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Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
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Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
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Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
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Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
Page 13
Page 14
Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
Page 14
Page 15
Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
Page 15
Page 16
Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
Page 16
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Suggested Citation:"Chapter 2 - Data Collection and Review." National Academies of Sciences, Engineering, and Medicine. 2008. Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. Washington, DC: The National Academies Press. doi: 10.17226/14189.
×
Page 17

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10 The goal of Task 1 of the NCHRP 12-70 Project was to col- lect, review, and interpret relevant practice, performance data, research findings, and other information needed to establish a starting point for subsequent phases of the Project. The work performed within this task included review of the current sta- tus the NCHRP 20-07 Project; literature searches; and con- tacts with individuals involved in the seismic design of retain- ing walls, slopes and embankments, and buried structures. Realizing that the final product for the Project needed to be a set of specifications that can be implemented by practicing engineers, the focus of this task was on the identification of approaches or ideas that could be implemented on a day-to- day basis by practicing engineers, rather than highly rigor- ous or numerically intensive methods that would be more suited for special studies. The results of this data collection and review task are summarized in four sections consisting of discussion of the earthquake design basis, key observa- tions from the literature review, results of contacts with var- ious individuals engaged in design, and a summary of con- clusions reached from this phase of the Project. Although this task was largely complete early in the Project, limited data collection and review continued throughout the dura- tion of the Project. 2.1 Earthquake Design Basis One of the key requirements for this Project was the deter- mination of an earthquake design basis. The earthquake design basis was important because it defined the level of ground motion that will occur at a site. The level of ground motion creates the “demand” side of the basic LRFD equation. As the earthquake design basis increases, the demand (or load) increases; and the capacity of the foundation needs to be pro- portionately larger to limit displacements and forces to accept- able levels. The earthquake design basis also established the performance expectations—for example, the amount of dis- placement that was acceptable. These performance expecta- tions will vary depending on the function of the retaining wall, slope and embankment, or buried structure. With the exception of California, the standard approach within AASHTO at the time of the NCHRP 12-70 Project involved use of a 500-year design earthquake (that is, approx- imately 10 percent chance of exceedance in a 50-year period). Individual states could adopt more stringent requirements for critical bridges. For example, the design basis used by the Washington Department of Transportation (WSDOT) for the new Tacoma Narrows Bridge was 2,500 years (that is, approximately 2 percent probability of exceedance in 50 years), as this bridge was considered a critical structure. Under the standard design approach, the structure (normally a bridge and its related abutment and wing walls) was designed to withstand the forces from the design earthquake without collapse, albeit damage could require demolition following the design event. The NCHRP 12-49 Project (NCHRP Report 472, 2003) attempted to increase the minimum design basis within AASHTO LRFD Bridge Design Specifications to a 2,500-year return period for the collapse-level event. The 2,500-year return period event has approximately a 2 percent probabil- ity of exceedance in 50 years. However, the recommended increase was not adopted for several reasons, including the potential cost of designing for the longer return period and a concern about the complexity of the recommended design process. A follow-up effort was undertaken by Dr. Roy Imbsen of Imbsen & Associates to modify the previous NCHRP 12-49 work, referred to as the NCHRP 20-07 Project (Imbsen, 2006). As part of this effort, the design return period was reconsidered. A consensus was reached by Dr. Imbsen and the AASHTO Highway Subcommittee on Bridges and Struc- tures on the earthquake design basis for both new and retro- fitted structures. This consensus involved a single level design with a return period of 1,000 years. The decision on the design return period established a basis for determining the approach to seismic design for the NCHRP C H A P T E R 2 Data Collection and Review

