National Academies Press: OpenBook

Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments (2008)

Chapter: Chapter 4 - Work Plan: Analytical Methodologies

« Previous: Chapter 3 - Problems and Knowledge Gaps
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Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 4 - Work Plan: Analytical Methodologies." 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 4 - Work Plan: Analytical Methodologies." 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 28
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Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 29
Page 30
Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 30
Page 31
Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 31
Page 32
Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 32
Page 33
Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 33
Page 34
Suggested Citation:"Chapter 4 - Work Plan: Analytical Methodologies." 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 34

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26 The goal of Task 3 for the NCHRP 12-70 Project was to iden- tify analytical methodologies that would be developed to address the knowledge gaps and problems presented in the previous chapters. The discussion of the work plan for analyti- cal methodology developments is presented under four major headings: • Seismic ground motions • Retaining walls • Slopes and embankments • Buried structures The discussion of seismic ground motion follows earlier discussions about the importance of the ground motions to the overall Project. As noted previously, decisions on seismic ground motion levels depended to a certain extent on conclu- sions reached during the NCHRP 20-07 Project, which was con- ducted as a separate contract. One of the principal investigators for the NCHRP 12-70 Project served as a technical advisor to the NCHRP 20-07 Project, enabling the NCHRP 12-70 Project to keep abreast of the ground motion recommendations and other components of the NCHRP 20-07 Project that could affect the NCHRP 12-70 Project. 4.1 Developments for Seismic Ground Motions The first area of development involved the ground motions used during the seismic design of retaining walls, slopes and embankments, and buried structures. The LRFD design pro- cedure involves comparing the capacity of the design element to the seismic demand for various limit states (that is, strength, service, and extreme). Establishing the seismic ground motion was a necessary step when defining the expected demand dur- ing seismic loading. The Project followed the recommendations from the NCHRP 20-07 Project in the definition of the seismic ground motion demand. The NCHRP 20-07 Project recommended adoption of the 1,000-year return period for the extreme limit state (that is, an event having a 7 percent probability of exceedance in 75 years). The NCHRP 20-07 guideline also focused its approach on the spectral acceleration at 1–second period (S1). This was an important development prompted by the observation that PGA is not a good param- eter to correlate with historical damage to structures. Measures of ground shaking at some intermediate period range (say spectral accelerations around 1 to 2 seconds) are a better indi- cator of displacement demand related to historical damage and hence more important for characterizing ground shaking for design. This is also true for designing retaining walls, slopes and embankments, and buried structures. In general, PGV is closely related to spectral accelerations at intermediate periods and, therefore, is a more appropriate measure of ground motion displacement demand than PGA, especially for cross correlation to the amplitude of ground deformations or permanent slope displacements. Also, re- cent seismological research suggested that lower levels of spectral acceleration at intermediate periods for CEUS com- pared to WUS, and these reductions are relevant to Project requirements. Historically, due to the absence of strong motion data from CEUS sites, seismic design criteria for projects in CEUS have generally been developed by applying the small PGA values from the CEUS sites to empirical WUS spectral shapes to define the target design spectrum for CEUS conditions. However, studies such as NUREG/CR-6728 conducted by the Nuclear Regulatory Commission (NRC) for nuclear power plant applications (NUREG, 2001) have shown that the dif- ferences in CEUS seismological conditions not only result in lower shaking levels (that is, lower PGA), but also result in much lower long-period content for CEUS sites. The NUREG/ CR-6728 studies have been adopted by the NRC in recogni- tion of the fundamental difference between requirements for seismological studies in CEUS versus historical WUS C H A P T E R 4 Work Plan: Analytical Methodologies

practice. Figure 4-1 presents the WUS and CEUS geograph- ical boundary following the USGS seismic-hazard mapping program. The boundary basically follows the Rocky Moun- tains passing through Montana, Wyoming, Utah, Arizona, then bending east through southern Colorado, New Mexico, and western Texas. Figure 4-2 presents results from a major study funded by NRC to identify differences in ground motion characteristics between WUS and CEUS for horizontal motions representa- tive of magnitude 6.5 events for generic soil sites. The NUREG/ CR-0098 spectral shape shown in Figure 4-2 is based on New- mark’s recommendation using historical strong motion data from WUS, while the spectral shape for CEUS was developed using procedures described in the NUREG/CR-6728 report based on up-to-date techniques for CEUS endorsed by NRC. The Regulatory Guide 1.60 is the historical design spectral shape originally used for designing nuclear power plants, now consid- ered overly conservative. In this figure both spectral displace- ment (RD) and peak spectral acceleration (PSA) at 1 second are normalized by PGA. 27 Figure 4-1. Boundary between WUS and CEUS. Figure 4-2. Spectral curve shapes for generic sites covering both WUS and CEUS (Sandia, 2004).

