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1This Final Report summarizes work that was carried out on National Cooperative Highway Research Program (NCHRP) Project 12-70 Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments. This project in- volved an effort to develop analysis and design methods and recommended load and resistance factor design (LRFD) spec- ifications for the seismic design of retaining walls, slopes and embankments, and buried structures. 1.1 Overall Project Objectives, Approach, and Schedule The overall objectives of the Project were to develop analysis and design methods and to prepare LRFD specifications and ex- ample problems for the design of retaining walls, slopes and em- bankments, and buried structures. These overall objectives were intended to address short-comings in AASHTO LRFD Bridge Design Specifications or in some cases the absence of a recom- mended design methodology in the LRFD Specifications. The approach used to address these two objectives was out- lined in a Working Plan submitted by the Project Team to NCHRP in May of 2004. The Working Plan is based on CH2M HILLâs proposal to NCHRP in November of 2003, with mod- ifications summarized in Attachment 2 of CH2M HILLâs letter dated January 13, 2004, to Dr. Robert Reilly of the Transporta- tion Research Board. Also included in this Working Plan was a Progress Schedule tied to the Project start date of March 29, 2004, and a Table of Deliverables for this Project. A copy of the Working Plan for the Project is included in Appendix A to Volume 1 of this Final Report. Five fundamental goals were identified during the plan- ning of the Project in 2004. These goals formed the basis for the work that was to be done during each Project activity. The five goals involved ⢠Improving existing or developing new analytical methods to overcome the shortcomings of existing technology, based on sound soil-structure interaction principles; ⢠Optimizing design approaches for both routine design and special design cases using more comprehensive methods; ⢠Avoiding hidden conservatism in design approaches; ⢠Ensuring applicability of specifications to seismic zones nationwide, including provisions for âno seismic designâ in low seismicity regions; and ⢠Satisfying LRFD philosophy and providing flexibility in establishing serviceability criteria. The approach for the Project initially focused on data col- lection and review during Task 1, leading to the documentation of problems and knowledge gaps in Task 2. The problems and knowledge gaps identified in Task 2 were used to recommend analytical methodology developments in Task 3, and a detailed work plan in Task 4. The results of these four tasks were summarized in Task 5, the first Interim Report. This phase of the work occurred within the first 9 months of the planned 39-month project duration. Following submittal of the first Interim Report and the NCHRP Oversight Panelâs review and approval of the work plan described in the first Interim Report, the approved work plan was implemented in Task 6. An outline of the LRFD specifications was prepared in Task 7, and the results of the analytical developments and LRFD specification outline were summarized in Task 8, which was identified as the second In- terim Report. The submittal of the second Interim Report concluded Phase 1 of the Project. The schedule for complet- ing the second Interim Report was originally planned to be approximately 22 months after the initiation of the Project; however, actual work took approximately 24 months. Phase 2 was initiated upon completion of Task 8. This phase involved Task Orders 9-12, where specifications, com- mentaries, and example problems were prepared and sub- mitted to the NCHRP Oversight Panel for review. The third Interim Report provided the first draft of the specifications, commentaries, and example problems, in accordance with the requirements of Task 10. Following receipt of comments C H A P T E R 1 Introduction
2from the NCHRP Oversight Panel, Task 11 was implemented. This task involved (1) making further modifications to the specifications, commentaries, and example problems; (2) ad- dressing the Oversight Panelâs comments on the third Interim Report, and (3) and preparing a Final Report. This work was scheduled to be completed after 35 months but took approx- imately 39 months. The final work activity in Phase 2 on the Project, Task 12, involved preparation of this Final Report and the revised spec- ifications, commentaries, and example problems. This task was finalized in November of 2007, approximately 44 months following initiation of the Working Plan in April of 2004. Fol- lowing this submittal, an additional example problem was completed, specifications and commentaries were revised, and the Final Report finalized in June 2008. Throughout work on each task within the Project there was a continuing effort to focus on the final product of the Project. This product involved a methodology that could be used in areas that are both highly seismic and relatively aseismic; that could be implemented by staff from DOTs, vendors, and con- sulting firms using existing software without the need for ex- tensive training; and that âmade senseâ relative to observed performance during past earthquakes. This theme was im- plemented throughout the Project, from start to finish. To the extent practical, this theme is followed in the presentation of each chapter of this Draft Final Report. 