11 12-70 Project. Specifically, ground motions associated with the 1,000-year return period could be used to identify the following: • Geographic areas that will not require special seismic design studies. For these areas there will be enough margin in the static design of retaining walls, slopes and embankments, and buried structures to accommodate seismic loading, unless special conditions (such as liquefaction) occur. • The type of analyses that will be required in more seismically active areas. For example, the decrease from the 2,500-year return period proposed in the NCHRP 12-49 Project to the 1,000-year return period resulted in smaller increases in ground motions. This meant that nonlinear behavior of soil was not as significant in any proposed design methodology as it would have been for the original NCHRP 12-49 Project recommendations. Another important recommendation made as part of the NCHRP 20-07 Project was to follow an NCHRP 12-49 recom- mendation to use the spectral acceleration from a response spectrum at 1 second (S1), rather than the PGA, as the param- eter for defining the seismic performance category. The spec- tral acceleration at 1 second was used for determining both the level of and the requirement for design analyses. Part of the motivation for this change was the observation that damage during earthquakes was better correlated to S1 than to PGA. By adopting S1 as the parameter for determining the level of and the requirements for design, the region where the thresh- old of seismic demand would be sufficiently low to avoid the need for specialized seismic demand analyses increased. There have been significant developments in the seismological com- munity in the past 10 years which concluded that the seismo- logical environment in CEUS differs from WUS in regards to the long-period content of earthquake ground shaking. For the same PGA, ground motion records from CEUS have much lower shaking intensity at longer periods of ground motion. The choice of using spectral acceleration at 1 second held the potential for minimizing the need for dynamic response analyses for many transportation structures. In order to simplify integration of the results of the NCHRP 12-70 Project with future editions of the AASHTO LRFD Bridge Design Specifications, developments resulting from the NCHRP 20-07 Project served as the basis when formulating analysis requirements for retaining walls, slopes and embank- ments, and buried structures. The relevant analysis require- ments included typical levels of ground shaking and spectral shapes for WUS and CEUS, which then defined the demand requirements for completing the design of retaining walls, slopes and embankments, and buried structures. While the preliminary decision on return period addressed one critical design need for the NCHRP 12-70 Project, the following additional changes regarding the earthquake design basis also needed to be considered by the NCHRP 12-70 Proj- ect or at least be coordinated with future work being done to implement the NCHRP 20-07 Project recommendations: • The shape of the spectrum to be used for design. Significant differences in spectral shapes occur between CEUS and WUS. These differences in spectral shape affect soil response in terms of either peak spectral acceleration or time histories from which design computations or response analyses are conducted. The previous AASHTO LRFD Bridge Design Specifications made no distinction between spectral shapes within the CEUS and WUS. The updated maps use the USGS Seismic Hazard Maps for a 1,000-year return period, thereby accounting for differences in spectral shape of characteristic earthquakes in CEUS versus WUS. • The method of introducing site effects on the rock motions developed for the 1,000-year earthquake return periods. The former site categories in the AASHTO LRFD Bridge Design Specifications were too qualitative in description to allow consistent use. The new site factors followed recommenda- tions given in the Federal Emergency Management Agency’s (FEMA) National Earthquake Hazards Reduction Program (NEHRP) reports and the International Building Code (IBC) documents, similar to what was recommended by the NCHRP 12-49 Project and consistent with South Car- olina Department of Transportation (SCDOT) guidelines prepared by Imbsen & Associates. • Performance expectation for the retaining walls, embank- ments and slopes, and buried structures under the 1,000-year event. For this event the amount of acceptable deformation depended on factors such as the potential consequences of the deformation (that is, to the retaining wall, roadway embankment or cut slope, or culvert), the potential need for and cost of repair, and the additional design requirements associated with the performance evaluation. A single set of design guidelines that captured all of these factors was not easily developed. 2.2 Literature Search Literature reviews were conducted for the three primary technical areas of the Project: retaining walls, slopes and embankments, and buried structures. The goal of the literature review was to do the following: • Identify the state-of-the practice in each of the areas of consideration, • Understand the basis for the methods being applied, including their assumptions and limitations, • Investigate alternative approaches that might be adopted during the development of analytical methodologies, • Establish some of the desirable features of analytical meth- ods that should be considered for development, and