28 Along with the difference in the PGA between WUS and CEUS sites, these figures show the drastic difference in the shaking hazard as measured by the peak spectral acceleration at 1 second (S1) or PGV between a WUS and a CEUS site. Such changes between the WUS and CEUS are also reflected in AASHTO 1,000-year maps. In view of the differences in ground motion characteris- tics, hence response spectra, between CEUS and WUS, as well as the NCHRP 20-07 Project recommendation to use the spectral acceleration at a 1-second period as the parame- ter for defining the level and requirements for bridge design, a focused ground motion study was conducted during the NCHRP 12-70 Project to establish a consistent approach for both projects. The NCHRP 12-70 ground motion study in- volved development of an analytical methodology that relates PGV and spectral acceleration at 1-second period (S1) and between PGV and PGA for CEUS and WUS. Effects of local soil conditions on the relationship between these ground motion parameters were avoided by developing the rela- tionships for NEHRP Site Class B conditions (that is, rock with a shear wave velocity between 2,500 and 5,000 feet per second), and then applying site coefficients to correct for soil conditions. This development was accomplished using an available ground motion database, including spectrum- compatible time history development reflecting differences in WUS and CEUS conditions. 4.2 Developments for Retaining Walls The next major area of development involved improved methods for estimating the forces on and the displacement response of retaining walls. The approach for evaluating the seismic displacement response of retaining walls consisted of using a limit equilibrium stability analysis in combination with the results of the seismic demand (ground motion) stud- ies described above. Analytical developments were required in three areas, as discussed in the following subsections. The focus of these developments was on rational methods for es- timating forces on and deformation of retaining walls located in CEUS and WUS. 4.2.1 Generalized Limit Equilibrium Analyses The problems and knowledge gaps associated with existing AASHTO Specifications for seismic earth pressure determi- nation have been summarized in the Chapter 3 discussion. Many problems are associated with the M-O equations used to compute seismic active and passive earth pressures for wall design. These problems include the inability of the M-O equa- tions to handle complex wall profiles, soil stratigraphies, and high seismic coefficients. With a few exceptions, these problems preclude practical modification of the M-O equations for general use. The problem for seismic active earth pressures can be overcome by the use of commercially available, limit- equilibrium computer programs—the same as used for the analysis of seismic slope stability. Current versions of many of these programs have the versatility to analyze conventional semi-gravity walls, as well as MSE, soil nail, or anchored walls. These analyses can be performed for complex wall profiles, soil stratigraphy, surcharge loading, and pseudo-static lateral earthquake loading. In the case of semi-gravity walls, values of earthquake- induced wall loads (PAE) induced by retained soils can be computed from a limit equilibrium stability analysis by cal- culating the maximum equivalent external load on a wall face (Figure 4-3) corresponding to a safety factor of 1.0. This con- cept, referred to as the generalized limit equilibrium (GLE) method, can be calibrated back to an idealized M-O solution for uniform cohesionless backfill, and has been used in prac- tice to replace M-O solutions for complex wall designs. The line of action of the external load can reasonably be assumed at the mid-height of the wall acting at an appropriate friction angle. In the case of MSE or soil nail walls, internal and exter- nal stability evaluations may be undertaken using limit equi- librium computer programs without the empiricism presently associated with AASHTO Specifications. Such an approach has been described by Ling et al. (1997). Potential computer programs for evaluating the GLE methodology were reviewed. One of the most valuable docu- ments for this review was a study by Pockoski and Duncan (2000) comparing 10 available computer programs for limit equilibrium analysis. Programs included in the study were UTEXAS4, SLOPE/W, SLIDE, XSTABLE, WINSTABL, RSS, Figure 4-3. Limit equilibrium method for estimating seismic active earth pressures.

