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Suggested Citation:"Chapter 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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 5 - Seismic Ground Motions." 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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35 This chapter summarizes the results of ground motion studies completed for the Project. The primary objectives of the ground motion studies were to • Provide a consistent basis for establishing ground motion to use during the seismic analysis of retaining walls, slopes and embankments, and buried structures; • Update Newmark charts for estimating permanent ground displacements of retaining walls and slopes to be consistent with the results of ground motion studies for CEUS and WUS; and • Establish correlations between PGV and spectral accelera- tion at a period of 1 second (S1) for use in the seismic analy- ses of retaining walls, slopes and embankments, and buried structures. Information in this chapter serves as input for the seismic response studies discussed in Chapters 6 through 9. These re- sults also form the basis of sections in Volume 2 containing rec- ommended specifications and commentaries in the AASHTO LRFD Bridge Design Specifications. 5.1 Seismic Loading Criteria The seismic design of bridges in the then current (2006) AASHTO LRFD Bridge Design Specifications was based on the peak ground accelerations and an appropriate response spec- trum for the site. This same general approach was reviewed during the NCHRP 12-70 Project for the seismic analyses of retaining walls, slopes and embankments, and buried struc- tures. However, criteria in the AASHTO LRFD Bridge Design Specifications were expected to change based on recommen- dations from the NCHRP 20-07 Project. Key changes recom- mended by the NCHRP 20-07 Project included (1) a change in the return period of the ground motion used for bridge de- sign from the existing 10 percent probability of exceedance in a 50-year period (that is, 475-year return period) to a 7 percent probability of exceedance in 75 years, which corresponded approximately to a 1,000-year return period; and (2) a change in the shape of the 5 percent damped response spectrum in the longer period range. The discussion of these seismic load- ing criteria in this section begins with a review of the update to the current AASHTO LRFD Bridge Design Specifications. This review is followed by a summary of the ranges of ground motions that can be expected in various regions of the United States and then the variation in response spectra for CEUS ver- sus WUS based on approaches recommended by the NCHRP 20-07 Project. 5.1.1 Update to AASHTO Seismic Ground Motion Criteria Seismic loading criteria used by the NCHRP 12-70 Project were taken from the criteria being developed for the seismic design of bridges within the NCHRP 20-07 Project Recom- mended LRFD Guidelines for the Seismic Design of Highway Bridges (Imbsen, 2006). At the time the NCHRP 12-70 Proj- ect work was being performed, preliminary feedback from the AASHTO T3 subcommittee was very favorable towards use of the 1,000 year return period and the NEHRP spectral shape concept. Rather than taking a separate approach or conducting a dual development, the NCHRP 12-70 Project assumed that the NCHRP 20-07 recommendations would be adopted at the AASHTO meeting in 2007. AASHTO mem- bers later adopted the ground motion changes during a vote in July of 2007. There were several good reasons for using the criteria de- veloped for the NCHRP 20-07 Project for the seismic design of retaining walls, slopes and embankments, and buried structures. First, it would be consistent with the approach being used by most transportation agencies and already used in part within the current AASHTO LRFD Bridge Design Specifications. Secondly, by using the same criteria as devel- oped for the NCHRP 20-07 Project, there was less chance for C H A P T E R 5 Seismic Ground Motions

36 confusion between guidelines being used for different parts of a project. Lastly, retaining walls, slopes and embankments, and buried structures are all components of the transportation network and by using the same criteria used by bridges, there was a common basis for judging risk to the transportation system. Key aspects of the NCHRP 20-07 Project related to ground motion criteria are summarized below: 1. The safety level earthquake was based on the USGS/ AASHTO seismic hazard mapping program. The recom- mended ground motion hazard level was a 7 percent prob- ability of exceedance in 75 years, corresponding roughly to a 1,000-year return period. The USGS was contracted by AASHTO to provide 1,000-year hazard maps and an implementation CD. 2. The map and implementation CD, with the proposed specifications developed by the NCHRP 20-07 Project team, were used by various state bridge departments for trial designs. These trials were carried out in 2006 and balloting for adoption by AASHTO was held in July of 2007. As noted above, this meant that much of the NCHRP 12-70 Project had to proceed on the basis that the NCHRP 20-07 recommendations would be adopted by AASHTO. 3. The approach recommended in the NCHRP 20-07 Proj- ect report involved developing a free-field ground surface design spectrum that served as the basic benchmark ground shaking criteria. The spectrum was defined on the basis of spectral acceleration (Sa) at three periods: 0.0, 0.2 and 1.0 seconds corresponding to the 1,000-year uniform- risk spectrum for a referenced soft rock condition. The three periods defined the PGA, short-period spectral ac- celeration (Ss), and the spectral acceleration at 1 second (S1), respectively. These spectral values are for soft rock site conditions where the average shear wave velocity within the upper 100 feet of geologic profile ranges from 2,500 to 5,000 feet per second (ft/sec), which is referred as Site Class B. 4. The above three spectral ordinates (that is, at 0.0, 0.2 and 1.0 seconds) are used to anchor a spectral curve shape. Figure 5-1 shows the resultant design acceleration re- sponse spectrum after adjusting the referenced soft rock spectrum for site soil effects. The adjustments for site ef- fects account for amplification or deamplification of the referenced rock motion for soil conditions at the site. This method of determining the spectrum is generally the same as that proposed earlier in the NCHRP 12-49 Proj- ect (NCHRP Report 472, 2002) and has been used in both the 2003 and 2006 International Building Code (IBC) for regulating the design of new buildings. The primary difference with the new approach adopted by AASHTO in July of 2007 from a ground motion stand- point is that it is using the 1,000-year return period, versus the 2,475-year return period recommended in NCHRP 12-49 and IBC 2003 and IBC 2006. (The IBC Figure 5-1. Design response spectrum constructed with the three- point method.