1.2 Project Background Work on the NCHRP 12-70 Project was initiated in April of 2004. The following three subsections provide background information for the work that has been accomplished. This background information includes a summary of plans for implementing the overall LRFD design methodology and overviews of interim conclusions from the work performed on the Project. The overview of conclusions helps provide a perspective for the development work that is being summarized in subsequent chapters. 1.2.1 Plans for Implementing the LRFD Design Methodology The work carried out for the NCHRP 12-70 Project must be consistent with the philosophy and format of the AASHTO LRFD Bridge Design Specifications and the seismic provisions for highway bridges. In this philosophy, âBridges shall be de- signed for specified limit states to achieve the objectives of constructibility, safety, and serviceability, with due regard to issues of inspectibility, economy, and aesthetics. . . .â In the LRFD procedure, margins of safety are incorporated through load (γp) factors and performance (or resistance, Ïr) factors. 1.2.1.1 Factors to Consider The basic requirement for this Project is to ensure that fac- tored capacity exceeds factored load as defined by the following equation for various limit states (or acceptable performance): where Ïr = performance factor; Rn = nominal resistance; γpi = load factor for load component I; and Qi = load effect due to load component i. During the initial phase of work for this Project, the LRFD methodology was not formerly introduced. Rather, the focus of the work was on the identification and evaluation of a de- sign methodology without load or resistance factors. Once the methodologies were developed and approved, then an approach for incorporating load and resistance factors was established relative to the recommended methodologies. Although work on the initial phase of work did not present recommendations on load and resistance factors to use with the proposed methodologies, consideration was given by the Project Team to how load and resistance factors might eventu- ally be used during seismic design. Ideally this approach would build on the load and resistance factors used in the conven- tional static load case presented in the current version of the AASHTO LRFD Bridge Design Specifications. For the static design case the appropriate load and resist- ance factors have been developed to yield a consistent margin of safety in the designed structure. This same logic needs to be followed for seismic loading to retaining walls, slopes and embankments, and buried structures. However, the approach for defining a consistent margin of safety is more difficult to define for the following reasons: ⢠The load factors and load cases (that is, on the right- hand-side of the above equation) had to be consistent with those recommended by the NCHRP Project 20-07 Recom- mended LRFD Guidelines for the Seismic Design of Highway Bridges (Imbsen, 2006). At the time the NCHRP 12-70 Proj- ect was initiated, the NCHRP 20-07 Project was establishing the appropriate earthquake loading return periodâsubject to the approval of the AASHTO Highway Subcommittee on Bridges and Structures (HSCOBS T-3) and eventually the AASHTO voting members. These recommendations would result in larger loads associated with a seismic event at a specific site relative to the then current AASHTO re- quirements, but the likelihood of the load occurring de- creased and would be relatively infrequent. Under this sit- uation use of a load factor on the seismic load was believed to be overly conservative. (The NCHRP 20-07 Project was originally referred to as the NCHRP 12-49 Update Project. Ï Î³r n pi iR Q⥠Σ ( )1-1
The intent of the NCHRP 12-07 Project was to revise rec- ommendations given in the NCHRP 12-49 Project (NCHRP Report 472, 2003) for use in updating seismic provisions in the AASHTO LRFD Bridge Design Specifications. One of the key recommendations initially made by the NCHRP 20-07 Project was to increase the return period for seismic design from the 500-year level in the then current (2006) LRFD specifications to a 1,000-year return period. The probability of occurrence for the 1,000-year event is approximately 7 percent in 75 years. This recommendation was approved by AASHTO in July of 2007, at the time that the NCHRP 12-70 Project report was being finalized.) ⢠From a resistance factor standpoint, design could be per- formed using either a limit equilibrium or displacement- based approach. The selection of resistance factors for these two cases will differ. For example, use of a resistance factor less than 1.0 often will result in a conservative design using limit equilibrium methods, but could lead to an unconser- vative design for a displacement-based approach. While the starting point involved use of load and resistance factors equal to 1.0, in certain geographic areas and for certain categories of design, use of a resistance factor less than 1.0 (that is, Ï < 1.0) was considered for simplifying the design process. An example of this was for the evaluation of seismic stability of slopes. If a deformational approach is not taken and the