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84 structures have experienced small permanent deformations such as bulging of the face and cracking behind the structure, but no collapse has occurred. A summary of seismic field per- formance is shown in Table 7-1. The inherent ductility and flexibility of such structures combined with the conservatism of static design procedures is often cited as a reason for the sat- isfactory performance. Nevertheless, as Bathurst et al. (2002) note, seismic design tools are needed to optimize the design Figure 7-20. Design procedure steps. of these structures in seismic environments. In the following sections, the current AASHTO design methods for external and internal stability are described, and From design examples and recognizing that static designs recommendations for modifications, including a brief com- have inherently high factors of safety, a recommendation to mentary of outstanding design issues, are made. eliminate step 7 and replace it by a simple clause to reduce the seismic coefficient from step 6 by a factor of 50 percent (as in the existing AASHTO Specifications) would seem realistic. 7.8.2 MSE Walls--Design Method This is particularly the case since the new displacement func- for External Stability tion gives values significantly less than the present AASHTO The current AASHTO design method for seismic external Specifications. stability is described in Article 184.108.40.206 in Section 11 of the Specifications, and is illustrated in Figure 7-21. The method 7.8 MSE Walls--Recommended evaluates sliding stability of the MSE wall under combined Design Methods static and earthquake loads. For wall inertial load and M-O The current AASHTO Specifications for MSE walls largely active earth pressure evaluations, the AASHTO method adopts are based on pseudo-static stability methods utilizing the M-O the Segrestin and Bastick (1988) recommendations, where the seismic active earth pressure equation. In this approach dy- maximum acceleration is given by: namic earth pressure components are added to static compo- nents to evaluate external sliding stability or to determine re- Am = (1.45 - A ) A (7-4) inforced length to prevent pull-out failure in the case of internal stability. Accelerations used for analyses and the concepts used where A is peak ground acceleration coefficient. for tensile stress distribution in reinforcing strips largely have However, as discussed in Appendix H, the above equation been influenced by numerical analyses conducted by Segrestin is conservative for most site conditions, and the wall height- and Bastick (1988), as described in Appendix H. (A copy of the dependent average seismic coefficient discussed in Figure 7-16 Segrestin and Bastick paper was included in earlier drafts of the in Section 7.5 is recommended for both gravity and MSE wall NCHRP 12-70 Project report. However, copyright restrictions precluded including a copy of the paper in this Final Report.) design. A reduced base width of 0.5H is used to compute the mass of the MSE retaining wall used to determine the wall inertial 7.8.1 Current Design Methodology load PIR in the AASHTO method (Equation 220.127.116.11-3). The In the past 15 years since the adoption of the AASHTO de- apparent rationale for this relates to a potential phase differ- sign approach, numerous publications on seismic design ence between the M-O active pressure acting behind the wall methodologies for MSE walls have appeared in the literature. and the wall inertial load. Segrestin and Bastick (1988) recom- Publications have described pseudo-static, limit equilibrium mend 60 percent of the wall mass compatible with AASHTO, methods, numerical methods using dynamic analyses, and whereas Japanese practice is to use 100 percent of the mass. model test results using centrifuge and shaking table tests. A A study of centrifuge test data shows no evidence of a phase comprehensive summary of much of this literature was pub- difference. To be consistent with previous discussion on non- lished by Bathurst et al. (2002). It is clear from review of this gravity cantilever walls, height effects, and limit equilibrium literature that consensus on a new robust design approach suit- methods of analysis, the total wall mass should be used to able for a revised design specification has yet to surface due to compute the inertial load. the complexity of the problems and ongoing research needs. The AASHTO LRFD Bridge Design Specifications for MSE Over the past several years, observations of geosynthetic walls separate out the seismic dynamic component of the force slopes and walls during earthquakes have indicated that these behind the wall instead of using a total active force PAE as types of structures perform well during seismic events. The discussed in Section 7.4. Assuming a load factor of 1.0, the
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85 Table 7-1. Summary of seismic field performance of reinforced soil structures (Nova-Roessig, 1999). 1 Reinforced Earth Co., 1990, 1991, 1994; 2 Collin et al., 1992; 3 Eliahu and Watt, 1991; 4 Stewart et al., 1994; 5 Sandri, 1994; 6 Sitar, 1995; 7 Tatsuoka et al., 1996; 8 Ling et al., 1997; 9 Ling et al., 1989; 10 Ling et al., 2001 following equation (Equation 18.104.22.168-2) is used to define is assumed that use was made of the approximation for KAE the seismic dynamic component of the active force: suggested by Seed and Whitman (1970), namely: PAE = 0.375 Am s H 2 (7-5) K AE = K A + 0.75kh (7-6) where where s = soil unit weight; and KA = static active pressure coefficient; and H = wall height. KAE = total earthquake coefficient. The use of the symbol PAE is confusing, as the seismic dy- Hence using the AASHTO terminology, namic increment is usually defined as PAE. Whereas it is PAE = (0.75 Am) × 0.5 sH2 not immediately evident how this equation was derived, it = 0.375Am s H2
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86 Figure 7-21. Seismic external stability of a MSE wall (AASHTO, 2007). Note that the Seed and Whitman (1970) simplified ap- ommended approach for MSE walls is a design procedure proach was developed for use in level-ground conditions. similar to that for gravity and semi-gravity walls (Section 7.6), If the Seed and Whitman simplification was, in fact, used to where a total active earthquake force is used for sliding sta- develop Equation (7-6), then it is fundamentally appropri- bility evaluations. ate only for level ground conditions and may underesti- It also is noted that the AASHTO LRFD Bridge Design Spec- mate seismic earth pressures where a slope occurs above the ifications suggest conducting a detailed lateral deformation retaining wall. analysis using the Newmark method or numerical modeling For external stability, only 50 percent of the latter force if the ground acceleration exceeds 0.29g. However, as dis- increment is added to the static active force, again reflecting cussed for gravity and semi-gravity walls, due to the inherently either a phase difference with inertial wall loads or reflect- high factors of safety used for static load design, in most cases ing a 50 percent reduction by allowing deformation potential yield seismic coefficients are likely to be high enough to min- as suggested for cantilever walls. In lieu of the above, the rec- imize potential sliding block displacements.