design approach also multiplies the resulting spectrum by a 2/3rd factor to account for the “reserve capacity” against collapse within most buildings.) The AASHTO procedure also involves anchoring the design spectrum at zero pe- riod (PGA) based on a 1,000-year return period hazard level. This approach compares to the IBC which assumes that the PGA is equal to 0.4 times the spectral acceleration at 0.2 seconds (that is, the short period spectral accelera- tion, Ss). The site coefficient used by AASHTO to adjust the PGA value (Fpga) for various soil classifications is iden- tical to the coefficient used for the 0.2-second, short pe- riod site factor (Fa) recommended by the NCHRP 12-49 Project and used by IBC. 5. Similar to NCHRP 12-49 and IBC 2006, the NCHRP 20-07 document provided two tables for site modification factors to be applied to the two spectral ordinates for other site soil/rock categories. Table 5-1 tabulates site coefficients (Fa) at the short period range (that is, at 0.0-second and 0.2-second periods), and Table 5-2 tabu- lates site coefficients (Fv) at the 1-second period. (AASHTO subsequently adopted a separate table for Fpga to be ap- plied to PGA. Values of Fpga are the same as Fa. Note also that AASHTO normalizes PGA to be dimensionless. The current version of AASHTO shows the same Fa and Fv values but without the units of gravitational acceleration (g).) The two site coefficient factors are applied to the three spectral ordinates from the new AASHTO 1,000- year maps and implementation CD for various site cat- egories in relation to the reference USGS Site Class B condition. 37 Table 5-1. Values of Fa as a function of site class and mapped short-period spectral acceleration. Table 5-2. Values of Fv as a function of site class and mapped 1 second period spectral acceleration.

– The spectral ordinate at 0.2 second defines a flat plateau with a constant spectral acceleration. This constant accel- eration branch of the spectral curve starts at 0.2 Ts where Ts is defined by the ratio of Sa at 0.2 seconds to Sa at 1 sec- ond. The long-period limit of the spectrum is governed by the intersection of the constant acceleration branch of the curve and the decreasing spectral acceleration branch of the response spectrum curve anchored at the 1-second ordinate. – The long-period range (decreasing spectral accelera- tion) is defined by the spectral ordinate at 1 second along with the assumption that the curve shape is in- versely proportional to period (T); that is, Sa α 1/T. This 1/T decrease is consistent with an assumption of con- stant spectral velocity. It also corresponds with a spec- tral displacement that increases linearly with the period of motion. (Note that the current IBC 2006 has a further provision where the 1/T decrease changes to a 1/T 2 de- crease. The period of this change differs across the United States, ranging from 4 seconds to 16 seconds. The change from 1/T to 1/T 2 was introduced for the de- sign of long-period structures, such as multistory build- ings, and for sloshing of large-diameter water reservoirs. A similar approach has not been taken by AASHTO for the design of long-period bridges. The maps in IBC 2006 are not applicable because they represent a return period of 2,475 years as opposed to the 1,000-year return period being recommended within the new AASHTO maps. It is presumed that the seismic design of long-span bridges would use site-specific evaluation methods in the ab- sence of maps similar to those in IBC 2006.) 5.1.2 Range of Ground Shaking Levels in the United States for Referenced Soft Rock A sensitivity analysis was conducted during the NCHRP 12-70 Project to determine the ground shaking levels for the 1,000-year return period at various locations in the United States. Site Class B soft rock reference condition was used for conducting this analysis. The purpose of the study was to establish the range in ground shaking levels that must be con- sidered during the seismic design of retaining walls, slopes and embankments, and buried structures—based on the rec- ommendations given in the NCHRP 20-07 Project. The 1,000-year hazard spectra used in this sensitivity study were generated by making use of the USGS interactive web- site, rather than the results of the USGS 1,000-Year Mapping Program. Although the USGS program was very close to completion at the time of this work, the results of the 1,000-year data were not available at the time the analyses were con- ducted (Fall 2005). Appendix C provides background infor- mation on the USGS interactive website. Figure 5-2 shows the results from this analysis; Table 5-3 tabulates these results. The figure shows the distinctly dif- ferent shapes of the response spectra in CEUS versus the WUS. In this figure, spectral curves for sites located in the more active WUS are shown by continuous lines, and sites for the less active CEUS are denoted by dashed lines. The difference between WUS and CEUS occurs along a distinc- tive boundary (see Figure 4-1) along the US Rocky Moun- tains. West of this boundary is referred to as the more seis- mically active WUS, and east is the less active CEUS. In general, ground shaking is higher in WUS as compared to CEUS, especially at longer periods (for example, 0.5 seconds or more). Other observations regarding the variation in ground mo- tion intensity between CEUS and WUS also were made from the sensitivity study, as summarized here. These observations are keyed to the spectral demand at the 1-second period, fol- lowing the approach taken in the NCHRP 20-07 Project, which makes use of spectral demand at 1 second for quantification of the seismic design category. 1. In general, the expected ground motion shaking level at 1-second period (S1), as measured by the 5 percent damped spectral acceleration for WUS typically ranges from 0.3 to 0.6g. In contrast for CEUS, the shaking level is much lower for S1—typically no more than 0.2g, even for relative active seismic areas near the cities of Memphis and Charleston. For many of the population centers, including New York and Boston, S1 is well below 0.1g—often being 0.05g or less. 2. There appears to be a larger range in ground shaking for CEUS sites as compared to WUS. For example, the design S1 for Seattle or Salt Lake City is approximately 50 percent of San Francisco and Los Angeles, the most active regions. In contrast for CEUS, the population centers in the Northeast 38 Figure 5-2. Variation in the 1,000-year benchmark soft rock spectra over the United States.