12 • Develop a list of potential example problems that could be used during validation studies and preparation of design examples. 2.2.1 Key References The literature review consisted of collecting and evaluating information already available to the Project Team, as well as electronic literature searches. One of the most effective search mechanisms was through use of Quakeline®, the search mechanism identified in the Multidisciplinary Center for Earthquake Engineering Research (MCEER) Center’s website (http://mceer.buffalo.edu/utilities/quakeline.asp). More than 140 abstracts have been downloaded and reviewed in the area of retaining walls dating from the past 10 years, more than 130 for seismic response of slopes and embankments, and more than 50 references for seismic response of pipelines and culverts. Copies of papers and reports were obtained for those references that appeared to contain unique information or results that are particularly relevant to the Project objectives. As noted in the intro- ductory paragraph to this chapter, this phase of the Project focused on references that could be used directly or indi- rectly to develop methodologies that could be implemented by practicing engineers. Some of the representative relevant articles and reports identified are summarized below. • Retaining Walls – “Analysis and Design of Retaining Structures Against Earthquakes.” Geotechnical Special Publication No. 80, ASCE, November, 1996. – Ausilio, E., E. Conte, and G. Dente. “Seismic Stability Analysis of Reinforced Slopes.” Soil Dynamics and Earth- quake Engineering, Vol. 19, No. 3, pp. 159–172, April 2000. – Bathurst, R. J., M. C. Alfaro, and K. Hatami. “Pseudo- Static Seismic Design of Geosynthetic Reinforced Soil Retaining Structures.” Asia Conference on Earthquake Engineering, Manila, Philippines, Vol. 2, pp. 149–160, March 2004. – Bathurst, R. J. and Z. Cai. “Pseudo-Static Seismic Analysis of Geosynthetic-Reinforced Segmental Retain- ing Walls.” Geosynthetics International, Vol. 2, No. 5, pp. 787–830, 1995. – Bathurst, R. J. and K. Hatami. “Seismic Response Analysis of a Geosynthetic Reinforced Soil Retaining Wall.” Geosynthetics International, Vol. 5, Nos. 1&2, pp. 127–166, 1998. – Bathurst, R. J., K. Harami, and M. C. Alfaro. “Geosyn- thetic Reinforced Soil Walls and Slopes: Seismic Aspects.” (S. K. Shukla Ed.): Geosynthetics and Their Applications, (2002) Thomas Telford Ltd., London, UK, pp. 327–392, November 2004. – Caltabiano, S., E. Cascone, and M. Maugeri. “Sliding Response of Rigid Retaining Walls.” In Earthquake Geotechnical Engineering: Proceedings of the Second Inter- national Conference on Earthquake Geotechnical Engi- neering; Lisbon, Portugal, 21–25 June 1999, Rotterdam: A. A. Balkema, 1999. – Cardosa, A. S., M. Matos Fernandes, and J. A. Mateus de Brito. “Application of Structural Eurocodes to Grav- ity Retaining Wall Seismic Design Conditioned by Base Sliding.” In Earthquake Geotechnical Engineering: Pro- ceedings of the Second International Conference on Earth- quake Geotechnical Engineering; Lisbon, Portugal, 21–25 June 1999, Rotterdam: A. A. Balkema, 1999. – Cascone, E. and M. Maugeri. “On the Seismic Behav- ior of Cantilever Retaining Walls.” In Proceedings of the 10th European Conference on Earthquake Engi- neering; Vienna, Austria, 28 August-2 September 1994, Rotterdam: A. A. Balkema, 1995. – Choukeir, M., I. Juran, and S. Hanna. “Seismic Design of Reinforced-Earth and Soil Nailed Structures.” Ground Improvement, Vol. 1, pp. 223–238, 1997. – Chugh, A. K. “A Unified Procedure for Earth Pressure Calculations.” In Proceedings of the 3rd International Conference on Recent Advances in Geotechnical Earth- quake Engineering and Soil Dynamics, St. Louis, 1995. – FHWA. “Manual for Design & Construction Monitor- ing of Soil Nail Walls.” U.S. Department of Trans- portation, Federal Highway Administration, Publica- tion No. FHWA-SA-96-069R, Revised October, 1998. – FHWA. “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design & Construction Guide- lines.” U.S. Department of Transportation Federal High- way Administration, National Highway Institute, Office of Bridge Technology, Publication No. FHWA-NHI-00- 043, March 2001. – Green, R. A., C. G. Olgun, R. M. Ebeling, and W. I. Cameron. “Seismically Induced Lateral Earthquake Pressures on a Cantilever Retaining Wall.” In Advancing Mitigation Technologies and Disaster Response for Life- line Systems: Proceedings of the Sixth U.S. Conference and Workshop on Lifeline Earthquake Engineering (TCLEE 2003), ASCE, Reston, VA, 2003. – Lazarte, C. A., V. Elias, D. Espinoza, and P. Sabatini. “Soil Nail Walls.” Geotechnical Engineering, Circular No. 7, March 2003. – Ling, H. I. “Recent Applications of Sliding Block The- ory to Geotechnical Design.” Soil Dynamics and Earth- quake Engineering, Vol. 21, No. 3, pp. 189–197, April 2001. – Ling, H. I., D. Leschinsky, and N. S. C. Nelson. “Post- Earthquake Investigation on Several Geosynthetic- Reinforced Soil Retaining Walls and Slopes during the

