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68 CHAPTER 7 Retaining Walls This chapter summarizes results of studies conducted for Figure 7-1 (FHWA 1996), which uses terminology adopted in the seismic analysis and design of retaining walls. The primary the AASHTO LRFD Bridge Design Specifications. The cut and objectives of these studies were to: fill designations refer to how the wall is constructed, not nec- essarily the nature of the earthwork (cut or fill) associated · Address limitations with current methods used to estimate with the wall. For example, a fill wall, such as a MSE wall or a seismic earth pressures on retaining walls. These limita- nongravity cantilever wall, may be used to retain earth fill for tions include difficulties in using the M-O equations for a major highway cut as illustrated in the representative Fig- certain combinations of seismic coefficient and backslope ures 7-2 to 7-5 showing wall types. This becomes an impor- above the retaining structure or for backfill conditions tant factor in the subsequent discussions related to external where soils are not cohesionless or are not uniform. seismic stability of such walls. · Develop guidance on the selection of the seismic coefficient Current AASHTO LRFD Bridge Design Specifications address used to conduct either a force-based or displacement-based seismic design of retaining wall types as summarized in the evaluation of the seismic performance of retaining walls. following paragraphs: There is considerable confusion in current practice on the selection of the seismic coefficient, particularly for different 1. Conventional gravity and semi-gravity cantilever walls wall types. (Article 11.6.5). The seismic design provisions cite the use · Provide recommendations on methodologies to use for the of the M-O method (specified in Appendix A, Article seismic analysis and design of alternate wall types that can A188.8.131.52) to estimate equivalent static forces for seismic be used to develop LRFD specifications. loads. Reductions due to lateral wall movements are per- mitted as described in Appendix A (A184.108.40.206). The approach taken to meet these objectives involved using 2. Nongravity cantilever walls (Article 11.8.6). Seismic design results from the ground motion and wave scattering studies provisions are not explicit. Rather reference is made to an discussed in the previous two chapters. Specifically, the ap- accepted methodology, albeit the M-O equations are sug- proach for determining ground motions and displacements gested as a means to compute active and passive pressures summarized in Chapter 5 provides the information needed provided a seismic coefficient of 0.5 times the site-adjusted for a force-based design and for determining retaining wall PGA is used. displacements. The information in Chapter 6 is used for mod- ifying the site-adjusted PGA to account for wave scattering ef- 3. Anchored walls (Article 11.9.6). Seismic design provi- fects. With this information two methodologies are provided sions are not explicit, and reference is made to M-O for the seismic analysis and design of retaining walls. The first method for cantilever walls. However, Article A220.127.116.11 involves use of the classic M-O equations, and the second indicates that, uses a more GLE methodology for cases where the M-O pro- For abutments restrained against lateral movement by cedure is not applicable or where an estimate of retaining wall tiebacks or batter piles, lateral pressures induced by inertia forces in the backfill will be greater than those given by the displacements is desired. Mononobe-Okabe analysis. The discussion goes on to suggest using a factor of 1.5 7.1 Current Design Practice in conjunction with site-adjusted PGA for design "where Various wall types are commonly used for transportation doubt exists that an abutment can yield sufficiently to systems. A useful classification of these wall types is shown in mobilize soil strength."
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69 Figure 7-1. Earth retaining system classification (after FHWA, 1996). Figure 7-2. Wall types (after FHWA, 1996).
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70 Completed MSE wall Geotextile wall Figure 7-3. MSE wall types (after FHWA, 1996). Figure 7-4. MSE walls--construction configurations.