are less than 25 percent of what would be expected for Memphis and Charleston (without considering the much higher shaking at the epicenter location at New Madrid). The relationship between spectral accelerations at 1 second and the PGA also is observed to differ between the CEUS and the WUS. A good rule-of-thumb is to assume that for the Class B soft rock ground shaking, PGA is related to S1 by the following relationship: (1) WUS Class B Rock Sites, PGA ≈ S1; and (2) CEUS Class B Rock Sites, PGA ≈ 2S1. 5.1.3 Variation in Spectral Shapes for Soil and Rock Sites in WUS versus CEUS The design response spectra shown in the previous section were developed from the USGS Hazard Mapping website for the referenced soft rock conditions. Figure 5-3 presents the normalized spectral curve shapes for the spectra shown in Figure 5-2. The differences between the spectral curve shapes for CEUS (shown in dashed lines) versus WUS (shown in con- tinuous lines) is quite evident in this figure. Beyond approx- imately 0.3 seconds, the ordinates for CEUS sites are gener- ally about half of the ordinates from WUS sites for the same period, with the exception of the Columbus, Ohio and the Minneapolis, Minnesota sites. These sites are extremely far from known seismic sources and are of extremely low design shaking levels. The spectral shapes shown in Figure 5-3 reflect the varia- tions in spectral shapes (that is, response spectra after nor- malizing by the design PGA) across the United States for a ref- erenced soft rock condition classified as Site Class B by the USGS. However, for sites where deposits of soil occur, the soft rock spectra need to be modified to local site soil conditions. For typical soil sites (commonly encountered in practical de- sign conditions), there tends to be a higher level of amplifica- tion for the intermediate period of response around 1 second. The effects of local soil amplification on the spectral shapes shown in Figure 5-3 also were evaluated. Following the NCHRP 20-07 Project guidelines, adjustments were made to the spectral ordinates at 0.2 (short) and 1-second (long) pe- riods. For this evaluation an adjustment factor for Site Class E site conditions (loose sand or soft clays with Vs < 650 ft/sec.) was used to evaluate the maximum potential effects of soil amplification on the spectral shapes. At lower shaking levels where maximum site amplification occurs, the site adjustment factors were 3.5 and 2.5, respectively, for the short-period and long-period adjustment factors. Figure 5-4 shows three spectral curve shapes developed from the above discussed sensitivity studies. These three curves are used to illustrate variations in the spectral curve shapes after allowing for differences between CEUS and WUS ground motions, as well as between rock and soil site effects. The three spectral curve shapes define an upper bound (UB), lower bound (LB), and intermediate (Mid) spectral shape— representing the combination of seismological variations 39 5% Damped Spectral Acceleration (g) WUS Sites EUS Sites Period (Second) Sa n Fr an ci sc o Lo s A ng el es Se at tle Sa lt La ke C ity Ph oe ni x N ew M ad rid M em ph is Ev an sv ill e Ch ar le st on N ew Y or k Co lu m bu s M in ne ap ol is 0.01 0.607 0.593 0.443 0.492 0.051 0.952 0.397 0.200 0.406 0.101 0.040 0.015 0.10 1.107 1.306 0.861 0.986 0.091 1.995 0.916 0.474 0.910 0.240 0.094 0.031 0.20 1.431 1.405 0.985 1.139 0.116 1.687 0.746 0.407 0.713 0.184 0.090 0.033 0.30 1.361 1.393 0.856 1.034 0.102 1.368 0.588 0.326 0.547 0.132 0.077 0.030 0.50 1.102 0.998 0.647 0.776 0.071 0.920 0.391 0.220 0.348 0.078 0.059 0.024 1.00 0.686 0.671 0.328 0.433 0.039 0.437 0.191 0.113 0.158 0.038 0.038 0.016 2.00 0.363 0.247 0.149 0.194 0.021 0.190 0.085 0.052 0.066 0.017 0.021 0.010 Deag Magnitude at 1-Sec 7.9 7.9 7.2 7.0 6.6 7.7 7.7 7.7 7.3 7.0 7.7 7.7 Deag Distance (Km) 11.5 12.0 7.0 1.7 171.0 17.2 59.7 164.2 23.5 413.9 616.6 939.3 Note: Spectral values shown in bold correspond to points SDS and SD1 in Figure 5-1. Table 5-3. 1000-year soft rock spectral ordinates.