Ji-Ji Earthquake of Taiwan.” Soil Dynamics and Earth- quake Engineering, Vol. 21, pp. 297–313, 2001. – Ling, H. I., D. Leschinsky, and E. B. Perry. “Seismic Design and Performance of Geosynthetic-Reinforced Soil Structures.” Geotechnique, Vol. 47, No. 5, pp. 933–952, 1997, Earthquake Engineering and Soil Dynamics, St. Louis, 1997. – Michalowski, R. L. and L. You. “Displacements of Reinforced Slopes Subjected to Seismic Loads.” Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 126, No. 8, pp. 685–694, August 2000. – Nova-Roessig, L. and N. Sitar. “Centrifuge Studies of the Seismic Response of Reinforced Soil Slopes.” Pro- ceedings of the 3rd Geotechnical Earthquake Engineer- ing and Soil Dynamics Conference, Special Publication No. 75, ASCE, Vol. 1, pp. 458–468, 1998. – Peng, J. “Seismic Sliding and Tilting of Retaining Walls in Kobe Earthquake.” M.S. Thesis, State University of New York at Buffalo, August 1998. – Prakash, S. and Y. M. Wei. “On Seismic Displacement of Rigid Retaining Walls.” Proceedings of the 3rd Interna- tional Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St. Louis, 1995. – Sakaguchi, M. “A Study of the Seismic Behavior of Geosynthetic Walls in Japan.” Geosynthetic International, Vol. 3, No. 1, pp. 13–30, 1996. – Sarma, S. K. “Seismic Slope Stability—The Critical Acceleration.” Proceedings of the 2nd International Conference on Earthquake Geotechnical Engineering, Lisbon, Vol. 3, pp. 1077–1082, 1999. – Seco e Pinto, P. S. “Seismic Behavior of Gravity Retain- ing Structures.” In Earthquake Geotechnical Engineer- ing: Proceedings of IS-Tokyo ‘95, The First International Conference on Earthquake Geotechnical Engineer- ing; Tokyo, 14–16 November 1995, Rotterdam: A. A. Balkema, 1995. – Simonelli, A. L. “Earth Retaining Wall Displacement Analysis under Seismic Conditions.” Proceedings of the 10th European Conference on Earthquake Engi- neering; Vienna, Austria, 28 August-2 September 1994, Rotterdam: A. A. Balkema, 1995. – Tatsuoka, F., M. Tateyama, and J. Koseki. “Behavior of Geogrid-Reinforced Soil Retaining Walls During the Great Hanshin-Awaji Earthquake.” Proceedings of the 1st International Symposium on Earthquake Geotech- nical Engineering, K. Ishihara, ed., Tokyo, pp. 55–60, 1995. – Tufenkjian, M. R. and M. Vucetic. “Seismic Stability of Soil Nailed Excavations.” Civil Engineering Depart- ment, UCLA School of Engineering and Applied Sci- ence, June 1993. • Slopes and Embankments – ASCE/SCEC. “Recommended Procedures for Imple- mentation of DMG Special Publication 117 Guidelines for Analyzing Landslide Hazards in California.” February 2002. – Ashford, S. A. and N. Sitar. “Seismic Coefficients for Steep Slopes.” Proceedings of the 7th International Con- ference on Soil Dynamics and Earthquake Engineering, pp. 441–448, 1995. – Dickenson, S. E., N. J. McCullough, M. G. Barkau, and B. J. Wavra. “Assessment and Mitigation of Lique- faction Hazards to Bridge Approach Embankments in Oregon.” Prepared for the Oregon Department of Transportation and Federal Highways Administration, November 2002. – Leshchinsky, D. and K. San. “Pseudo-Static Seismic Stability of Slopes: Design Charts.” Journal of Geotechni- cal Engineering, ASCE, Vol. 120, No. 9, pp. 1514–1532, September 1994. – Ling, H. I. “Recent Applications of Sliding Block Theory to Geotechnical Design.” Soil Dynamics and Earth- quake Engineering, Vol. 21, No. 3, pp. 189–197, April 2001. – Loukidis, D., P. Bandini, and R. Salgado. “Stability of Seismically Loaded Slopes Using Limit Analysis.” Geo- technique, Vol. 53, No. 5, pp. 463–479, June 2003. – Martin, G. “Evaluation of Soil Properties for Seismic Stability Analyses of Slopes.” Stability and Performance of Slopes and Embankments II: Proceedings of a Spe- cialty Conference Sponsored by the Geotechnical Divi- sion of the American Society of Civil Engineers, Vol. 1, pp. 116–142, 1992. – Munfakh, G. and E. Kavazanjian. “Geotechnical Earth- quake Engineering, Reference Manual.” Federal High- way Administration, National Highway Institute, 1998. – Rogers, J. D. “Seismic Response of Highway Embank- ments.” In Transportation Research Record 1343, TRB, National Research Council, Washington, D.C., 1992, pp. 52–62. – Sarma, S. K. “Seismic Slope Stability—The Critical Acceleration.” Proceedings of the 2nd International Conference on Earthquake Geotechnical Engineering, Lisbon, Vol. 3, pp. 1077–1082, 1999. – Simonelli, A. “Displacement Analysis in Earth Slope Design Under Seismic Conditions.” Soil Dynamics and Earthquake Engineering VI, pp. 493–505, 1993. – Simonelli, A. and E. Fortunato. “Effects of Earth Slope Characteristics on Displacement Based Seismic Design.” Proceedings of the 11th World Conference on Earthquake Engineering, CD-ROM-1017, 1996. – Simonelli, A. and C. Viggiano. “Effects of Seismic Motion Characteristics on Earth Slope Behavior.” 1st Inter- 13

