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5 the most practical approaches to seismic design. A growing Buried Structures trend towards the use of more rigorous modeling methods, Simple-to-use design methods for medium-to-large- such as the computer code FLAC (Itasca, 2007), for the size culverts and pipes under the effect of transverse evaluation of retaining structures, slopes and embank- seismic racking deformations, taking into account ments, and buried structures has occurred recently. While soil-structure interaction effects. FLAC and similar software appear to provide a more rig- Guidance on how to select transient ground defor- orous modeling of various soil and soil-structure prob- mation (or strain) parameters for design and analysis lems, these more numerically intensive procedures do not purposes. Development of a consistent and rational procedure appear to be suitable for development of day-to-day design methodologies required by this Project. for buried structures subject to various forms of per- Task 2: Problems and Knowledge Gaps. On the basis of manent ground displacement (PGD), including lat- the work carried out for this task, primary development eral spreading, embankment slope movements or needs were identified. These needs included common flow, and faulting. needs that applied to all three of the Project areas (retain- Task 3: Work Plan--Analytical Methodologies. Informa- ing walls, slopes and embankments, and buried structures) tion from Tasks 1 and 2 was used to identify types of ana- and area-specific developments, as summarized here: lytical methodology developments required. These devel- Common Needs opments resulted in work product elements shown in Better definition of the ground motions that should Table 1-1. This summary is a modified version of Exhibit 6 be used during design, including appropriate adjust- of the Working Plan for the NCHRP 12-70 Project. ments for ground motion incoherency, strain ampli- Task 4: Work Plan--Performance Strategy. A strategy for tude, and ground motion amplification/deamplifica- accomplishing the Development of Analytical Methodolo- tion. gies was provided in Task 4. As noted in the NCHRP re- Development of screening procedures that advise search project statement, Task 4 also included the identifi- the designer when sufficient margin exists within cation of example applications and parametric studies that the static design to preclude the need for seismic were to be performed, including the comparison with ex- analyses. isting methods. The performance strategy that was identi- Guidance on the selection of soil strength properties fied served as a basis for the work that was conducted in Task 6, as reported in the second Interim Report. that should be used during seismic design. Retaining Walls Numerical procedure that avoided deficiencies in the 1.2.3 Overview of Conclusions M-O procedure at high acceleration levels and high from Second Phase of Work back slope angles and that handled mixed soil (c-) The second phase of the work covered Tasks 6 through 8 of conditions. The recommendation was to use either the Working Plan. This work was documented in the 2nd wedge-based equations or a limit-equilibrium stabil- Interim Report. ity program to determine the forces needed for seis- Work on Task 6 involved developments in the four areas mic design. summarized below. The discussions in the following chapters Charts for estimating wall displacement for repre- provide details in each of these four areas of development. sentative areas of the United States (for example, CEUS versus WUS). Ground Motion Parameters. Procedures for selecting Guidance on the selection of the seismic coefficient ground motion parameters for use in seismic design were for limit-equilibrium and displacement-based design evaluated, and recommendations for the selection of ground and the variation of this coefficient with wall height. motions to use in the seismic response studies were devel- Slopes and Embankments oped. Ground motion conditions characteristic of both Procedures for determining the appropriate seismic WUS and CEUS were considered during this development. coefficient and its variation with slope height. Retaining Walls. An approach for evaluating the behavior Charts for estimating displacement for representative of retaining walls during seismic events was identified, and areas of the United States (for example, CEUS versus evaluations of this approach were carried out. This approach WUS). (These charts are the same as those used for considered the global stability of walls, as well as the forces to estimating the displacement of conventional rigid be used in structural design. Various types of retaining walls gravity walls.) were considered during this evaluation, including semi- Procedures for introducing the effects of liquefaction. gravity, nongravity cantilever (for example, sheet pile and Procedures for treating rock slopes. soldier pile), MSE, anchored, and soil nail walls.

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6 Table 1-1. Proposal for work product elements. Type of Investigation Purpose Methods or Concepts Evaluate Suitability of Limit Offer to end users the means for Examples showing evaluation of Equilibrium Computer Program based improved methodology for establishing seismic earth pressures based on on Method of Slices for Determination design seismic earth pressure readily available limit equilibrium of Lateral Earth Pressures magnitudes for mixed soil conditions, computer programs for representative steep backslopes, and high ground wall types (gravity, nongravity, motions. anchored, MSE, nail), including comparisons to existing chart solutions. Analyses of MSE Walls Develop revised design methodology A single integrated design method for MSE walls based on limit equilibrium computer programs is envisaged Analyses to Develop Design Charts Provide a rational basis for selecting Separate charts or equations for for Estimating Height-Dependent seismic coefficient as a function of WUS and CEUS earthquakes Seismic Coefficient both wall height and slope height for different soil conditions Analyses to Update Design Charts for This design chart will provide end Methodology that accounts for Estimating Slope and Wall Movement users the means of estimating slope differences in WUS and CEUS Displacements and wall movements as a function of earthquakes yield acceleration, PGA, and PGV. Analyses to Develop Design Provide design guidance and Design approaches for rigid Approaches for Permanent and specifications culverts/pipelines and one for flexible Transient Ground Deformation for culverts/pipelines Culverts and Pipelines Slopes and Embankments. Methods for evaluating the seis- the information might be incorporated within the context of mic stability of natural slopes and constructed embankments the existing LRFD specifications. were identified and reviewed. A deformation-based approach Task 8, which involved preparation of the second Interim for evaluating the seismic performance of slopes and em- Report, completed the second phase of the work. The second bankments was developed based on the ground motion Interim Report was submitted to NCHRP for review by the parameters established for the Project. NCHRP Oversight Panel. Comments and suggestions from Buried Structures. Procedures for evaluating the response the NCHRP Oversight Panel were subsequently discussed of buried pipelines and culverts during seismic loading also during a meeting between the Oversight Panel and the Project were identified and evaluated. These procedures were ex- Team in May of 2006. tended from an approach used to evaluate the seismic per- The levels of effort for the four areas of development were not formance of large-diameter, vehicular tunnels. Both the equal. More priority was placed on topics where the risk was transient and permanent movements of the ground were considered highest during seismic events, as summarized below: considered in these evaluations. The types of buried pipelines ranged from flexible materials to rigid pipelines. Vehicle Retaining Walls. This topic was assigned the highest pri- tunnels are not considered. ority, as problems associated with the design of retaining walls, and in particular the use of the Mononobe-Okabe Results of the work on Task 6 constituted the majority of equations, is a continued source of uncertainty for design- work completed in this phase. However, the work also included ers. Part of the reason for assigning this topic the highest an outline for the LRFD specifications, designated as Task 7 priority is the potential consequences of retaining wall fail- within the Working Plan. The objective of Task 7 was to outline ures during a seismic event. Retaining wall damage and oc- a methodology for implementing the recommended approach casionally failures after earthquakes have been observed, and to seismic design in a format similar to that used within the the repair of these walls can be time consuming and costly. current LRFD specifications. This outline built on the then cur- Finally, the category of retaining walls involves a number of rent (2005 and 2006) AASHTO LRFD Bridge Design Specifica- different cases, ranging from gravity to anchored walls. The tions where possible. However, some of the topics addressed seismic response of these cases differs in the way that seismic during this Project were not covered within the existing LRFD demands develop within the wall, as well as the manner specifications. For these cases suggestions were made on how that these demands are resisted.