(that is, between WUS and CEUS) and potential soil condi- tions variations (that is, Category B, C, D, and E sites). The physical representation of the three shapes shown in Figure 5-4 is: • The LB spectral curve shape was developed from the soft rock spectrum for the New York City site, a CEUS site. • The UB spectral curve shape was developed for a San Fran- cisco site, a WUS site, after applying the Site Class D soil fac- tor to the San Francisco reference soft rock spectrum. • The Mid spectral curve shape is the soft rock spectrum directly developed for San Francisco The spectral curve ordinates at 1-second period now reflect about a factor of 4.5 variation between the UB versus the LB shaking conditions reflecting amplification of the intermedi- ate period (that is, about 1 second) motion due to site soil re- sponse effects. As discussed later, spectrum-compatible mo- tions will be generated for the three spectral curve shapes that then will be used for slope and retaining wall scattering 40 Figure 5-3. Spectral curve shapes from spectra presented in Figure 5-2. Figure 5-4. Spectral curve shapes adopted for further ground motion studies.

(coherency) analyses. The scattering analyses will be used to examine height-dependent average acceleration factors. 5.2 Newmark Displacement Correlations The following section provides a summary of work done to refine Newmark-displacement correlations that will be used in the retaining wall, slopes and embankments, and buried structures analyses discussed in later chapters. These correlations often are presented in the form of charts or equations that can be used by the designer to estimate the amount of displacement based on an acceleration ratio at a site. The acceleration ratio is defined as the ratio of the ac- celeration at which a slope or retaining wall starts to slide to the peak ground acceleration. The current AASHTO LRFD Bridge Design Specifications has a discussion of the Newmark method in Appendix A of Section 11. Various updates of the Newmark relationship have been made. One of the more recent relationships was developed as part of the NCHRP 12-49 Project (NCHRP Report 472, 2002). The fol- lowing subsections present refinements to the NCHRP 12-49 work based on a strong motion database that covers CEUS, as well as WUS. 5.2.1 Approach for Updating Newmark Charts One major step in establishing performance criteria for de- sign purposes is to estimate the displacement of a retaining structure or slope due to the design earthquake. When a time history of the design earthquake is available, earthquake- induced displacements can be calculated using the Newmark’s sliding block method. This approach involves integrating the earthquake record twice for the region above the yield accel- eration, where the yield acceleration is the point where the factor of safety in sliding is 1.0. For routine retaining struc- tures or slope designs, however, a design motion time his- tory is often not available, and the designer relies on design motion parameters such as PGA and PGV. Research has shown there is a reasonable correlation be- tween these ground motion parameters and calculated per- manent displacement from the Newmark method. A rela- tionship that was developed for the NCHRP 12-49 Project was updated using the records from recent earthquakes. To establish a nationwide relationship for permanent displace- ment, it was necessary to use ground motions with charac- teristics representative of CEUS and WUS earthquake records in the analyses. A database of strong ground motion records was used to study the design ground motion criteria for the NCHRP 12-70 Project. The main characteristics of this database: • Include over 1,800 strong motion records (horizontal and vertical components); • Contain records from recent (before 2001) large-magnitude earthquakes around the world (events in Japan, Turkey, and Taiwan); • Represent earthquake records in WUS and CEUS; and • Contain earthquake records for rock and soil site conditions. This strong motion database has been used to update the cor- relations between permanent seismic displacement (Newmark Sliding Block Method) and strong motion record characteris- tics developed during the NCHRP 12-49 Project. The update involved accounting for the much larger database compared to the limited database used by Martin and Qiu (1994) in devel- oping the charts shown in the NCHRP 12-49 Project report. The database also was used to check relationships for PGV based on S1, as described later in this chapter. 5.2.2 Description of Ground Motion Database The ground motion database was developed from the strong motion catalog compiled as part of the United States Nuclear Regulatory Commission (USNRC) publication NUREG/ CR-6728 Technical Basis for Revision of Regulatory Guidance on Design Ground Motions: Hazard- and Risk-Consistent Ground Motion Spectra Guidelines (McGuire et al., 2001). The catalog is available on two CDs, one for WUS and the other one for CEUS. Data are compiled in terms of magnitude, distance, and soil type bins, as follows: • Two regions: WUS and CEUS; • Two site conditions: rock and soil; • Three magnitude bins: 4.5–6, 6–7, and 7–8; and • Four distance bins: 0–10 km, 10–50 km, 50–100 km, and 100–200 km. The earthquake records are reasonably distributed in the range of practical interest. Figure 5-5 shows the distribution of the strong motion records in the catalog. Each record includes the following data: • Acceleration, velocity, and displacement time histories; • Relative displacement, relative velocity, pseudo relative velocity, absolute acceleration, and pseudo absolute accel- eration spectra (5 percent damped); and • Time interval and duration of Arias intensity for various ranges. It should be noted that due to the limited number of record- ings east of the Rocky Mountains, a majority of CEUS records are based on WUS records with a scaling factor. 41

