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OCR for page 88
88 distribution from the existing AASHTO approach with that Methods range from more complex FLAC computer analy- determined using the tributary area of strips in the inertial ses to simplified methods based on limit equilibrium and active zone. Newmark sliding block analyses. Bathurst et al. (2002) sum- A computer program MSEW (ADAMA, 2005) has been marizes a number of these methods. Approaches based on developed and is commercially available to design MSE walls limit equilibrium and Newmark sliding block methods are using the current AASHTO LRFD Bridge Design Specifica- also described, for example, by Ling et al. (1997) and Paulsen tions. An application of the program to design a representa- and Kramer (2004). Comparisons are made in the latter two tive wall is provided in Appendix I, where the older allowable papers with centrifuge and shaking table test results, with stress design (ASD) specifications are compared to the LRFD some degree of success. However, the explicit application specifications. A modest seismic coefficient of 0.1 is used for of these performance-based methods in the AASHTO LRFD design. Slightly longer reinforcing strips are needed for the Bridge Design Specifications at the present time is premature. LRFD design, and seismic loading does not impact the de- sign. The suggested recommendations to modify the seismic 7.9 Other Wall Types design procedure (acceleration coefficients and tensile load distribution) cannot be directly incorporated in the program, Three other wall types were considered during this Project: but changes to the source code could be made with little effort, (1) nongravity cantilever walls, (2) anchored walls, and (3) soil and the design impact of the changes examined by studying nail walls. The treatment of these walls has been less detailed several examples. than described above for semi-gravity and MSE walls. Part The work plan in Chapter 4 identified a methodology in- of this reduced effort is related to the common characteris- volving the application of limit equilibrium programs for as- tics of the nongravity cantilever, anchored, and soil nail walls sessing internal stability of MSE walls. In particular the com- to the walls that were evaluated. The following subsections puter programs, SLIDE and ReSSA (Version 2), were going provide a summary of the recommended approach for these to be used to conduct detailed studies. After performing a wall types. limited evaluation of both programs, the following concerns were noted relative to their application to AASHTO LRFD 7.9.1 Nongravity Cantilever Walls Bridge Design Specifications: These walls include sheet pile walls, soldier pile and lagging 1. Since static and seismic design methodologies should desir- walls (without anchors), and secant/tangent pile walls. Each of ably be somewhat consistent, the adoption of such programs these walls is similar in the sense that they derive their resist- for seismic design means that a similar approach should ance to load from the structural capacity of the wall located be used for static design. This would require a major revi- below the ground surface. The heights of these walls typically sion to the AASHTO static LRFD design methodology. range from a few feet to as high as 20 to 30 feet. Beyond this 2. Whereas the use of ReSSA (Version 2) for static analyses height, it is usually necessary to use anchors to supplement has been compared successfully to FLAC analyses by the stiffness capacity of the wall system. The depth of the wall Leshchinsky and Han (2004), similar comparisons have below the excavation depth is usually 1.5 to 2 times the height not been identified for seismic loading problems. Such of the exposed wall face. comparisons would provide more confidence in the use of a limit equilibrium program to simulate the mechanics of 7.9.1.1 Seismic Design Considerations loading. In particular the main concern is the distribution of seismic lateral forces to reinforcing strips from the limit The conventional approach for the seismic design of these equilibrium analyses. It would be of value if in future cen- walls is to use the M-O equations. Article C11.8.6 of the trifuge tests, for example, strips could be instrumented to AASHTO LRFD Bridge Design Specifications indicates that measure loads during seismic loading. a seismic coefficient of kh = 0.5A is to be used and that wall inertial forces can be ignored. In this context A is the peak In view of the these concerns, adoption of limit equilibrium ground acceleration for the site based on the AASHTO haz- analyses is not currently recommended for MSE internal sta- ard map and the site classification. The use of the 0.5 factor bility analysis, although future research on their potential implies that the wall is able to move, although this is not ex- application is warranted. plicitly stated. As discussed in previous sections, the original Deformation design approaches are not identified for inter- development of the 0.5 factor assumed that the wall could nal stability in the AASHTO Specifications. Such methods are move 10A (in inches), which could be several inches or more complex as they involve sliding yield of reinforcing strips or and which would often be an unacceptable condition for this possible stretch in the case of geosynthetic grids or geotextiles. class of walls.