SNAIL, and GOLDNAIL. Example problems in the Pockoski and Duncan report addressed design and analysis of MSE, soil nail and anchored (tieback) walls, and examined issues such as ease of use, accuracy, and efficiency. However, the Pockoski and Duncan study considered only static loading conditions. The programs MSEW (based on AASHTO Specifications for MSE walls) and ReSSA (a limit equilibrium program for re- inforced soil slopes), both developed by ADAMA Engineer- ing Inc. (ADAMA, 2005a and b) and licensed to the FHWA, also were considered in this review. An application of the latest version of ReSSA has been illustrated in a paper by Leshchinsky and Han (2004) and compared to FLAC analyses. Based on the review of the above report by Pockoski and Duncan, information from some of the software suppliers, and discussions with various researchers and practitioners, the programs SLIDE, MSEW, and ReSSA (2.0) appeared to be the best suited for use in the analytical methodology develop- ment of the Project. Checks with an alternate program were also performed to confirm the flexibility of the methodology being recommended for development. Application examples are further discussed in Chapter 7. In the case of semi-gravity walls validation of the GLE approach with the closed-form M-O solutions is discussed in Chapter 7. Parametric studies and examples of design applications to representative walls including wall-height effects and deformation analyses (discussed in Sections 4.2.2 and 4.2.3, respectively), along with comparative examples using existing AASHTO design methods, also are discussed in Chapter 5 and 6. 4.2.2 Wall Height-Dependent Seismic Coefficient The next area of analytical methodology development involved a sound technical procedure for selecting the seis- mic coefficient to be used in the limit equilibrium approach. The current practice in selecting the seismic coefficient as- sumes rigid body soil backfill response where the seismic coefficient is defined by the peak ground acceleration oc- curring at a point in the free field. For wall heights in excess of approximately 30 feet, this rigid-body assumption can be questioned. Figure 4-4 presents two schematic diagrams illustrating the issues pertaining to the seismic coefficient used for wall pres- sure determination compared to the free-field motion at a point on the ground surface. For simplicity, a massless re- taining wall is used to eliminate the inertial response of the retaining wall, thereby resulting in a relatively simple prob- lem involving inertial response of the retained fill acting on the wall. For this problem the soil mass behind the retaining wall is governed by incoherency in the ground motion at dif- ferent points of the soil mass. The acceleration time history response at different points in the soil mass will be different from each other. Total force acting when normalized by the soil mass within the failure plane gives rise to an equivalent seismic coefficient for wall design. As the retaining wall height and the lateral dimension of the mass increase, an increasing degree of the high fre- quency content of the ground motion will be eliminated. Hence, the seismic coefficient for earth pressure determina- tion should be a function of wall height, as well as a function of the frequency content of the ground motion record. High frequency-rich ground motions tend to be more incoherent and result in a lower seismic coefficient. This observation also means that the seismic coefficient should decrease for the low, long-period content of CEUS motion records as compared to WUS, or for rock motion records as opposed to soft soil site records. This analytical development to quantify the effects of incoherency (also referred to as scattering or wave scattering in this Final Report) involved use of a library of spectrum- compatible time histories representing a range of conditions, including earthquake magnitudes, soil versus rock sites, and CEUS versus WUS locations. This information was used to evaluate the dependence of the seismic coefficient on wall height. Coherency (wave-scattering) analyses were con- ducted, and then the acceleration time histories for various failure mechanisms were integrated to evaluate the relation of seismic coefficient versus the original reference PGA and spectral acceleration at 1 second (S1). The wave scattering analyses were conducted for multiple wall heights (for exam- ple, 30-foot, 60-foot, and 100-foot heights). The variation in seismic coefficient was established as a function of time, thereby defining “seismic coefficient time histories” for dif- ferent locations behind the retaining wall. 29 Figure 4-4. Effects of spatially varying ground motions on seismic coefficient.