5.2.3 Permanent Displacement Data Permanent displacement is a characteristic of the strong motion record, as well as the ratio of the structure yield ac- celeration to peak ground acceleration in the sliding mass (ky/kmax) of the subject structure. Using the strong motion records in the USNRC catalog, permanent displacements have been calculated for ky /kmax values in the range of 0.01 to 1. A nonsymmetrical displacement scheme was assumed in these analyses, meaning that the displacement occurs in one direc- tion and is not reversible. Figure 5-6 shows the concept of the Newmark sliding block method for calculation of permanent displacements due to earthquake time histories. 5.2.4 Microsoft Access Database To evaluate the correlations between different parameters in the USNRC earthquake catalog, an Access database has been developed. The database comprises two tables, one for storage of basic record information (INFOTAB), and a second table (NEWMARK) for storage of permanent displacement data. Figure 5-7 shows a schematic diagram for the ground motion 42 Figure 5-5. Distribution of the magnitude and distance from source for the records in the USNRC Earthquake Catalog. Figure 5-6. Illustration of Newmark’s sliding block method for estimation of permanent displacement due to earthquake.

information database, and Table 5-4 gives a description of each field in the Access database. The developed database can be used to efficiently explore correlations between different record char- acteristics. It also can be used to prepare data sets required for various statistical analyses. 5.2.5 Spectral Acceleration Characteristics To compare strong motion records from different re- gion, magnitude, and soil type bins, the normalized spec- tral acceleration and normalized relative density graphs are plotted for each bin. The average spectrum for each region- site condition for different magnitude ranges was calcu- lated. The average normalized spectra are presented in Figures 5-8 and 5-9. Results in Figures 5-8 and 5-9 show the following trends: • Records with higher magnitudes generally have higher am- plitude in the long-period range. • Records for WUS and CEUS generally have different spec- tral shapes. WUS records have higher normalized ampli- tudes in lower frequency (long-period) ranges, while CEUS records have higher amplitudes in high frequency (low- period) ranges. • The difference in spectral shape between WUS and CEUS records is more evident for the rock records. • Having larger amplitudes at long periods implies that for the same PGA, the earthquake records in WUS will have larger PGV, therefore inducing larger displacements in the structure. 5.2.6 Correlation between PGV and S1, PGA and M Several correlations between PGV and other ground mo- tion parameters such as S1, PGA, and M were developed dur- ing this study. After reviewing recent publications related to this subject, a revised form of a PGV correlation suggested by Abrahamson (2005) for the estimation of PGV from spectral acceleration at one second (S1) was selected for use, as dis- cussed in Section 5.3. It is expected that in the future, USGS will publish recom- mended PGV values for different locations nationwide. In that case the S1-PGV correlation will be replaced in favor of design PGV values, and the designers can use Newmark displace- ment correlations directly using the USGS-recommended PGV values. 5.2.7 Newmark Sliding Block Displacement Correlations Various researchers have proposed different correlations for predicting the permanent displacement of earth structures subjected to seismic loading. A summary and comparison of some of these correlations can be found in a paper by Cai and Bathurst (1996). The majority of these correlations are based on the results of direct Newmark sliding block analyses on a set of strong motion records. Martin and Qiu (1994) used the following general form for estimation of Newmark displacement: Using a database of earthquake records with a magnitude range between 6.0 and 7.5, published by Hynes and Franklin (1984), Martin and Qiu concluded that the correlation with M (magnitude) is negligible. The following simplified equation was proposed by Martin and Qiu and adopted in NCHRP 12-49 Project: where d = permanent displacement in inches, ky = yield acceleration, d k k k k A Vy y= ( ) −( )− − −6 82 10 55 5 08 0 86 0. max . max . . .86 1 66M . ( )5-2 d C k k k k A V My a y a a a a = ( ) −( )max max ( )1 2 3 4 51 5-1 43 Figure 5-7. Strong motion information database model.

