Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments (2008) / Chapter Skim
Currently Skimming:
Pages 68-95
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 68... ...
The information in Chapter 6 is used for modifying the site-adjusted PGA to account for wave scattering effects. With this information two methodologies are provided for the seismic analysis and design of retaining walls.
|
From page 69... ...
69 Figure 7-1. Earth retaining system classification (after FHWA, 1996)
|
From page 70... ...
70 Figure 7-4. MSE walls -- construction configurations.
|
From page 71... ...
, the seismic active earth pressure coefficient (KAE) , the seismic passive earth pressure (PPE)
|
From page 72... ...
Figure 7-8 shows the effect of backfill slope angle on KAE as a function of seismic coefficient, and illustrates the design dilemma commonly encountered of rapidly increasing earth pressure values with modest increases in slope angles. Figure 7-9 indicates the underlying reason, namely the fact that the failure plane angle α approaches that of the backfill slope angle ω, resulting in an infinite mass of the active failure wedge.
|
From page 73... ...
If the depth of embedment is limited, as in the case of many gravity, semi-gravity, and MSE walls, the importance of the passive earth pressure to overall equilibrium is small, and therefore, using the static passive earth pressure is often acceptable. In the case of nongravity cantilever walls and anchored walls the structural members below the excavation depth depend on the passive earth pressure for stability and therefore the effects of seismic loading on passive earth pressures can be an important contribution.
|
From page 74... ...
The preferred approach for passive earth pressure determination is to use log spiral procedures, similar to the preferred approach for gravity loading. Shamsabadi et al.
|
From page 75... ...
Figure 7-12. Seismic coefficient charts for c- soils for 40.
|
From page 76... ...
7.5 Height-Dependent Seismic Design Coefficients Current AASHTO LRFD Bridge Design Specifications use peak ground acceleration in conjunction with M-O analysis to compute seismic earth pressures for retaining walls. Except for MSE walls where amplification factors as a function of peak ground acceleration are used, based on studies by Segrestin and Bastick (1988)
|
From page 77... ...
. Idriss modulus and damping versus shearing strain curves for rock.
|
From page 78... ...
Additional parametric studies were subsequently conducted to evaluate the effects of wall heights, impedance Seed and Idriss modulus and damping curves were used to represent shearing strain effects. • Nine ground motions consistent with the discussions in Chapter 5, including the two used by Segrestin and Bastick.
|
From page 79... ...
. 7.5.2 Results of Impedance Contrast and Nonlinearity Evaluations Results of the studies summarized above and described in Appendix G generally follow trends similar to the wave scattering studies described in Chapter 6.
|
From page 80... ...
. taries for the AASHTO LRFD Bridge Design Specifications, the use of a simple linear function to describe reductions in average height-dependent seismic coefficients, as shown in Figure 7-16, is recommended.
|
From page 81... ...
For wall heights greater than 100 feet, α coefficients may be assumed to be the 100-foot value. Note also for practical purposes, walls less than say 20 feet in height and on very firm ground conditions (B/C foundations)
|
From page 82... ...
For wall heights greater than 20 feet, the use of heightdependent seismic coefficients is recommended to determine maximum average seismic coefficients for active failure zones, and may be used to determine kmax for use in Newmark sliding block analyses. In effect, this represents an uncoupled analysis of deformations as opposed to a fully coupled dynamic analysis of permanent wall deformations.
|
From page 83... ...
1. Establish an initial wall design using the AASHTO LRFD Bridge Design Specifications for static loading, using appropriate load and resistance factors.
|
From page 84... ...
In this approach dynamic earth pressure components are added to static components to evaluate external sliding stability or to determine reinforced length to prevent pull-out failure in the case of internal stability. Accelerations used for analyses and the concepts used for tensile stress distribution in reinforcing strips largely have been influenced by numerical analyses conducted by Segrestin and Bastick (1988)
|
From page 85... ...
( ) 7-6 85 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 Table 7-1.
|
From page 86... ...
If the Seed and Whitman simplification was, in fact, used to develop Equation (7-6) , then it is fundamentally appropriate only for level ground conditions and may underestimate seismic earth pressures where a slope occurs above the retaining wall.
|
From page 87... ...
The internal inertial force in the AASHTO method is calculated using the acceleration Am defined in Section 7.8.2 for the external stability case. As previously discussed, the acceleration equation used for external stability evaluations is too conservative for most site conditions, and the use of the wall-height dependent average seismic coefficient concept discussed in Section 7.5 is recommended.
|
From page 88... ...
Slightly longer reinforcing strips are needed for the LRFD design, and seismic loading does not impact the design. The suggested recommendations to modify the seismic design procedure (acceleration coefficients and tensile load distribution)
|
From page 89... ...
For soldier piles the effective width of the structural element below the base of the wall is assumed to be from 1 to 3 pile diameters to account for the wedge-shape form of soil reaction. The upper several feet of soil are also typically neglected for static passive earth pressure computation.
|
From page 90... ...
Include cohesion component as 90 Figure 7-23. Seismic passive earth pressure coefficient based on log spiral procedure (c soil cohesion, soil total unit weight, and H is height)
|
From page 91... ...
Article 11.9.6 indicates that, "the provisions in Article 11.8.6 shall apply." The referenced article deals with nongravity cantilever walls, and basically states that the M-O equations should be used with the seismic coefficient kh = 0.5A. Various other methods also have been recommended for the seismic design of anchored walls: • The FHWA report Geotechnical Earthquake Engineering (FHWA, 1998a)
|
From page 92... ...
If 1 to 2 inches are not tolerable or cannot develop, then use the full seismic coefficient. Estimate earth pressure for passive loading using Figures 7-23 to 7-25 or the equations developed by Shamsabadi et al.
|
From page 93... ...
• A detailed design example based on the recommended approach is presented. The earlier FHWA report Geotechnical Earthquake Engineering (FHWA, 1998a)
|
From page 94... ...
The methodologies introduce new height-dependent seismic coefficients, as discussed in Chapters 5 and 6 and further refined in Section 7.5 for these analyses. Results of the work completed for retaining walls includes charts showing the effects of cohesion within the soil on the seismic earth pressure coefficients that were developed.
|
From page 95... ...
More problems were observed for rigid gravity walls and nongravity cantilever walls, often because of the lack of seismic design for these walls. The methodologies suggested in this chapter should help improve the seismic performance of these walls in the future.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.