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104 eral spreading. These relationships are based on empirical ever, in the absence of a consensus approach within the pro- correlations between observed lateral displacement, earth- fession for handling this issue, it is difficult to provide specific quake parameters, and soil conditions. This approach is guidance. The current difficulty in developing an approach typically applied near rivers or other locations where slopes results from uncertainties in two areas: (1) the capacity of the are gentle and a free face might exist. Generally, results from soil in its liquefied state, particularly where there are static these methods are considered most suitable for screen- shearing stresses (that is, sloping ground effects) for the site ing of potential displacement issues and involve too much and also where the soil could dilate under large deformations, uncertainty for design. and (2) the ground motions to use after the seismic wave trav- · An approach was suggested in the NCHRP 12-49 Project els through the liquefied soil. While numerical methods, such (NCHRP Report 472, 2002) for addressing liquefaction of as DESRA (1978), are available to address the latter issue, bridge abutments. This approach includes the effects of these methods are limited in availability to designers. foundation pile pinning. Combinations of earthquake The approach used to address liquefaction during seismic magnitude, site-adjusted PGA, and SPT blowcounts are slope and embankment design has and likely will continue to used to decide whether the liquefaction analysis is required. require more research. Until a consensus is reached within the A residual strength is assigned to the liquefied layer using profession, the NCHRP 12-70 Project team recommends using either of two empirical relationships (Seed and Harder, the methodology summarized in the NCHRP 12-49 Project, 1990; Olson and Stark, 2002). While this approach is rela- but providing more cautionary words on the limitations of tively simple to apply, it is often criticized that it relies on this method. triggering relationships for liquefaction and does not prop- erly account for the dilation effects that occur under large 8.6 Conclusions ground displacement. Results of recent centrifuge research programs also indicate the methodology may not replicate This chapter summarizes the approach recommended for important mechanisms that occur during seismic loading. the seismic analysis and design of slopes. The methodology · Various computer models, such as FLAC, also are used uses conventional limit equilibrium slope stability analysis commonly to investigate the seismic stability problem methods, in combination with the Newmark method for es- where liquefiable soils have been identified. These methods timating displacements. Relative to existing methods, the seem to be used extensively by designers, often without approach: having a particularly good understanding or appreciation for the uncertainties of the model. One significant criticism · Incorporates the results of wave scattering and ground mo- of this method is that thin layers that lead to ground dis- tion studies summarized in Chapters 5 and 6, including an placement during liquefaction are often ignored. equation that relates the PGV to the spectral acceleration · The NCHRP 20-07 Project initially suggested that the en- at one second. tire issue of liquefaction could be ignored if the magnitude · Uses a new set of equations for estimating displacements of the design earthquake is less than a value of approxi- that were calibrated against the USNRC strong motion data- mately 6.5. The controlling magnitude was taken from a base, making the equations applicable to the CEUS as well as study conducted by Dickenson et al. (2002) for the Oregon the WUS. Department of Transportation. It is likely that Dickenson and his co-authors did not intend for his work to be used The proposed method is thought to be relatively simple to in this manner, and preliminary feedback from the geo- use and easily adopted by designers. The primary outstand- technical community suggested that this approach was too ing issues are (1) the use of this method to develop a "no unconservative for adoption by AASHTO. analysis" approach and (2) an appropriate methodology for introducing liquefaction potential into the analysis. Interim There is little doubt that liquefaction-related slope insta- approaches for addressing each of these issues are given in the bility is an important consideration in some locations. How- chapter; however, further research on each is required.