy, site) effects on ground motions: (1) the attenuation relation- basin effects, regional geology, topographic relief, and soil- ship approach, (2) the code-factor approach, and (3) the site foundation-structure interaction (SFSI). The study of basin response analysis approach. effects, regional geology, and topographic relief impacts on ground motions are primarily in the domain of engineering The attenuation relationship approach uses attenua- seismology and remains primarily in the realm of research. tion relationships or ground motion prediction equations Code-based factors (e.g., Eurocode 8; EC8 2000) have been (GMPE) that consider local site effects, including soil con- introduced to account for these effects, but site-specific evalu- ditions. While older attenuation equations distinguish only ation of these effects for highway facilities is rare and will between soil and rock, recently published Next Generation not be discussed in this study. Attempts have been made to Attenuation (NGA) relationships (e.g., Abrahamson et al. capture some of these effects on a more limited basis through 2008) can provide ground motion prediction as a function of the use of 2-D and 3-D analyses and will be briefly addressed shear wave velocity (Vs), including velocities based on ASCE in that context only. The study of SFSI effects is an area under 7-type site classes that are also adopted by IBC 2006 (Inter- rapid development, mostly by structural engineers. An over- national Code Council 2006). In this approach, a response view of these effects relevant to geotechnical engineers can spectrum is developed and can be used directly in a spectral be found in Kramer (1996) and more recently in Kramer and analysis. If needed, ground motions would have to be sepa- Stewart (2004) and is beyond the scope of this study. rately generated through some form of spectral matching, which will be discussed later. This study focuses on evaluation of local soil deposit- related site effects as illustrated in Figure 1. The presentation The second approach for assessing soil effects on is primarily focused on horizontally layered soil deposits, ground motions computes rock outcrop (surface rock) including other soil deposits and earthen structures that can response spectrum using a rock attenuation equation and be approximated as the horizontally layered soil deposits. then modifies the rock spectrum by generic soil amplifica- Both total and effective stress conditions are addressed. The tion factors such as the FPGA (used in AASHTO); Fa, and focus of this study is on practical applications relevant to Fv factors in Tables 11.4-1 and 11.4-2 of ASCE 7; or other design and analysis of highway facilities. published sources such as EPRI (Electric Power Research Institute 1993), Rodriguez-Marek et al. (2001), or Stewart et al. (2003). As in the first approach, a response spectrum is developed and can be used directly in a spectral analy- sis. If needed, ground motions can be generated separately through some form of spectral matching, which will be discussed later. The third approach calls for evaluation of local site effects by conducting a detailed site response analysis using computer software. The site response analysis approach for evaluation of site effects on ground motions is widely used (see also the survey synthesis section, chapter four). This approach is favored by Geotechnical Earthquake Engineers as it takes into account the unique geotechnical character- istic (i.e., "seismic signature") of a site. The approach uses back analysis of numerous case histories and works well FIGURE 1 Framework of site response analysis. when the profile has significant impedance contrast and

6 when material (model) parameters are established through INFORMATION REQUIRED FOR SITE RESPONSE a reasonable site characterization effort (e.g., Kwok et al. ANALYSIS 2006, 2008). A number of numerical techniques are available for site A site response analysis is commonly performed under response analysis, including 1-, 2-, and 3-D equivalent-linear many conditions: (1) when soil conditions cannot be rea- (frequency domain) and nonlinear (time domain) analysis sonably categorized into one of the standard site conditions; approaches. All these techniques require a common set of (2) when empirical site factors for the site are not avail- information and input. able (e.g., such as site class F); (3) when special ground conditions govern the design (e.g., soil liquefaction, seis- The input to site response analysis requires (1) input mic settlement, lateral spreading, and slope stability); (4) ground motion time histories; (2) identification of subsur- for any case where the objective is to obtain ground motions face conditions, including geometry, stratification, and considered to be more representative of the local geologic depth to bedrock and groundwater; and (3) specification and seismic site conditions than motions obtained from the of basic and advanced material properties for each layer first two approaches; or (5) where (nonlinear) SFSI analysis of subsurface soil and of bedrock, such as unit weight and is undertaken. shear wave velocity (or low-strain shear modulus) and shear modulus and damping as a function of shear strain. More The attenuation relationship approach outlined above is advanced analyses require additional soil properties (e.g., addressed in greater detail elsewhere (e.g., Kramer 1996; saturated hydraulic conductivity and wet and saturated unit Kramer and Stewart 2004; Abrahamson et al. 2008). The weight), model (i.e., curve-fitting) parameters, and hysteretic code-factor approach is also addressed in these references and viscous damping model parameters (Rayleigh damping and in relevant codes (e.g., AASHTO 2010a; IBC 2006). parameters for frequency dependent formulations). The third approach for evaluation of soil effects on ground motions, commonly referred to as site response analysis, is Not all of the above-listed input information has the same discussed in detail in this study. influence on the results of the site response analysis. In most cases, input ground motions have the most influence on the The report presumes that the reader has basic familiarity results of site response analysis. The near-surface shear with earthquake engineering, geotechnical earthquake engi- wave velocity profile and material nonlinearity (e.g., the neering, and soil behavior. The reader is referred to a book modulus reduction and damping ratio curves) are, as noted such as Geotechnical Earthquake Engineering by Kramer by Roblee et al. (1996), the parameters that predominantly (1996) and Dynamics of Structures by Chopra (2006) for this control ground motion response expressed by the accelera- background information. tion response spectrum. Detailed information on the "uncer- tainty" related to soil property evaluation and assessment of spatial variability of ground motions can be found in Jones SITE RESPONSE ANALYSIS METHODS et al. (2002) and in Kwok et al. (2007). Site response analysis methods can be classified by the Input Ground Motion Time Histories domain in which calculations are performed (frequency domain or time domain), the sophistication of the constitu- It is generally recognized that the selection of input ground tive model employed (linear, equivalent-linear, and/or non- motion is one of the primary contributors, if not the primary linear), whether effects of pore water pressure generation are contributor, to uncertainty in site response analysis. Various neglected or not (total-stress and effective-stress analyses, codes and design guidance documents outline procedures respectively), and the dimensionality of the space in which for selection of design ground motions. For example, ASCE analysis is performed (1-D, quasi 2-D, 2-D, and 3-D). Other (2006) (ASCE 7-05, Chapter 21, Section 21.1.1) requires that considerations in classifying site response analysis methods "at least five recorded or simulated rock outcrop horizontal include modeling of cyclic reduction and degradation in a ground motion acceleration time histories be selected from total-stress mode. events having magnitudes and fault distances that are con- sistent with those that control the MCE [Maximum Con- The following section describes the input required for site sidered Earthquake]." To further minimize the uncertainty response analysis, followed by a discussion of the various related to selection of design ground motions, ASCE 7-05 methods available for site response analysis with increasing also requires that the time histories be scaled such that the complexity: (1) frequency domain equivalent-linear analy- average acceleration response spectrum of each time history sis; (2) nonlinear time domain total stress analysis; and (3) is approximately at the level of the MCE rock acceleration nonlinear time domain effective-stress analysis. response spectrum over the period range of significance to

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 428: Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions identifies and describes current practice and available methods for evaluating the influence of local ground conditions on earthquake design ground motions on a site-specific basis.

The report focuses on evaluating the response of soil deposits to strong ground shaking.

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