30 The resultant seismic coefficient time histories were used for conducting Newmark sliding block analyses for wall deforma- tion studies. More meaningful seismic coefficients for pseudo- static earth pressure design were established by relating the acceleration ratio in the Newmark analysis to a limiting perma- nent displacement value (say at 6 inches) from the conducted analyses. The resultant product of this effort was charts of seis- mic coefficient versus PGA for different wall heights. Charts of wall height-dependent seismic coefficient versus 5 percent damped spectral acceleration at 1 second (S1) also were devel- oped. The latter charts might have better technical merit as discussed earlier regarding fundamental differences between PGA versus S1. 4.2.3 Deformation Analyses As part of this effort, an updated analytical methodology was developed for estimating wall deformations during seismic loading as a function of yield acceleration. This approach was allowed within the then current (2006) AASHTO Specifica- tions; however, the equation used for estimating displacements was based on a limited database. The following approach was taken from the updated ana- lytical methodology: 1. Semi-gravity walls: Using the computed time histories as- sociated with the wall height seismic coefficient studies, Newmark sliding block charts showing displacements ver- sus the ratio of yield acceleration to the peak ground ac- celeration (ky/kmax) were determined. (Note that ky is the acceleration that results in a factor of safety of 1.0; kmax is the PGA adjusted for local site effects. The kmax term is equivalent to As in the current AASHTO LRFD Bridge Design Specifications. The seismic coefficient for retaining wall design is commonly defined in terms of k rather than PGA to indicate a dimensionless seismic coefficient. The use of k to define seismic coefficient during wall design is followed in this Project.) These charts are a function of S1, which relates strongly to PGV. The charts in turn were used to reassess the suitability of the 50 percent reduction factor in peak acceleration included within AASHTO for pseudo-static wall design. As noted previously, the 50 per- cent reduction is based on acceptable horizontal displace- ment criteria, where walls are free to slide. For walls sup- ported by piles, displacement limits need to be integrated with pile performance criteria associated with pile capac- ity. In such cases, questions related to pile pinning forces and their influence on yield accelerations of the wall-pile system also need to be considered. 2. MSE walls: Deformation analyses to assess performance criteria for MSE walls are clearly more complex than for semi-gravity walls due to the flexibility of the wall system. A valuable source of reference material on this topic has been documented in a University of Washington Master of Science thesis by Paulsen (2002), where an equivalent Newmark sliding block analysis was developed to accom- modate the additional deformations arising from rein- forcing strip deformation and slip. However, parameter selection for the model was empirical and based on cali- brations from centrifuge and shake table tests. Whereas the model was promising, it was insufficiently mature for practical application at this time. FLAC analyses also have been performed to evaluate deformation behavior under seismic loading, and may be applicable for analysis of spe- cial cases. However, with respect to AASHTO Specifica- tions, the analytical methodology attempted to relate the proposed pseudo-static limit equilibrium analyses to de- formation performance criteria in an empirical way, based on existing case histories and model tests, and the ap- proach described by Ling et. al. (1997). 4.3 Developments for Slopes and Embankments The next major area of development involved methods for evaluating the seismic performance of cut slopes and fill em- bankments. Relative to the development needs for retaining walls, these needs were not as significant. In most cases suit- able analytical methodologies already existed for evaluating the seismic response of slopes and embankments, but these methods were not documented in the AASHTO LRFD Bridge Design Specifications, suggesting that much of the work re- lated to slopes and embankments involved adapting current methodologies into an LRFD specification and commentary. Although development needs for slopes and embankments were less than for the other two areas, three developments were required, as summarized below: • Develop a robust set of Newmark displacement charts for slope displacement evaluations, reflecting both differences between WUS and CEUS and the influence of slope height. In this respect, the analysis approach was similar to that previously described for walls. However, additional param- eters were needed in examining the coherence of inertial loads over potential sliding masses, including slope angle and shear wave velocities of slope material, and strength parameters ranging from those for cut slopes to fills. The analysis program used for wave scattering analyses involved QUAD-4M (1994). • Develop a screening method for determining areas requir- ing no seismic analysis. The screening method depended on a combination of the level and duration of ground shaking, the geometry of the slope, and the reserve capacity that the slope has under static loading. A critical consideration in