owner wants to base the evaluation strictly on a com- parison of soil capacity to seismic loads, the current approach would be to confirm that the factor of safety is greater than 1.1 to 1.2 for an acceleration coefficient of 0.5 times the peak ground acceleration (PGA) at the ground surface. (Many ap- plications in geotechnical engineering are based on factors of safetyâwhere the resistance of the soil is compared to the forces causing failure. When using LRFD methods for the same design, it is often more meaningful to refer to the ca- pacity to demand (C/D) ratio rather than the factor of safety. The use of C/D ratio also is consistent with terminology used by bridge engineers. Discussions in this report will refer to C/D ratio and factor of safety interchangeably.) This same ap- proach can be taken in the context of LRFD design, but in this case the resistance factor is defined by the reciprocal of the factor of safety used, assuming that the load factor is equal to 1.0 for the reasons stated above. With this in mind the thrust of the work was to formulate the LRFD specifications in terms of the following three considerations: 1. Identifying the limit states to be considered during the earthquake load case. 2. Defining the expected performance of the designed system for each of the limit states defined in item (1) above. 3. Outlining the design analysis procedure and capacity criteria. The various limit states to be examined were categorized into three areas. The first involved the evaluation of the global stability of the overall site, which includes requirements for slope stability and similar mechanisms. The next dealt with the design of the foundation system for external stability (that is, sliding, overturning, and bearing) to ensure that the size of the foundation and the implied geotechnical (that is, overall soil) capacity was sufficient. The last involved the design for internal structural stability to ensure that structural compo- nents functioned properly under the increased dynamic load from the earthquake. Depending on whether a design project involved a retaining wall, a slope or embankment, or a buried structure, an assessment of one or more of these limit states may not be required. For example, the limit state for seismic design of slopes and embankments only involves global stability, while the buried structure only considers internal stability. 1.2.1.2 Relationship to Design Process From past earthquake experience, most cases of observed or postulated failures relate to intolerable structural damage, as opposed to excessive overall movement, especially for retaining walls and buried structures. These structures are inherently more sensitive to movement relative to above- ground structures. Also, most freestanding retaining walls (that is, other than bridge abutments) can undergo a signifi- cant degree of movement without adversely impacting their intended functions. Therefore, the most germane LRFD design issue was to as- sure structural integrity, commonly referred to as designing for the internal stability of the earth retaining system. When de- signing for structural integrity, the geotechnical engineer will define the seismic loading criteria and conducts soil-structure interaction analyses, as needed, for characterizing foundation stiffness and damping parameters. The responsibility of actual design usually falls to the structural designer. The structural en- gineer typically will bear the responsibility for conducting the structural response analyses and will make use of the recom- mendations regarding seismic loading and foundation stiffness in a global model. The structural designer would be the one who actually goes through the LRFD design process in check- ing the structural capacity versus demand, and eventually will sign the structural drawings. Requirements in other sections of the AASHTO LRFD Bridge Design Specifications are followed when conducting structural analyses and design checks. Note that this general approach is not always the case. For some wall types, such as the Mechanically Stabilize Earth (MSE) or soil nail walls, the geotechnical engineer also may be responsible for the internal stability as well. In this case the geo- technical engineer would select reinforcing or soil nail size, and confirm that the stresses imposed by seismic loading are acceptable relative to LRFD requirements. 3
Understanding the role of the geotechnical and structural engineers is rather important, and this Project needed to clar- ify these roles in the process of preparing the LRFD specifica- tions. These roles also need to be understood in the definition of load and resistance factors to use during design. Since in- dependent groups often are responsible for the design elements, each group needs to have a basic understanding of what is being conveyed by the load or resistance factor that is being used for seismic design. 