Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 133

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 132
132 generalized limit equilibrium method using conventional slope ect was to assume that the entire mass within the rein- stability software, for cases involving layered soils. Charts that forcing strips would respond as a rigid mass, and there- included the effects of soil cohesion on seismic active and pas- fore should be included within the sliding analyses and sive pressures were developed. A key consideration within the internal stability evaluation. This approach can lead to methodology was the amount of movement that would develop very large inertial forces, which may not develop be- or could occur during seismic loading, and how this movement cause of the flexibility of the MSE wall system. As noted would affect the seismic demand on the retaining wall. in the section on MSE wall design, there are also signif- A number of retaining wall topics were identified as requir- icant issues regarding the approach used to estimate ing further evaluation or investigation. These topics fall into two tensile forces in the reinforcement during internal sta- categories: (1) generic issues and (2) wall-specific issues: bility evaluations, and there is a need to upgrade the two standard software packages, MSEW and ReSSA, 1. Generic issues, relating to the demand and capacity once a consensus is reached on the approach used to de- evaluations sign MSE walls. Part of the design issue associated with Simplified methods of estimating seismic passive earth MSE walls is how to properly account for the flexibility pressures, particularly for cases involving cohesion, of the wall system in the method of analysis being used. should be developed. Rigorous procedures involving the Additional research on the use of the generalized limit use of log spiral methods are recommended and charts equilibrium approach and evaluation of deformations showing typical results are provided. However, the log to define wall performance also is needed. spiral approach to passive pressure determination is not Nongravity cantilever walls and anchored walls both in- easily performed, and in the absence of simple log spiral volved a similar question on whether movement of the methods, the designer is likely to resort to less accurate soil wedge behind the retaining wall will be sufficient to Coulomb or even Rankine methods of estimating passive allow use of a lower seismic coefficient. For both wall earth pressures. types the approach being recommended, assumes there The potential for shear banding in cohesionless soils lim- is no amplification of ground motions behind the re- iting the development of seismic active earth pressures taining wall and that the wall will displace enough to sup- needs to be researched. This idea has been suggested by port using a seismic coefficient in design that has been Japanese researchers and by some researchers in North reduced by 50 percent. The potential for amplification of America (for example, R. J. Bathurst and T. M Allen) as forces to values higher than the free-field ground mo- potentially limiting the development of seismic earth tions is a particular concern for the anchored walls. pressures. The concept is that failure during seismic The process of pretensioning each anchor to a design loading will occur along the same failure surface as de- load ties the soil mass together, and though the strands veloped during static active earth pressure mobilization, or bars used for prestressing can stretch, there is a fun- rather than changing to some flatter slope angle. This damental question whether the wall-tendon-grouted mechanism would limit the development of seismic anchor zone can be simplified by eliminating any inter- active earth pressures to much lower values than cur- action effects. rently calculated. Unfortunately, the amount of infor- Whereas soil nail walls appear to be relatively simple in mation supporting this concept is currently limited, terms of overall seismic design, there are still fundamen- though it does appear to have some promise. tal questions about the development of internal forces 2. Wall-specific issues within the soil mass during seismic loading. These ques- The inertial force associated with the soil mass above tions include whether the internal forces are transferred the heel of a semi-gravity cantilever wall remains a de- to the nails in the same manner as during static loading. sign issue. The recommendations in this report assume The AASHTO LRFD Bridge Design Specifications also that the only seismic force that must be considered is needs to be supplemented with specific discussions on the incremental earth pressure from the active failure the static design of soil nail walls, and then these static wedge, and that the soil mass above the heel of the wall provisions need to be reviewed relative to provisions does not provide any additional seismic load to the appropriate for seismic loading. stem of the wall. Detailed finite element analyses could help resolve this issue. 10.3 Slopes and Embankments Several issues were identified for MSE walls, including the amount of inertial mass that should be considered The seismic design of slopes and embankments was identi- for sliding analyses and for the internal design of the re- fied as a more mature area of seismic design, where both sim- inforcing system. The approach taken during this Proj- plified limit equilibrium and displacement-based approaches