the development of a screening method was the identifica- tion of potentially liquefiable soils and how these condi- tions would be handled in the evaluation. Guidelines were developed for the NCHRP 12-49 Project for treating the stability of approach fills located on liquefiable soils; these methods served as a starting point for this Project as well. • As no LRFD approach for the static design of slopes exists, a commentary that addressed strength parameter selection for static and seismic design and was consistent with approaches to retaining wall design was developed as part of this Project. Based on the literature review and identification of knowledge gaps summarized in Chapters 2 and 3, the work on slopes and embankments was limited to soil conditions and did not include rock slopes. The stability of rock slopes during seismic loading is controlled by the specific fracturing patterns of the rock, making a generic approach for the eval- uation of the seismic stability of rock slopes beyond what could be accomplished by this Project. For this reason it was concluded that the topic of rock slope stability during seismic loading should be addressed by site-specific evaluations. 4.4 Developments for Buried Structures The final area of development involved a methodology for dealing with buried culverts and pipe structures. It was rec- ognized that the seismic hazard to buried culverts and pipes can be classified as being caused by either peak ground dis- placement or TGD resulting from wave propagation. How- ever, there was no existing seismic design methodology or guidelines for the design of culvert or pipe structures in Sec- tion 12 of the AASHTO LRFD Bridge Design Specifications. Design and analysis procedures have been proposed by some researchers and design engineers for pipelines (for ex- ample, gas and water) or tunnel (that is, transportation or water) systems. While some of these procedures can be used for the design and analysis of culvert and pipes (for example, the trans- verse racking/ovaling deformation of the section), others cannot be directly applied because (1) culverts and pipes are typically of limited length, (2) culverts and pipe structures are typically constructed within a built-up embankment, and (3) the characteristics of peak ground displacement and its effects on culvert and pipes are phenomenologically complex. The analytical methodology development for buried struc- tures involved the following main elements: • Develop analysis procedures for TGD. – Guidelines on the selection of design TGD parameters. – Methods for estimating transverse racking/ovaling deformations (provide design charts as well as recom- mended step-by-step procedure). – Validation of design charts by numerical analysis. – Apply procedures to an established range of problems. – Develop screening guidelines to provide a basis for screen- ing culverts and pipelines relative to their need for fur- ther seismic evaluation (that is, define the “no-analysis required” criteria). • Identify analysis procedures for peak ground displacement. – Guidelines on the selection of design peak ground dis- placement parameters (for example, spatial distribution of ground motions and soil stiffness parameters). – Effects of soil slope slumping, liquefaction-induced lat- eral spread and settlements, and fault rupture. 4.4.1 Analysis Procedures for TGD The response of a buried linear structure can be described in three principal types of deformations: (a) axial deforma- tions, (b) curvature deformations, and (c) ovaling (for circu- lar cross section) or racking (for rectangular cross section) deformations as shown in Figures 4-5 and 4-6. The axial and curvature deformations are induced by com- ponents of seismic waves that propagate along the culvert/ pipe axis. Current design and analysis methodologies for pipelines and tunnel systems were developed typically for long, linear structures. Culverts and pipe structures for trans- portation applications, however, are typically of limited length. For this condition the transient axial/curvature deformations should generally have little adverse effects on culvert/pipe structures and, therefore, design and analysis provisions may not be required for these two modes of TGD effects. This pre- liminary assumption, however, was further evaluated during the completion of the initial phase of this study and verified by numerical analysis. The ovaling/racking deformations are induced along the transverse cross section when seismic waves propagate per- pendicularly to the culvert/pipe axis. The design and analysis methodology develop by Wang (1993) can be readily applied for culverts with circular or rectangular cross sections. For example, the simple design chart shown in Figure 4-7 allows quick determinations of induced culvert/pipe racking/ovaling deformations. Previous observations have suggested that smaller diameter pipes (or small diameter highway culverts) are more resistant to ovaling deformations than the larger culvert structures. A further investigation of the factors resulting in this differ- ent performance between large and small buried structures was evaluated. Once identified, these factors were reflected in the screening guidelines discussed above. In addition, the proposed analytical methodology development attempted to identify simplified procedures for noncircular and non- rectangular sections. It was anticipated that parametric numer- ical analyses would be required for developing these simplified procedures. 31

32 Figure 4-6. Ovaling/racking deformations. MBF Figure 4-5. Axial/curvature deformations. Another important aspect for evaluating the TGD effects on culvert/pipe structures was to determine the appropriate design ground motion parameters to characterize the ground motion effects. It has long been recognized that PGA is not a good parameter for buried underground structures. Instead, PGV is a good indicator for ground deformations (strains) in- duced during ground shaking. This is particularly important because given the same PGA value, the anticipated PGV for CEUS would typically be much smaller than that for the WUS. Results based on the PGA versus PGV study presented earlier in the work plan for the retaining walls, slopes, and embank- ments were used for the culvert structures.

