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87 7.8.3 MSE Walls--Design Method wall-height dependent average seismic coefficient concept for Internal Stability discussed in Section 7.5 is recommended. In the AASHTO method, the total inertial force is distributed The current AASHTO design method for seismic internal to the reinforcements in proportion to their effective resistant stability is described in Article of Section 11 of the lengths Lei as shown on Figure 7-22. This approach follows the AASHTO Specifications, and is illustrated in Figure 7-22. finite element modeling conducted by Segrestin and Bastick The method assumes that the internal inertial forces gener- (1988), and leads to higher tensile forces in lower reinforce- ating additional tensile loads in reinforcements act on an ment layers. This is the opposite trend to incremental seismic active pressure zone assumed to be the same for the static loading used by AASHTO for external stability evaluations loading case. A bilinear zone is defined for inextensible re- based on the M-O equation. In the case of internal stability inforcements such as metallic strips and a linear zone for evaluation, Vrymoed (1989) used a tributary area approach extensible strips. Whereas it could reasonably be anticipated that assumes the inertial load carried by each reinforcement that these active zones would extend outwards for seismic layer increases linearly with height above the toe of the wall cases, as for M-O analyses, numerical and centrifuge mod- for equally spaced reinforcement layers. A similar approach els indicate that the reinforcement restricts such outward was used by Ling et al. (1997) in limit equilibrium analyses. movements, and only relatively small changes in location This concept would suggest that longer reinforcement lengths are seen. could be needed at the top of walls with increasing accelera- The internal inertial force in the AASHTO method is cal- tion levels, and the AASHTO approach could be unconserv- culated using the acceleration Am defined in Section 7.8.2 for ative. In view of this uncertainty in distribution that has been the external stability case. As previously discussed, the ac- widely discussed in the literature, a suggested compromise is celeration equation used for external stability evaluations is to distribute the inertial force uniformly within the reinforce- too conservative for most site conditions, and the use of the ment. In essence, this represents an average of the tensile load Figure 7-22. Seismic internal stability of a MSE wall (AASHTO, 2007).