14 national Conference on Earthquake Geotechnical Engi- neering, pp. 1097–1102, 1995. – Stewart, J. P., T. F. Blake, and R. A. Hollingsworthe. “A Screen Analysis Procedure for Seismic Slope Stabil- ity.” Earthquake Spectra, Vol. 19, Issue 3, pp. 697–712, August 2003. – Wahab, R. M. and G. B. Heckel. “Static Stability, Pseudo-Static Seismic Stability and Deformation Analy- sis of End Slopes.” Proceedings of the 2nd International Conference on Earthquake Geotechnical Engineering, Lisbon, Portugal, Vol. 2, pp. 667–672, 1999. – Wartman, J. et al. “Laboratory Evaluation of the New- mark Procedure for Assessing Seismically-Induced Slope Deformations.” Proceedings of the 2nd International Conference on Earthquake Geotechnical Engineering, Lisbon, Portugal, Vol. 2, pp. 673–678, 1999. • Buried Structures – American Lifelines Alliance. “Seismic Fragility Formula- tions for Water System.” Part 1—Guidelines and Part 2— Appendices, April 2001. – ASCE. “Guidelines for the Seismic Design of Oil and Gas Pipeline Systems.” American Society of Civil Engi- neers, Committee on Gas and Liquid Fuel Lifelines of the ASCE Technical Council on Lifeline Earthquake Engineering, 1994. – Hamada, M., R. Isoyama, and K. Wakamatsu. “Liquefaction-Induced Ground Displacement and Its Related Damage to Lifeline Facilities.” Soils and Foun- dations, Special Issue, 1996. – Holzer, et al. “Causes of Ground Failure in Alluvium dur- ing the Northridge, California, Earthquake of January 17, 1994.” Technical Report NCEER-96-0012, 1996. – Johnson, E. R., M. C. Metz, and D. A. Hackney. “Assess- ment of the Below-Ground Trans-Alaska Pipeline Fol- lowing the Magnitude 7.9 Denali Fault Earthquake.” TCLEE, Monograph 25, 2003. – MCEER. “Response of Buried Pipelines Subject to Earth- quake Effects.” MCEER Monograph Series No. 3, 1999. – NCEER. “Highway Culvert Performance during Earth- quakes.” NCEER Technical Report NCEER-96-0015, November 1996. – NCEER. “Case Studies of Liquefaction and Lifeline Per- formance during Past Earthquakes.” Technical Report NCEER-92-0001, Volume 1, M. Hamada, and T. D. O’Rourke Eds., 1992. – O’Rourke, M. J. and X. Liu. “Continuous Pipeline Sub- jected to Transient PGD: A Comparison of Solutions.” Technical Report NCEER-96-0012, 1996. – O’Rourke, M. J. and C. Nordberg. “Longitudinal Per- manent Ground Deformation Effects on Buried Con- tinuous Pipelines.” Technical Report NCEER-92-0014, 1996. – O’Rourke, T. D. “An Overview of Geotechnical and Lifeline Earthquake Engineering.” Geotechnical Special Publication No. 75—Geotechnical Earthquake Engineer- ing and Soil Dynamics III, ASCE, Vol. 2, 1999. – O’Rourke, T. D., S. Toprak, and Y. Sano. “Factors Affecting Water Supply Damage Caused by the North- ridge Earthquake.” Proceedings, 6th US National Con- ference on Earthquake Engineering, Seattle, WA, 1998. – Pease, J. W. and T. D. O’Rourke. “Seismic Response of Liquefaction Sites.” Journal of Geotechnical and Geo- environmental Engineering, ASCE, Vol. 123, No. 1, pp. 37–45, January 1997. – Shastid, T., J. Prospero, and J. Eidinger. “Southern Loop Pipeline—Seismic Installation in Today’s Urban Envi- ronment.” TCLEE, Monograph 25, 2003. – Youd, T. L. and C. J. Beckman. “Performance of Corru- gated Metal Pipe (CMP) Culverts during Past Earth- quakes.” TCLEE, Monograph 25, 2003. 2.2.2 General Observations Results of this literature review determined that a significant amount of information has and continues to be published on the topics of seismic design and performance of retaining walls, slopes and embankments, and buried structures. These publi- cations cover all facets of seismic design and performance from simplified to highly rigorous numerical methods, laboratory testing with shake tables and centrifuges, and case histories, though the number falling into this last category is relatively limited. Whereas the amount of literature is significant, the advances in design methodology have been relatively limited over the past 10 to 20 years. New methodologies often have been refinements of procedures suggested many years before. What might be con- sidered the only significant advance is the common application of various numerical methods to investigate seismic response. • Limit-equilibrium computer codes are available from var- ious vendors for evaluation of global stability of retaining walls, slopes and embankments, and the permanent dis- placement component of buried structures. These codes allow the designer to consider various internal and exter- nal forces, with seismic forces included as a horizontal force coefficient. Results from these analyses include criti- cal failure surfaces and factors of safety for global stability. • A more limited number of finite element and finite dif- ference codes also are being used now to estimate the dis- placement of soils or soil-structure systems during seis- mic loading. These more rigorous numerical procedures allow consideration of various geometries, time-dependent loads, and soil properties whose strength changes with cycles of loading.

