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25 different performance requirements and loading criteria equation or a limit-equilibrium stability program to being used or proposed from different studies. A consistent determine the forces needed for stability. methodology and design criteria compatible with the other Charts for estimating wall displacement for representa- components of the highway facilities have yet to be devel- tive areas of the United States (for example, CEUS ver- oped for the culvert structures. sus WUS). Guidance on the selection of the seismic coefficient for limit-equilibrium and displacement-based design and 3.4 Conclusions its variation with wall height. Knowledge gaps and problems identified in the literature Slopes and embankments review, through discussions with various individuals at DOTs Procedures for determining the appropriate seismic and those conducting research in the area, and through the coefficient and its variation with slope height. completion of Task 2 have not identified any additional or Charts for estimating displacement for representative new knowledge gaps or problems; the ones cited above are areas of the United States (for example, CEUS versus relatively well-known and documented. It appeared that in WUS). (These charts are the same as those used for esti- most cases, existing simplified methodologies with appropri- mating the displacement of conventional rigid gravity ate improvements and documentation could be used to walls.) address these knowledge gaps and problems. Procedures for introducing the effects of liquefaction. While many problems could be handled by existing sim- Procedures for treating rock slopes. plified methodologies, the complexity of some topics, such as Buried structures the seismic design of geosynthetic MSE walls, was seen as Simple-to-use design charts for medium-to-large-size more complex than originally anticipated. This complexity culverts and pipes under the effect of transverse seismic resulted in part from the changing approach to the static racking deformations, taking into account soil-structure design of this wall type. It also appeared that the seismic interaction effect. design of other wall types, such as soil nail walls, still lacked Guidance on how to select transient ground defor- the rigor needed to be considered state-of-the-practice. As mation (or strain) parameters for design and analysis noted in the discussion of earthquake design basis, current purposes. practice with some of these wall types involved sufficient con- Development of a consistent and rational procedure servatism in the ground motion specification, as well as the for buried structures subject to various forms of PGD, inherent conservatism in static design, that these shortcom- including lateral spread, embankment slope movements ings were not a serious design issue. In fact, overall current or flow, and faulting. design methods have worked surprisingly well. On the basis of the work carried out for this task, the pri- An overall need for the three areas was a screening procedure mary development needs were identified as follows: that would provide guidance to the designer as to when a seis- mic analysis could be neglected, because the reserve capacity for Retaining walls static design was sufficient to meet seismic demands during Numerical procedure that avoided deficiencies in the the design seismic event. Further, guidance was needed on M-O procedure at high acceleration levels and steep back- the selection of appropriate ground motions to use for seismic slopes and that handled mixed soil (c-) conditions. The design and the determination of appropriate soil strengths to recommendation was to use either a wedge-equilibrium use in the capacity estimate.