kmax = the maximum seismic acceleration in the sliding block, A = peak ground acceleration (in/sec2), and V = peak ground velocity (in/sec). A correlation based on Equation (5-2), but in logarithmic form, was used for estimation of Newmark displacement from peak ground acceleration and peak ground velocity. Writing Equation (5-2) in logarithmic form resulted in the following equation: Using a logarithmic transformation of the data helped to stabilize the variance of residuals and normalize the variables, hence improving the correlation in the entire range of the parameters. The coefficients for Equation (5-3) were estimated using regression analysis. The permanent displacement data from log log log l max maxd b b k k b k k b y y( ) = + ( ) + −( ) + 0 1 2 3 1 og log ( )maxk b( ) + ( )4 PGV 5-3 44 Table Field Description INFOTAB NO Earthquake event number INFOTAB EARTHQUAKE Earthquake event name INFOTAB YEAR Event year INFOTAB MODY Event date INFOTAB HRMN Event time INFOTAB MAG Earthquake magnitude INFOTAB OWN Station owner INFOTAB STNO Station number INFOTAB STATION Station name INFOTAB DIST Closest distance from source INFOTAB GEOM Geomatrix site classification code INFOTAB USGS USGS site classification code INFOTAB HP Filter corner frequency, high INFOTAB LP Filter corner frequency, low INFOTAB PGA Peak ground acceleration INFOTAB PGV Peak ground velocity INFOTAB PGD Peak ground displacement INFOTAB DUR Duration INFOTAB FILENAME Record file name INFOTAB PAA1S Pseudo spectral acceleration at 1 second INFOTAB PRV1S Pseudo relative velocity at 1 second INFOTAB RD1S Relative displacement at 1 second INFOTAB PAAMAX Peak pseudo spectral acceleration INFOTAB PRVMAX Peak pseudo relative velocity INFOTAB RDMAX Peak relative displacement INFOTAB DUR95 5%-95% Arias intensity duration INFOTAB REGION Region (WUS or CEUS) INFOTAB SITE Site type (Soil/Rock) NEWMARK FILENAME Record file name NEWMARK REGION Region (WUS or CEUS) NEWMARK SITE Site type (Soil/Rock) NEWMARK DIR Record direction (horizontal/vertical) NEWMARK MAG Earthquake magnitude NEWMARK PGA Peak ground acceleration NEWMARK KYMAX ky/kmax (ratio of yield acceleration to PGA) NEWMARK DISP Calculated permanent (Newmark) displacement Note: Rock/Soil Definitions ≈A and B for rock, C, D and E for soil based on NEHRP classification. Table 5-4. Description of different fields in the access ground motion database.

45 Figure 5-8. Average normalized spectral acceleration for rock records. Figure 5-9. Average normalized spectral acceleration for soil records.

the previously mentioned database were used in the regression analysis. The regression analyses were performed for different regions (WUS/CEUS) and site conditions (rock/soil), resulting in four different correlations. The correlations are presented in Equations (5-4) to (5-7). The units in Equations (5-4) to (5-7) are displacement (d) in inches, PGA in g, and PGV in in/sec. WUS-Rock: with a standard error of 0.22 log10 units. WUS-Soil: with a standard error of 0.22 log10 units. CEUS-Rock: with a standard error of 0.31 log10 units. CEUS-Soil: with a standard error of 0.23 log10 units. When using the above equations, the term kmax is the peak ground acceleration coefficient (PGA) at the ground surface log . . log . logmax mad k k k ky y( ) = − − ( ) + −1 49 0 75 3 62 1 x max. log . log ( ) ( ) − ( ) + ( )0 85 1 61k PGV 5-7 log . . log . logmax mad k k k ky y( ) = − − ( ) + −1 31 0 93 4 52 1 x max. log . log ( ) ( ) − ( ) + ( )0 46 1 12k PGV 5-6 log . . log . logmax mad k k k ky y( ) = − − ( ) + −1 56 0 72 3 21 1 x max. log . log ( ) ( ) − ( ) + ( )0 87 1 62k PGV 5-5 log . . log . logmax mad k k k ky y( ) = − − ( ) + −1 55 0 75 3 05 1 x max. log . log ( ) ( ) − ( ) + ( )0 76 1 56k PGV 5-4 modified by the Site Class factor for peak ground acceleration (Fpga). The current AASHTO LRFD Bridge Design Specifica- tions define the site-adjusted PGA as As. For this Project kmax is used rather than As to be consistent with the common prac- tice in geotechnical earthquake engineering of using k as the seismic coefficient during seismic earth pressure and slope stability evaluations. 5.2.8 Comparison Between Correlations A comparison between correlations for different regions and site conditions has been performed. The comparison was car- ried out for two cases, assuming PGV (in/sec) = 30 × PGA (in/sec2) and PGV (in/sec) = 60 × PGA (in/sec2), respectively. These comparisons are shown in Figures 5-10 through 5-17. The results from these comparisons are summarized as follows: • Figures 5-10 and 5-11 show the comparison between rock and soil correlations for WUS region [Equations (5-4) and (5-5)] for PGV = 30 × kmax and PGV = 60 × kmax, respectively. • Figures 5-12 and 5-13 show the comparison between the rock and soil correlations for CEUS region [Equations (5-6) and (5-7)] for PGV = 30 × kmax and PGV = 60 × kmax, respectively. • Figures 5-14 and 5-15 compare WUS-Rock and CEUS-Rock correlations [Equations (5-4) and (5-6)]. • Figures 5-16 and 5-17 show the comparison between Martin-Qiu correlation and WUS-Rock correlation [Equa- tions (5-2) and (5-4)]. These comparisons show that the CEUS-Rock correlation results in smaller displacements in comparison to other cor- relations, including the Martin-Qiu correlation. It should be 46 Figure 5-10. Comparison between WUS-Rock and WUS-Soil correlations for PGV = 30  kmax.

Figure 5-11. Comparison between WUS-Rock and WUS-Soil correlations for PGV = 60  kmax. Figure 5-12. Comparison between CEUS-Rock and CEUS-Soil correlations for PGV = 30  kmax. Figure 5-13. Comparison between CEUS-Rock and CEUS-Soil correlations for PGV = 60  kmax.