1.2.1.3 Example of LRFD Reserve Capacity Concept In formulating the LRFD guidelines, consideration needs to be given to a prevalent consensus among practitioners, es- pecially in state highway departments, that retaining walls, slopes and embankments, and buried structures generally have performed very well during seismic eventsâeven though many constructed structures have not been designed for the earth- quake load case. The main reason for this relates to the fact that the capacity of most retaining walls, slopes and embank- ments, and buried structures provides sufficient reserve to re- sist some level of earthquake loading when they are designed for static loading. This observation needed to be kept in mind when formulating the LRFD specifications in order that the proposed approach was determined to be reasonable to engi- neers using the methodology. As an illustration of this point, Dr. Lee Marsh, who served on the Technical Advisory Panel for the NCHRP 12-70 Project, quantified the level of reserve structural capacity for a hypo- thetical wall, to put the design process in perspective. In the course of a design, retaining walls are designed for global and external stability (that is, the process of checking for sufficient soil capacity for the global system), as well as for internal stress in the structural components. Dr. Marsh conducted a set of analyses to determine the reserve structural capacity for a standard wall that had been designed for a static load condi- tion. For simplicity, Dr. Marsh conducted the analyses for a nongravity cantilever sheet pile wall to focus on structural in- tegrity issues, rather than involving additional complexity associated with other nonstructural failure modes such as sliding failure through the soil at the base of a semi-gravity wall. Such mechanisms introduce an additional load fuse which might further reduce the earthquake design load to a lower value than the case associated with sheet pile walls. Results of these analyses are included in Appendix B. The sensitivity study conducted by Dr. Marsh indicates the following: 1. Most existing retaining walls, even when they only are de- signed for static loading, have sufficient reserve structural ca- pacity to withstand an appreciable level of earthquake load. 2. If a retaining wall has been designed to satisfy typical re- quirements for static loading, the inherent capacity will withstand about 0.12g pseudo-static loading, based on a very conservative capacity associated with first yield, with the most conservative assumption on wave scattering (that is, 1.0 as discussed in Chapter 6), and the most con- servative nonyielding structural performance criteria. 3. Under a less conservative interpretation, more suitable for correlating to historical structural damage from past earth- quakes, the inherent capacity is likely to be much higher, to a PGA at the ground surface as high as 0.68g. This case cor- responds to a scattering factor (see Chapter 6) equal to 0.5, and nominal yielding is allowed. 4. Even for a nonyielding limit state, a scattering factor equal to 0.5 can be justified for most design situations, espe- cially for much of the central and eastern United States (CEUS), where the characteristic ground shaking has lower, long-period ground motion content. In this situ- ation the retaining wall can withstand a site-adjusted PGA of 0.24g. For the 1,000-year return period ground motion criterion that was adopted by AASHTO in July of 2007, most regions in the CEUS, other than the New Madrid and the Charleston regions, will be required to design for a PGA at the ground surface of about 0.1g or lower. For much of the Western United States (WUS), outside of California, Alaska, and the Pacific Northwest, design would be for a PGA at the ground surface of about 0.2g. Based on the above cited reserve struc- tural capacity study, along with results from dynamic analy- ses of retaining walls, many of the regions in the CEUS and WUS can use simplifying screening criteria to eliminate the need for overly complicated seismic analyses. 1.2.2 Overview of Conclusions from Initial Phase of Work The initial phase of work involved Tasks 1 through 5 of the Working Plan. A number of conclusions were reached in this early work, and these conclusions formed the framework for the work plan that was implemented in Task 6 and reported in the 1st Interim Report. Highlights from Tasks 1 through 4 are summarized here: ⢠Task 1: Data Collection and Review. The conclusions from this task were that the methodologies available to design professionals within departments of transportation (DOTs) and consultants for the DOTs are primarily limited either to pseudo-static methods, such as the Mononobe-Okabe (M-O) method for the design of retaining structures and the limit equilibrium method of slope stability analysis, or to simplified deformation methods (for example, New- mark charts or analyses). Although these methods have limitations, as discussed in later chapters of this Draft Final Report, improvements in these methodologies still offer 4