A number of observations relative to the overall goals of this Project can be made from the results of the literature review. Further discussion is provided in Chapter 3. • Retaining Walls – M-O equations are used almost exclusively to estimate seismic active and passive earth pressure. Little atten- tion seems to be given to the assumptions inherent to the use of the M-O equations. The seismic coefficient used in the M-O equation is assumed to be some percent of the free field ground acceleration—typically from 50 to 70 percent—and the soils behind the retaining structure are assumed to be uniform. – There is widespread acceptance, particularly in Europe, of displacement-based methods of design, although it is recognized that displacements are sensitive to the nature of earthquake time histories. – Only limited experimental data exist to validate the forces estimated for the design of retaining walls. These data are from shake tables and centrifuge tests. In most cases they represent highly idealized conditions relative to normal conditions encountered during the design of retaining walls for transportation projects. – The overall performance of walls during seismic events has generally been very good, particularly for MSE walls. This good performance can be attributed in some cases to inherent conservatism in the design methods cur- rently being used for static loads. • Slopes and Embankments – Except in special cases the seismic stability analysis for slopes and embankments is carried out with commer- cially available limit-equilibrium computer codes. These codes have become very user friendly and are able to handle a variety of boundary conditions and internal and external forces. – Limited numbers of laboratory and field experiments have been conducted to calibrate methods used to esti- mate seismic stability or displacements. These experi- ments have used centrifuges to replicate very idealized conditions existing in the field. Usually the numerical method is found to give reasonable performance esti- mates, most likely because of the well-known boundary conditions and soil properties. – Slope and embankment performance during earthquakes has varied. Most often slopes designed for seismic load- ing have performed well. The exception has been where liquefaction has occurred. The most dramatic evidence of seismically induced slope instability has occurred for oversteepened slopes, where the static stability of the slope was marginal before the earthquake. • Buried Structures – A number of procedures have been suggested for the design of culverts and pipelines. Most often these pro- cedures have been based on post-earthquake evalua- tions of damage to water and sewer pipelines. The procedures consider both the TGD and PGD. Most examples of damage are associated with PGD. Pressures on the walls of buried structures are typically estimated using conventional earth pressure equations, including the M-O equations for seismic loading. – Experimental studies have been conducted with cen- trifuges and shake tables to estimate the forces on cul- verts and pipes that result from seismically induced PGD. Only limited attention has been given to experi- mental studies involving the effects of TGD on pipelines and culverts. – Observations from past earthquakes suggest that per- formance of culverts and pipe structures located beneath highway embankments has generally been good. This good performance is most likely associated with the design procedures used to construct the embankment and backfill specifications for the culverts and pipes. Typical specifications require strict control on backfill placement to assure acceptable performance of the culvert or pipe under gravity loads and to avoid settle- ment of fill located above the pipeline or culvert, and these strict requirements for static design lead to good seismic performance. – The most common instances of culvert or pipe structure damage during past earthquakes is where lateral flow or spreading associated with liquefaction has occurred. In these situations the culvert or pipe has moved with the moving ground. 2.3 DOT, Vendor, and Consultant Contacts Contacts were made with staff on the Project Team, staff in geotechnical groups of DOTs, vendors, and other con- sultants to determine the availability of design guidelines to handle seismic design of retaining walls, slopes and embank- ments, and buried structures. During these contacts, an effort also was made to determine the normal approach fol- lowed when performing seismic design and analyses of retaining walls, slopes and embankments, and buried struc- tures. This was viewed as a key step in the data collection and review process, as the procedures used by this group of practitioners represent the current state-of-the practice and should form the starting point for the development of any new methodology. Some of the key design guides and references identified from these contacts are summarized here: • Caltrans: Contacts with California Department of Trans- portation (Caltrans) personnel focused on the design 15