As a final consideration, there is an on-going proposal (NCHRP Project 15-28) to upgrade the computer program CANDE-89 to incorporate the LRFD design methodology. CANDE-89 is a comprehensive design/analysis tool for the cross section design and analysis (in two-dimensional plane- strain domain) of buried structures, particularly culverts. The seismic effects of transient racking/ovaling deformations on culverts and pipe structures must be considered additional to the normal load effects and preferably could be incorporated into the updated CANDE analysis. In Chapter 10 recom- mendations on proposed seismic design methodologies to be incorporated into the CANDE program are made. It is antic- ipated that an option would be required in the CANDE pro- gram to allow ground displacement profile as a loading input to the CANDE analysis. 4.4.2 Analysis Procedures for Permanent Ground Deformations (PGD) Various approaches for analysis or design of pipeline sys- tems (for gas and water) have been proposed under the effect of PGD including those to account for the effects of liquefaction- induced lateral spread, slope deformations (slump), post- liquefaction settlements, and fault displacements. Significant disparity exists among these approaches. There are also dif- ferent performance requirements and loading criteria being used or proposed for different studies. A consistent method- ology and design criteria compatible with other components of the highway facilities are yet to be developed for the culvert and pipe structures. In general, there are three major steps for evaluating the PGD effects: (1) determine the PGD patterns (that is, spa- tial distributions) using the site-specific subsurface condi- tions encountered at the culvert location; (2) derive the suitable soil stiffness accounting for the dynamic as well as cyclic effects (for example, softening due to liquefaction and repeated loading cycles; and hardening due to increased strain rates); and (3) evaluate the structural response to the PGD taking into consideration soil-structure interaction effects. In estimating the PGD patterns for liquefaction-induced lateral spread, slopes/embankment slumping, and post- liquefaction settlements, the procedures developed for re- taining walls, slopes, and embankments can be used. Fault rupture has a relatively low occurrence frequency. It is gen- erally difficult to design for the effects of fault rupture unless the fault displacement is small or the backfill within the soil envelope consists primarily of properly designed compress- ible material to accommodate the fault displacement. As part of this study, general guidelines on design strategy for coping with large PGD, based on various previous project experiences gained from tunnel and pipeline design, were identified. 4.5 Summary In summary, the proposed analytical methodology devel- opment plan resulted in work product elements shown in Table 4-1. This summary is a modified version of Exhibit 6 of the Working Plan for the NCHRP 12-70 Project. 33 Figure 4-7. Earthquake-induced structural transient racking/ovaling deformations.

34 Table 4-1. Work product elements. Type of Investigation Purpose Establish Basis for Determining Ground Motions Suitable for CEUS and WUS Identifies consistent approach for defining ground motions to use for seismic evaluation of retaining walls, slopes and embankments, and buried structures, including modifications that account for permanent displacements. Develop Design Charts for Estimating Height-Dependent Seismic Coefficient Provides a rational basis for selecting seismic coefficient as a function of both wall height and slope height for different soil conditions. Update Design Charts for Estimating Slope and Wall Movement Displacements Provides end users the means of estimating slope and wall movements as a function of yield acceleration, PGA, and PGV. Evaluate Suitability of Limit Equilibrium Computer Program based on Method of Slices for Determination of Lateral Earth Pressures Offers end users the means for improved methodology for establishing design seismic earth pressure magnitudes for mixed soil conditions, steep backslopes, and high ground motions. Identify Method for Designing Nongravity Cantilever Walls and Anchored Walls Using Limit Equilibrium and Displacement-Based Methods Establishes a basis for estimating seismic earth pressures to use for wall design and provides a simplified approach for conducting displacement-based analyses. Review Basis for Estimating Seismic Performance of MSE Walls Proposes revisions to design methodology based on conclusions from evaluations carried out for this Project, as appropriate. Document Approach for Evaluating Seismic Stability of Slopes and Embankments Provides documentation for limit equilibrium and displacement-based approach for evaluating seismic stability of slopes. Develop Design Approaches for Permanent and Transient Ground Deformation for Culverts and Pipelines Provides design guidance and specifications.

Next: Chapter 5 - Seismic Ground Motions »
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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