the most practical approaches to seismic design. A growing trend towards the use of more rigorous modeling 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 appear to provide a more rig- orous modeling of various soil and soil-structure prob- lems, these more numerically intensive procedures do not appear to be suitable for development of day-to-day design methodologies required by this Project. ⢠Task 2: Problems and Knowledge Gaps. On the basis of the work carried out for this task, primary development needs were identified. These needs included common needs that applied to all three of the Project areas (retain- ing walls, slopes and embankments, and buried structures) and area-specific developments, as summarized here: â Common Needs Better definition of the ground motions that should be used during design, including appropriate adjust- ments for ground motion incoherency, strain ampli- tude, and ground motion amplification/deamplifica- tion. Development of screening procedures that advise the designer when sufficient margin exists within the static design to preclude the need for seismic analyses. Guidance on the selection of soil strength properties that should be used during seismic design. â Retaining Walls Numerical procedure that avoided deficiencies in the M-O procedure at high acceleration levels and high back slope angles and that handled mixed soil (c-Ï) conditions. The recommendation was to use either wedge-based equations or a limit-equilibrium stabil- ity program to determine the forces needed for seis- mic design. Charts for estimating wall displacement for repre- sentative areas of the United States (for example, CEUS versus WUS). Guidance on the selection of the seismic coefficient for limit-equilibrium and displacement-based design and the variation of this coefficient with wall height. â Slopes and Embankments Procedures for determining the appropriate seismic coefficient and its variation with slope height. Charts for estimating displacement for representative areas of the United States (for example, CEUS versus WUS). (These charts are the same as those used for estimating the displacement of conventional rigid gravity walls.) Procedures for introducing the effects of liquefaction. Procedures for treating rock slopes. â Buried Structures Simple-to-use design methods for medium-to-large- size culverts and pipes under the effect of transverse seismic racking deformations, taking into account soil-structure interaction effects. Guidance on how to select transient ground defor- mation (or strain) parameters for design and analysis purposes. Development of a consistent and rational procedure for buried structures subject to various forms of per- manent ground displacement (PGD), including lat- eral spreading, embankment slope movements or flow, and faulting. ⢠Task 3: Work PlanâAnalytical Methodologies. Informa- tion from Tasks 1 and 2 was used to identify types of ana- lytical methodology developments required. These devel- opments resulted in work product elements shown in Table 1-1. This summary is a modified version of Exhibit 6 of the Working Plan for the NCHRP 12-70 Project. ⢠Task 4: Work PlanâPerformance Strategy. A strategy for accomplishing the Development of Analytical Methodolo- gies was provided in Task 4. As noted in the NCHRP re- search project statement, Task 4 also included the identifi- cation of example applications and parametric studies that were to be performed, including the comparison with ex- isting methods. The performance strategy that was identi- fied served as a basis for the work that was conducted in Task 6, as reported in the second Interim Report. 1.2.3 Overview of Conclusions from Second Phase of Work The second phase of the work covered Tasks 6 through 8 of the Working Plan. This work was documented in the 2nd Interim Report. Work on Task 6 involved developments in the four areas summarized below. The discussions in the following chapters provide details in each of these four areas of development. ⢠Ground Motion Parameters. Procedures for selecting ground motion parameters for use in seismic design were evaluated, and recommendations for the selection of ground motions to use in the seismic response studies were devel- oped. Ground motion conditions characteristic of both WUS and CEUS were considered during this development. ⢠Retaining Walls. An approach for evaluating the behavior of retaining walls during seismic events was identified, and evaluations of this approach were carried out. This approach considered the global stability of walls, as well as the forces to be used in structural design. Various types of retaining walls were considered during this evaluation, including semi- gravity, nongravity cantilever (for example, sheet pile and soldier pile), MSE, anchored, and soil nail walls. 5
⢠Slopes and Embankments. Methods for evaluating the seis- mic stability of natural slopes and constructed embankments were identified and reviewed. A deformation-based approach for evaluating the seismic performance of slopes and em- bankments was developed based on the ground motion parameters established for the Project. ⢠Buried Structures. Procedures for evaluating the response of buried pipelines and culverts during seismic loading also were identified and evaluated. These procedures were ex- tended from an approach used to evaluate the seismic per- formance of large-diameter, vehicular tunnels. Both the transient and permanent movements of the ground were considered in these evaluations. The types of buried pipelines ranged from flexible materials to rigid pipelines. Vehicle tunnels are not considered. Results of the work on Task 6 constituted the majority of work completed in this phase. However, the work also included an outline for the LRFD specifications, designated as Task 7 within the Working Plan. The objective of Task 7 was to outline a methodology for implementing the recommended approach to seismic design in a format similar to that used within the