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89 Most nongravity cantilever walls are flexible and there- One important difference for this class of walls relative to fore the customary approach to static design is to assume that gravity walls and MSE walls is that the capacity of the wall active earth pressure conditions develop. The amount of depends on the passive pressure at the face of the structural movement also will be sufficient to justify use of the M-O unit: either the sheet pile or the soldier pile. For static loading, equation for estimating seismic active earth pressures. How- the passive pressure is usually estimated from charts as shown ever, rather than the 0.5 factor currently given in the AASHTO in Article 3.11.5.4 of the AASHTO LRFD Bridge Design Speci- Specifications, it is suggested that the wave scattering fac- fications. For soldier piles the effective width of the structural tors described in Section 7.5 of this chapter be used. For element below the base of the wall is assumed to be from 1 to typical nongravity cantilever walls, which have a height of 3 pile diameters to account for the wedge-shape form of soil 25 feet or less, this means that the factor will range from 0.8 reaction. The upper several feet of soil are also typically ne- to 0.9 rather than 0.5. glected for static passive earth pressure computation. This is The decision whether to use the 0.5 factor currently given done to account for future temporary excavations that could in AASHTO will depend on the amount of permanent move- occur. In view of the low likelihood of the excavation occur- ment of the nongravity cantilever wall that is acceptable dur- ring at the time of the design earthquake, this approach can ing the design seismic event. If the structural designer reviews be neglected for seismic load cases. the design and agrees that average permanent wall movements Under seismic loading a reduction in the seismic passive of 1 to 2 inches at the excavation level are acceptable, the seis- pressure occurs. This reduction can be estimated using M-O mic coefficient used for design (after reducing for scattering equation for passive pressures (Equation A11.1.1.1-4). How- effects) can be further reduced by a factor of up to 0.5. ever, as noted earlier in this chapter, the M-O equation for The acceptability of the 0.5 factor is based on several passive earth pressures is based on a granular soil and Coulomb considerations: failure theory. Various studies have shown that Coulomb theory can be unconservative in certain situations. The M-O Allowable stresses within the wall are not exceeded during equation also does not include the contributions of any cohe- the earthquake and after the earthquake, since there is sive content to the soil. Similar to the previous discussion for likely to be at least 1 to 2 inches of permanent wall move- active pressures, the effects of cohesion on the passive earth ment at the excavation level. pressure have been found to be significant. Weather conditions at the site will allow several inches of As an alternative to the M-O passive pressure equation, the outward movement to develop. If pavements, sidewalks, seismic passive earth pressure can be estimated using the charts or protective barriers prevent outward movement of 1 to in Figures 7-23 through 25. These charts show the relationship 2 inches, then the reduction of 0.5 would not seem to be between KPE and kh as a function of the normalized soil cohe- appropriate. sion. The charts were developed using log spiral procedures, Aesthetics of the wall after permanent movement are ac- following the methodology published by Shamsabadi et al. ceptable. Often there will be some rotation with the move- (2007). The interface friction for these charts is 0.67 . Proce- ment at the excavation line, resulting in a wall that is lean- dures described by Shamsabadi et al. can be used to estimate ing outward. This wall may be structurally acceptable but the seismic passive coefficient for other interface conditions. it may result in questions whether the fill is falling over. Significant deformation is required to mobilize the pas- Movement at the excavation level or at the top of the wall, sive pressure, and therefore, for static design, the resulting which will likely be at least 1 to 2 several inches because of passive pressure coefficient is often reduced by some amount rotation, do not damage utilities or other infrastructure to control deformations. For static loading the reduction is located above or below the wall. usually 1.5 to 2. In the absence of specific studies showing otherwise, this same reduction may be appropriate for the Another important consideration is the characteristics of seismic loading case in a limit equilibrium analysis, to limit the soil being supported. Nongravity cantilever walls are the deformation of the nongravity cantilever. This approach normally constructed using a top-down method, where the would be taken if using the computer programs SPW 911 structural support system is installed (that is, sheet pile or or SWALSHT. soldier pile) and then the earth is excavated from in front of Alternately, a numerical approach, such as followed within the structural members. In many cases the natural soil behind the computer program PY WALL (Ensoft, 2005) can explicitly the wall will have some cohesive content. As discussed in account for the displacement through the use of p-y springs. Section 7.3, the active earth pressure can be significantly re- Programs such as L-PILE and COM624 also can be used to duced if the soil has a cohesive component. If site explorations make these analyses, although appropriate consideration needs can confirm that this cohesive component exists, then it makes to be given to the development of p-y curves. These programs sense that the design method accounts for this effect. are not specifically set up for evaluating seismic response

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90 Figure 7-23. Seismic passive earth pressure coefficient Figure 7-24. Seismic passive earth pressure coefficient based on log spiral procedure (c soil cohesion, based on log spiral procedure (cont.) (c soil cohesion, soil total unit weight, and H is height). soil total unit weight, and H is height). but can be used to evaluate seismic performance by intro- ducing appropriate soil pressures and reactions consistent with those expected to occur during a seismic event. Appen- dix K describes a study that was part of the NCHRP 12-70 Project that demonstrates the use of the general beam-column approach to evaluate nongravity cantilever retaining walls under seismic loading. Included within the Appendix K dis- cussion are recommendations on p- and y-multipliers to de- velop p-y curves for continuous (sheet pile) retaining walls. 7.9.1.2 Seismic Design Methodology The following approach is suggested for design of non- gravity cantilever walls: 1. Perform static design following the AASHTO LRFD Bridge Design Specifications. 2. Establish the site peak ground acceleration coefficient (kmax) and spectral acceleration S1 at 1 second from the 1,000-year maps adopted by AASHTO (including appropriate site soil modification factors). 3. Determine the corresponding PGV from correlation equa- tions between S1 and PGV (provided in Chapter 5). Figure 7-25. Seismic passive earth pressure coefficient 4. Modify kmax to account for wall-height effects as de- based on log spiral procedure (cont.) (c soil cohesion, scribed in Section 7.6. Include cohesion component as soil total unit weight, and H is height).