16 requirements for retaining walls and the approach used to evaluate seismic slope stability. Caltrans personnel con- firmed that the retaining wall design requirements are doc- umented in the Caltrans Bridge Design Specifications dated August 2003. Specifications include a 14-page Part-A on General Requirements and Materials and 106-page Part-B on Service Load Design Method, Allowable Stress Design. Some of the key design requirements for retaining walls include the following: – A minimum factor of safety of 1.3 for static loads on overall global stability. – A minimum factor of safety of 1.0 for design of retain- ing walls for seismic loads. – Seismic forces applied to the mass of the slope based on a horizontal seismic acceleration coefficient (kh) equal to one third of the site-adjusted PGA, the expected peak acceleration produced by the maximum credible earth- quake. Generally, the vertical seismic coefficient (kv) is considered to equal zero. Caltrans specifications go on to indicate that if the factor of safety for the slope is less than 1.0 using one-third of the site-adjusted PGA, procedures for estimating earthquake- induced deformations, such as the Newmark Method, may be used provided the retaining wall and any supported structure can tolerate the resulting deformations. • WSDOT: Initial contacts with WSDOT’s geotechnical staff focused on WSDOT’s involvement in develop- ing technical support for load and resistance factors used in geotechnical design. While this work was not specifi- cally directed at seismic loading, both the methodology and the ongoing work through the AASHTO T-3 group appeared to be particularly relevant to Phase 2 of this Project. WSDOT efforts included evaluation of load and resistance factors through Monte Carlo simulations. Subsequent discussions took place with WSDOT on seis- mic design methods for retaining walls in general and MSE walls in particular. One key concern on the part of WSDOT was how to incorporate load and resistant factors in the seismic design process. This concern was particularly critical in the use of the M-O procedure for determining seismic earth pressures. WSDOT found that if no resistance factors were applied to the dynamic case, as suggested in NCHRP 12-49 Project report and other similar documents, it was possible that the seis- mic earth pressure will be lower than the static earth pressure determined using load and resistance factors in the AASHTO LRFD Bridge Design Specifications. WSDOT also provided a preliminary copy of their draft seis- mic design requirements for retaining walls, slopes, and embankments. – For pseudo-static analyses, WSDOT proposed using a horizontal seismic coefficient equal to 0.5 times the site-adjusted PGA with a target factor of safety of 1.1. Newmark-type analyses were allowed where an esti- mate of deformations was needed. – Seismic earth pressures on walls were determined using the M-O equations. WSDOT staff specifically pointed out the difficulties that they have had in dealing with high acceleration values and steep back slopes when using the M-O equations. • ODOT and ADOT&PF: Both the Oregon Department of Transportation (ODOT) and the Alaska Department of Transportation and Public Facilities (ADOT & PF) have recently worked on developing guidelines for addressing the effects of liquefaction on embankment stability. Some of this information is useful for addressing the response of slopes in liquefiable soils. • Vendors: Design methods used by several vendors of MSE walls (for example, Keystone, Hilfiker, and Mesa) were reviewed. Generally, these vendors followed methods recommended by FHWA. Both the inertial force within the reinforced zone and the dynamic earth pressure from M-O earth pressure calculations were used in external sta- bility evaluations. Guidelines also were provided for eval- uation of internal stability in the approach used by some vendors. • Consultants: Contacts also were made with geotechnical engineers and structural designers to determine what they perceived as the important issues for seismic design of retaining walls, slopes and embankments, and buried structures. Below is a list of some of the issues identified from this limited survey: – There was consensus that there needs to be clarification on the responsibility between geotechnical engineers and structural engineers in the overall design process. The view was that a lack of communication occurs between the two parties resulting in much confusion at times. – The design practice varied tremendously from state to state and from project to project on many fundamental requirements, including whether retaining walls need to be designed for the seismic load case at all. A com- mon practice was to design retaining walls for static loading only with its inherent factor of safety, and many designers believed that retaining walls have per- formed well in past earthquakes and traditional static design practice and its inherent conservatism were adequate. – A major objective in future effort should be to devote some effort to clarifying basic steps involved in design- ing retaining walls. – Pseudo-static methods are typically used to evaluate sta- bility of slopes and embankments during seismic load- ing. There seems to be a divergence of opinion on the