current LRFD specifications. This outline built on the then cur- rent (2005 and 2006) AASHTO LRFD Bridge Design Specifica- tions where possible. However, some of the topics addressed during this Project were not covered within the existing LRFD specifications. For these cases suggestions were made on how the information might be incorporated within the context of the existing LRFD specifications. Task 8, which involved preparation of the second Interim Report, completed the second phase of the work. The second Interim Report was submitted to NCHRP for review by the NCHRP Oversight Panel. Comments and suggestions from the NCHRP Oversight Panel were subsequently discussed during a meeting between the Oversight Panel and the Project Team in May of 2006. The levels of effort for the four areas of development were not equal. More priority was placed on topics where the risk was considered highest during seismic events, as summarized below: ⢠Retaining Walls. This topic was assigned the highest pri- ority, as problems associated with the design of retaining walls, and in particular the use of the Mononobe-Okabe equations, is a continued source of uncertainty for design- ers. Part of the reason for assigning this topic the highest priority is the potential consequences of retaining wall fail- ures during a seismic event. Retaining wall damage and oc- casionally failures after earthquakes have been observed, and the repair of these walls can be time consuming and costly. Finally, the category of retaining walls involves a number of different cases, ranging from gravity to anchored walls. The seismic response of these cases differs in the way that seismic demands develop within the wall, as well as the manner that these demands are resisted. 6 Type of Investigation Purpose Methods or Concepts Evaluate Suitability of Limit Equilibrium Computer Program based on Method of Slices for Determination of Lateral Earth Pressures Offer to end users the means for improved methodology for establishing design seismic earth pressure magnitudes for mixed soil conditions, steep backslopes, and high ground motions. Examples showing evaluation of seismic earth pressures based on readily available limit equilibrium computer programs for representative wall types (gravity, nongravity, anchored, MSE, nail), including comparisons to existing chart solutions. Analyses of MSE Walls Develop revised design methodology for MSE walls A single integrated design method based on limit equilibrium computer programs is envisaged Analyses to Develop Design Charts for Estimating Height-Dependent Seismic Coefficient Provide a rational basis for selecting seismic coefficient as a function of both wall height and slope height for different soil conditions Separate charts or equations for WUS and CEUS earthquakes Analyses to Update Design Charts for Estimating Slope and Wall Movement Displacements This design chart will provide end users the means of estimating slope and wall movements as a function of yield acceleration, PGA, and PGV. Methodology that accounts for differences in WUS and CEUS earthquakes Analyses to Develop Design Approaches for Permanent and Transient Ground Deformation for Culverts and Pipelines Provide design guidance and specifications Design approaches for rigid culverts/pipelines and one for flexible culverts/pipelines Table 1-1. Proposal for work product elements.
⢠Slopes and Embankments. This topic was assigned a lower priority for several reasons. First, many times the seismic design of slopes and embankments is ignored, as the cost of mitigating potential problems is often far more than the cost of repairing damage after an earthquake. A second reason is the factor of safety (FS) used for the static design of slopes (for example, FS = 1.3 to 1.5 for permanent slopes) is often observed to be sufficient to cover stability during small to medium seismic events (where liquefaction is not an issue). Finally, failure of a slope often involves minimal risk to the highway users and the failed slope can usually be quickly repaired. ⢠Buried Structures. This topic is given a lower priority pri- marily because the consequences of failure are often limited. Nevertheless, the current AASHTO LRFD Bridge Design Specifications is deficient in that no guidelines are provided, even for those designers who might want to consider seismic loading. One of the other important considerations during the sec- ond phase of work was developments that were occurring in the area of ground motions. At the time of the work, current AASHTO LRFD Bridge Design Specifications (2006) provided guidance on the determination of ground motions required for design; however, the guidance was being modified as part of a separate NCHRP project to update the current LRFD seismic provisions. This work was being performed within NCHRP 20-07 Project being conducted by Imbsen & Associates (Imbsen, 2006). Part of the recommended update involved changing from the then current 500-year earthquake (that is, 10 percent probability of occurrence in 50 years) to a 1,000 year design basis (approximately 7 percent in 75 years). (Various probabilities of occurrence are associated with the nominal 1,000-year return period. For a 75-year exposure period, the exceedance probability is approximately 7 percent. This ex- ceedance probability is also approximately 5 percent for a 50-year exposure period.) Included within the proposed up- date was a focus on using the spectral acceleration at 1 second (S1) as a basic proxy for ground motion. Realizing the plans within the NCHRP 20-07 Project, as well as a fundamental need for velocity information for some of the methodologies being proposed as part of the NCHRP 12-70 Project, a signif- icant focus was given to the development of a set of rational ground motion parameters to use during the seismic design and analysis of retaining walls, slopes and embankments, and buried structures. 