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96 CHAPTER 8 Slopes and Embankments This chapter summarizes the results of embankment and nificantly in terms of geometry, material properties, and slope stability studies that were carried out for the Project. groundwater conditions. In most cases the constructed slopes The primary objectives of these studies were to: will be relatively uniform in soil conditions, though the con- structed material will vary from sands and gravels to fill that Develop a methodology for evaluating the seismic re- has high fines content (that is, cohesive soil content). On the sponse of embankments and slopes that can be easily used other hand the natural slopes will usually be highly variable, by designers; with layers that range from gravels to clays and often the Account for the results of ground motion and wave scat- groundwater will be located within the slope. tering studies presented in Chapters 5 and 6 in the pro- posed approach; and 8.1.1 Engineered Slopes and Embankments Provide comments on the use of the proposed methodol- ogy in low seismicity areas, where a "no analysis" approach These slopes generally will be constructed from an imported may be appropriate for the seismic analysis and design of material. Depending on the geographic area, the imported embankments and slopes. materials can be predominantly sands or gravels or they can have a high percentage of cohesive soil. The slopes are com- The proposed methodology is intended for use in con- pacted and will usually exhibit good strength characteristics. structed embankments or naturally occurring soil slopes. As Slope angles often will range from 2H:1V (horizontal to ver- noted in Section 4.3, rock slopes are not being considered in tical) to flatter than 3H:1V. Height of the slope can vary from this development. a few feet to over 50 feet. A common example of these slopes This chapter begins with a brief summary of the types of would be the approach fill used at either end of a bridge. slopes and embankments commonly encountered during These approach fill slopes would be on the order of 30 feet in transportation projects. This discussion is followed by a brief height. summary of current practice, a summary of the methodology These slopes are perhaps the easiest to evaluate from the being proposed, and an example application of this method- standpoint that the fill is defined, and therefore determina- ology. The chapter is concluded with a discussion of other tion of material properties is more straight-forward. If the fill considerations relative to the seismic analysis and design of is cohesionless, the friction angle () will normally be 35 de- slopes and embankments. As with previous chapters, the ap- grees or higher. If the fill has appreciable fines content, the proach identified in this chapter will form the basis of the compacted strength often will be in excess of 2,000 psf. The proposed specifications, commentaries, and example prob- groundwater location for most of these slopes will be at some lems given in Volume 2 of this Final Report. distance below the base of the fill. The designs of these slopes become problematic if the embankment fill is being placed on a soft or liquefiable foundation material. In these cases the de- 8.1 Types and Performance termination of the strength of the foundation material under of Slopes static and seismic loading becomes a key consideration dur- Two general classes of slopes need to be considered for the ing the analysis. methodology development: natural slopes and constructed or The geotechnical investigation of the engineered fill gener- engineered slopes. These two categories of slopes will vary sig- ally will be limited to investigating the characteristics of the