Figure 5-14. Comparison between WUS-Rock and CEUS-Rock correlations for PGV = 30  kmax. Figure 5-15. Comparison between WUS-Rock and CEUS-Rock correlations for PGV = 60  kmax. noted that the correlations for other regions (that is, CEUS- Soil, WUS-Rock, and WUS-Soil) result in relatively similar displacement levels slightly greater than the Martin-Qiu correlation. Consequently correlations were combined for these data leading to a mean displacement correlation given by: All data except CEUS-Rock: with a standard error of 0.23 log10 units. log . . log . logmax mad k k k ky y( ) = − − ( ) + −1 51 0 74 3 27 1 x max. log . log ( ) ( ) − ( ) + ( )0 80 1 59k PGV 5-8 5.2.9 Confidence Level The displacement correlations discussed in previous sec- tions were based on a mean regression curve on the observed data. For design purposes a higher confidence level than the mean curve (the mean curve corresponds to 50 percent con- fidence level) is often selected. A common practice is to use the mean curve plus one standard deviation, which approxi- mately corresponds to a confidence level of 84 percent. Fig- ures 5-18 and 5-19 show the 84 percent confidence intervals for permanent displacement based on site-adjusted peak ground acceleration coefficient of 0.3 and PGV = 30 × kmax and 48

PGV = 60 × kmax, respectively, with respect to the mean design curve given by Equation (5-8). 5.2.10 Design Recommendations For design applications, Equation (5-8) for soil and rock sites for WUS and CEUS and Equation (5-6) for CEUS rock sites are recommended. The regression curves shown on Figure 5-18 and Figure 5-19 suggest that 84 percent confi- dence levels in displacement evaluations could be reason- ably approximated by multiplying the mean curve by a factor of 2. 5.3 Correlation of PGV with S1 A procedure for establishing the PGV for design from the spectral acceleration at one second (S1) also was developed for the Project. For earth and buried structures, PGV provides a direct measure of the ground deformation (as opposed to ground shaking parameters represented by the spectral am- plitude) and is a more meaningful parameter than PGA or spectral accelerations for designing against kinematic loading induced by ground deformation. Also PGV is a key parame- ter used for Newmark deformation analysis, as described in Section 5.2. 49 Figure 5-16. Comparison between Martin-Qiu and WUS-Soil correlations for PGV = 30  kmax. Figure 5-17. Comparison between Martin-Qiu and WUS-Soil correlations for PGV = 60  kmax.

The initial approach taken to develop the PGV-S1 correla- tion involved performing statistical studies of the USNRC database. However, the resulting correlation exhibited con- siderable scatter. Subsequently a correlation being devel- oped by Dr. Norm Abrahamson of the Pacific Gas and Electric Group in San Francisco was identified through dis- cussions with seismologists involved in ground motion studies. Dr. Abrahamson forwarded a draft paper that he was writing on the topic. (A copy of the draft paper was originally included in Appendix D. Copyright restrictions prevented in- cluding this draft as part of the Final Report for the NCHRP 12-70 Project.) In the draft of the Abrahamson’s paper, the following re- gression equation was recommended for determining PGV based on the spectral acceleration at 1 second (S1) and the magnitude (M) of the earthquake. where PGV is in units of cm/sec, S1 is spectral acceleration at T = 1 sec in units of g, and M is magnitude. Dr. Abrahamson reported that this equation has a standard deviation of 0.38 natural log units. Because the strong motion database used in Dr. Abraham- son’s regression analyses consists of exclusively the WUS database, an evaluation was performed to determine whether the above regression equation would be valid for representative ln . . ln . ln .PGV( ) = + ( ) + ( ) +( )3 97 0 94 0 013 2 931 1 2S S + 0 063. ( )M 5-9 50 Figure 5-18. Mean Newmark displacement and 84% confidence level, PGA = 0.3g, PGV = 30  kmax. Figure 5-19. Mean Newmark displacement and 84% confidence level, PGA = 0.3g, PGV = 60  kmax.

51 Figure 5-20. Comparison between Abrahamson PGV equation with all data in NUREG/ CR-6728. CEUS records. The NUREG/CR-6728 strong motion data, as discussed in Section 5.2.4, was used to evaluate the validity of the Abrahamson PGV equation shown above. Figures 5- 20 through 5-24 present comparisons between the results of the Abrahamson PGV equation and the strong motion data- base from NUREG/CR-6728. The following conclusions can be made from Figures 5-20 through 5-24: 1. The Abrahamson PGV equation gives reasonable predic- tions using the NUREG/CR-6728 database, even though the strong motion database from CEUS is characterized by much lower long-period ground motion content. Part of the reason is that the spectral acceleration at 1 second has been used as a dependent variable in the regression equa- tion. The reasonableness of the comparisons occurs when rock and soil conditions are separated for the CEUS and the WUS. 2. Magnitude (M) appears to play a very small role in affect- ing the predicted PGV result. For example, there is very lit- tle change (that is, barely 10 percent) in the resultant PGV value as the magnitude M changes from 5.5 to 7.5. The in- sensitivity of magnitude, as well as the potential difficulty and/or ambiguity in establishing the deaggregated magni- tude parameter for many CEUS sites where the seismic sources are not well defined, was discussed with Dr. Abra- hamson (2005). From a practical perspective, it was con- cluded that the PGV correlation could be significantly simplified by eliminating the parameter M from Equation (5-9). Dr. Abrahamson concurred with this suggestion. 3. During discussions with Dr. Abrahamson, various other versions of the PGV predictive equation were discussed. Other versions involve using spectral acceleration at the 3-second period. These equations are more suitable for capturing peak ground velocity if there is a strong velocity pulse from near-fault earthquake records. However, for applications involving the entire United States, especially for CEUS, these near-fault attenuation equations are not believed to be relevant or appropriate at this time. Dr. Abrahamson reported that his research found that PGV is strongly correlated with the spectral acceleration at 1 second (S1); therefore, the attenuation equation used S1 to anchor the regression equation. Dr. Abrahamson commented that be- sides the 1-second spectral acceleration ordinate, other spec- tral values around 1 second might be used to improve the PGV prediction; however, from his experience, the PGA (that is, peak ground acceleration or spectral acceleration at zero- second period) has a frequency too far off for correlating with PGV, and this difference tends to increase the error in the regression equation. From these comments, a decision was made to use the PGV equation based solely on the 1-second spectral acceleration ordinate (S1). In all the presented figures, the PGA amplitudes are depicted in four different categories.