1.2.4 Overview of Conclusions from Third Phase of Work The third phase of work involved Tasks 9 and 10: the de- velopment of specifications, commentaries, and example problems. Results of this work were summarized in the third Interim Report. Specifications and commentaries were presented in three sections: ⢠Section X: Retaining Walls. This section provided proposed specifications and commentaries for six types of retaining walls: (1) rigid gravity and semi-gravity (conventional) walls, (2) nongravity cantilever walls, (3) anchored walls, (4) mechanically stabilized earth (MSE) walls, (5) prefabri- cated modular walls, and (6) soil nail walls. With the excep- tion of soil nail walls, design methods for gravity loads for each of these wall types were covered within the current AASHTO LRFD Bridge Design Specifications. ⢠Section Y: Slopes and Embankments. This section pro- vided proposed specifications and commentaries for the seismic design of slopes and embankments. The specifica- tions covered natural slopes and engineered fills. A method- ology for addressing sites with liquefaction potential was included in the specifications. Current AASHTO LRFD Bridge Design Specifications do not provide specific guid- ance on the methods used to evaluate the stability of slopes under gravity and live loads. In this case the specifications and commentaries used the standard of geotechnical prac- tice as the starting point for design. ⢠Section Z: Buried Structures. This section covered the seismic design of culverts and drainage pipes. The discus- sion focused on the design for transient ground displace- ments (TGD) and included mention of the requirements for design for PGD. Generally, the ability of the culvert or drainage pipe to withstand PGD depends on the amount of permanent ground movement that occurs during the seismic event. Procedures given in Section Y provide a means for estimating these displacements. Culverts and drainage pipes will generally move with the ground; there- fore, movement of more than a few inches to a foot will often damage the pipe or culvert. Also included within the third Interim Report were (1) an appendix presenting charts for estimating seismic active and passive earth pressure coefficients that included the contri- butions from cohesion and (2) an appendix summarizing the design of nongravity cantilever walls using a beam-column displacement method. Contents of the third Interim Report were reviewed with the NCHRP 12-70 Oversight Panel. The focus of the panel discus- sions was on the organization of the specifications and the ex- ample problems that needed to be completed to support the development of the specifications. This feedback was used to modify the specifications and commentaries and to update the example problems. A fourth Interim Report was prepared to document this information. The NCHRP Oversight Panel 7
provided comments on the fourth Interim Report, and these comments have been addressed where possible in this Final Report. 1.3 Organization of Final Report This Final Report is organized into two volumes. The first volume, titled Final Report, is a compilation of information presented previously in the first, second, third, and fourth Interim Reports; it is published as NCHRP Report 611. The second volume, titled Recommended Specifications, Com- mentaries, and Example Problems, presents the proposed specifications, commentaries, and example problems for the retaining walls, slopes and embankments, and buried structures. 1.3.1 Volume 1âFinal Project Report This volume has 10 chapters following Chapter 1 Introduc- tion. These chapters were taken from interim reports prepared as the Project was completed. The Draft Final Report serves as documentation for the work as it was being performed during the Project and provides the basis for information presented in the recommended specifications, commentaries, and example problems. ⢠Chapter 2âData Collection and Review summarizes re- sults from the literature review for the three principal areas of development (that is, retaining walls, slopes and em- bankments, and buried structures). This summary includes conclusions reached from discussions with individuals rep- resenting selected DOTs, vendors, and consultants regard- ing the availability of seismic design guidelines for each of the three principal areas of development. ⢠Chapter 3âProblems and Knowledge Gaps involves a dis- cussion of knowledge gaps and problems associated with current design methodologies for each of the three areas. These knowledge gaps and problems were identified on the basis of the literature review and discussions with repre- sentatives from DOTs, vendors, and other consultants summarized in Chapter 2, as well as