seismic coefficient to use during these analyses and an acceptable factor of safety. – Design of buried structures (that is, pipelines and cul- verts) is normally limited to a check on liquefaction potential, on the potential for flotation, and an evalua- tion of slope stability or lateral flow. Where lateral soil movement was expected, the buried structure was either considered expendable or ground treatment methods were used to mitigate the potential for lateral ground movement. An interesting observation from these contacts was that the approach used by transportation agencies, specifically DOTs, seemed to lag the methodologies being used by many con- sultants. This is particularly the case for the seismic design of slopes, where the common practice was to limit the seismic stability analyses to the abutment fill using pseudo-static methods. With the possible exception of some DOTs, such as Caltrans and WSDOT, there was some hesitation towards using deformation methods. It also seemed that free-stand- ing retaining walls and buried structures most often were not designed for seismic loading. This was due in part to the lack of generally accepted design guidelines and the general costs associated with the implementation of additional design requirements. As a final note, it was commonly accepted by most practi- tioners involved in designing retaining walls and underground structures that earth structures have performed well in past earthquakes, even for the higher ground shaking levels in WUS. These observations suggested that the seismic design requirement for earth structures should not burden the designer with overly complex and often over costly designed systems. A very important part of the NCHRP 12-70 Project was to take advantage of recent seismological studies and seismic performance observations to avoid unwarranted conservatism and to reduce the region of the country requir- ing seismic loading analyses. 2.4 Conclusions Conclusions from this task were that the methodologies available to design professionals within DOTs and consult- ants for the DOTs are primarily limited either to pseudo- static methods, such as the M-O equations for estimating seismic earth pressures on retaining structures and the limit- equilibrium method of slope stability analysis, or to simplified deformation methods (for example, Newmark charts or analy- ses). Although these methods have limitations, improvements in these methodologies still offer the most practical approaches to seismic design. A growing trend towards the use of more rigorous model- ing methods, such as the computer code FLAC (Itasca, 2007), for the evaluation of retaining structures, slopes and embank- ments, and buried structures has occurred recently. While FLAC and similar software provide a more rigorous model- ing of these problems and can be a very powerful method of analysis, these more numerically intensive procedures do not appear to be suitable for development of design methodolo- gies required by this Project. Rather they offer methodologies either to check the simplified procedures appropriate for con- ventional design or to evaluate special loading conditions and special geometries. Even in these special cases, these more rig- orous procedures can be prone to significant inaccuracies when the person using the software does not have a good understanding of conditions that could affect results. As discussed in the next chapter, it also was apparent from the review of the literature that some areas of seismic design were relatively mature, with design methods provided and gen- erally accepted. The design of slopes and embankments is an example of this. But other areas were less well under- stood even for static loading. Design of geosynthetic walls falls into this category. This difference in “design maturity” added to the complexity of the NCHRP 12-70 Project, as the intent of the NCHRP 12-70 Project was to have design guides consistent with and build upon static design methods. 17

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Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 611: Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments explores analytical and design methods for the seismic design of retaining walls, buried structures, slopes, and embankments. The Final Report is organized into two volumes. NCHRP Report 611 is Volume 1 of this study. Volume 2, which is only available online, presents the proposed specifications, commentaries, and example problems for the retaining walls, slopes and embankments, and buried structures.

The appendices to NCHRP Report 611 are available online and include the following:

A. Working Plan

B. Design Margin—Seismic Loading of Retaining Walls

C. Response Spectra Developed from the USGS Website

D. PGV Equation—Background Paper

E. Earthquake Records Used in Scattering Analyses

F. Generalized Limit Equilibrium Design Method

G. Nonlinear Wall Backfill Response Analyses

H. Segrestin and Bastick Paper

I. MSE Wall Example for AASHTO ASD and LRFD Specifications

J. Slope Stability Example Problem

K. Nongravity Cantilever Walls

View information about the TRB Webinar on Report 611: Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments: Wednesday, February 17, 2010

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