52 Figure 5-22. Comparison between Abrahamson PGV equation with only NUREG/CR-6728 CEUS soil data. Figure 5-21. Comparison between Abrahamson PGV equation with only NUREG/CR-6728 CEUS rock data.

53 Figure 5-23. Comparison between Abrahamson PGV equation with only NUREG/CR-6728 WUS rock data. WUS-SOIL 100 10 PG V (in /s) 1 0.1 0.001 0.01 0.1 S1(g) 1 10 0.0<PGA<0.1 0.1<PGA<0.2 0.2<PGA<0.3 0.3<PGA Norm Mean Norm Mean-1s Norm Mean+1s Figure 5-24. Comparison between Abrahamson PGV equation with only NUREG/CR-6728 WUS soil data.

From these plots, the trend of increasing PGV with S1 is very evident; however, there is no discernible trend for PGA. In addition to presenting the median PGV equation, Fig- ures 5-20 through 5-24 show the mean-plus and the mean- minus one standard deviations. These lines use the standard deviation coefficient of 0.38 as suggested by the Abrahamson PGV equation. The use of the standard deviation coefficient of 0.38 implies that the mean-plus one standard deviation and the mean-minus one standard deviation will be 1.46 and 0.68 of the median PGV values. From the five figures presented in this section, the follow- ing relationship was selected for estimating PGV for design analyses, with the equation reduced to the following expres- sion in log10 units rather than natural log basis: where PGV = inches/sec and For design purposes Equation (5-10) was later simplified to the following equation. Equation (5-10) was developed by using the mean-plus one standard deviation prediction (shown in heavy thick lines in the five figures for an M = 7.5 event). 5.4 Conclusions The work presented in this chapter forms the basis of the ground motion determination used during the seismic analy- sis and design of retaining walls, slopes and embankments, and buried structures. The results of the ground motion stud- ies were developed by interpreting existing strong motion data relative to recommendations that were made for the up- date of the AASHTO LRFD Bridge Design Specifications. PGV in 5-11sec ( )( ) = 55 1F Sv C S S1 10 1 10 14 82 2 16 0 013 2 30 2 93= + + +[ ]. . log . . log . 2 PGV 0.3937 10 5-100.434C1= × ( ) Earthquake ground motion studies described in this chap- ter are based on an earthquake with a 7 percent probability of exceedance in 75 years (that is, the 1,000-year return period), consistent with the recommendations adopted by AASHTO in July of 2007. The 1,000-year earthquake ground motions are available in maps and from an implementation CD de- veloped by the USGS for AASHTO. As shown in this chapter, the recommended 1,000-year return period is a significant change from the existing AASHTO Specifications, in terms of PGA and spectral shape for WUS and CEUS locations. These differences need to be considered when conducting seismic analysis and design for retaining walls, slopes and embank- ments, and buried structures, and therefore these ground motion discussions form an important component of the overall NCHRP 12-70 Project. The information from ground motion review also was used to update Newmark displacement correlations, as also de- scribed in this chapter. Newmark displacement correlations will be used for estimating the displacement of retaining walls, slopes and embankments, and buried structures, as discussed in later chapters. The update in the displacement correlations considered ground motions that will typically occur in CEUS as well as WUS. Again both the PGA and spectral shape were important considerations during the development of these correlations. Results of the Newmark displacement studies led to two equations [Equation (5-6) for CEUS rock sites and Equation (5-8) for WUS soil and rock sites and CEUS soil sites] and two charts (Figures 5-18 and 5-19) for use in design. As a final component of the ground motion studies, a cor- relation between PGV and spectral acceleration at 1 second (S1) was developed. This information is needed within the Newmark displacement correlations developed for this Proj- ect, as well as for evaluating the transient response of buried structures. Equation (5-10) presents the correlation. Results of the equation are compared with records from the USNRC strong motion database to show the reasonableness of the rec- ommended equation. For design purposes Equation (5-10) was later simplified to Equation (5-11). The simplified equa- tion provided a reasonable approximation of the data. 54

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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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