the Project Teamâs ex- perience on related retaining wall, slope and embankment, and buried structure projects in seismically active areas. ⢠Chapter 4âWork Plan: Analytical Methodologies describes the work plan for developing analytical methodologies that was recommended for addressing the knowledge gaps and problems outlined in Chapter 3. The proposed analytical methodologies included development of methods for quan- tifying the determination of seismic demand, as well as the methods used to determine the capacity during seismic load- ing for each area of development. ⢠Chapter 5âSeismic Ground Motions summarizes results from the ground motion studies. These results include a re- view of the seismic loading criteria developed for the Project. This discussion also covers information on the ground motion revisions being proposed at the time (and since adopted) to the current AASHTO LRFD Bridge Design Spec- ifications, the range of ground shaking levels that new seis- mic maps show, and the variation in response spectra between WUS and CEUS. The review of seismic loading cri- teria is followed by summaries of (1) the Newmark dis- placement correlations that were developed and (2) the cor- relation between peak ground velocity (PGV) and spectral acceleration at one second (S1). Information in this chapter serves as basic input data for the following studies. ⢠Chapter 6âHeight-Dependent Seismic Coefficient involves a summary of the results of the height-dependent seismic coefficient that was developed for use in the analysis of re- taining walls, as well as slopes and embankments. This sum- mary covers effects of ground motion incoherency, referred to as wave scattering analyses, for slopes and for retaining walls, and it provides guidance on the intended application of the scattering solutions. ⢠Chapter 7âRetaining Walls describes the current design process, including the use of the Mononobe-Okabe equa- tions and the limitations of this approach. This discussion is followed by a summary of the potential effects of cohe- sive soil content on seismic earth pressures estimated by the Mononobe-Okabe method and a generalized limit- equilibrium approach for determining seismic active earth pressures. The next discussions cover results of a study of impedance contrasts and nonlinear effects on seismic design coefficients and the use of a displacement-based design ap- proach for gravity, semi-gravity, and MSE walls. The chapter concludes with specific comments on the design of gravity and MSE walls and some general guidance on the design of nongravity cantilever, anchored, and soil nail walls. ⢠Chapter 8âSlopes and Embankments reviews the current approach used for the seismic design of slopes and em- bankments. This review is followed by a recommended displacement-based approach for evaluating seismic sta- bility. The recommended approach provided a basis for developing screening methods where no analysis is re- quired or where a factor of safety approach is preferred. ⢠Chapter 9âBuried Structures covers the recommended approach for the TGD design of buried pipes and culverts. The discussions in this chapter review the general effects of earthquake loading and the potential failure modes. A brief summary of the seismic design practice is given, and then the proposed methodology is defined. This methodology covers ovaling of circular conduits, racking of rectangular conduits, and then results of a series of parametric and ver- ification studies. ⢠Chapter 10âRecommendations for Future Work summa- rizes a number of topics not resolved during the Project 8
and are believed to warrant further study. These topics range from identification of methods for quantifying the amount of cohesion that can be counted on during design to methods for describing the liquefaction strength of soils located beneath embankments. ⢠Chapter 11âReferences lists the references used during the Project. This report also includes a number of appendices with sup- porting documentation for the work presented in Chapters 2 through 9. 1.3.2 Volume 2âRecommended Specifications, Commentaries, and Example Problems This volume includes recommended specifications, com- mentaries, and example problems as summarized below. The background for some, but not all, of the methods described in Volume 2 is included in Volume 1. Some methods outlined in the specifications and commentaries and used in the ex- ample problems were developed as the specifications, com- mentaries, and example problems were being completed. This work occurred after the completion of work described in Volume 1. ⢠Specifications and Commentaries summarize the recom- mended specifications and commentaries after revisions to address (1) the NCHRP Oversight Panelâs comments on drafts of the specifications and commentaries and (2) mod- ifications made by the Project Team after completing ex- ample problems. Some topics such as slope stability did not currently have an independent section or subsection within the AASHTO LRFD Bridge Design Specifications, but rather were scattered within the various sections. The approach for including the work developed during the NCHRP 12-70 Project became, therefore, more of a challenge. ⢠Example Problems show the steps necessary to complete a seismic design following the methods proposed for this Project. 9
