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Table 8-2. Results of ground displacement estimates for example
stability evaluation.
Ground Motion Displacement (inches)
Static C/D 10% in 50 2% in 50
Parameter Slope Angle Ratio kyield 7% in 75 Years Years Years
Upper Bound Till ( = 42 degrees)
Case 1 1H to 1V 0.9 NA NA NA NA
Case 2 1.5H to 1V 1.3 0.13 6-9 3-5 14-18
Case 3 2H to 1V 1.7 0.25 <1 <1 3-4
Upper Bound Till ( = 38 degrees, c = 200 psf)
Case 1 1H to 1V 1.2 0.09 12-19 7-11 26-32
Case 2 1.5H to 1V 1.6 0.26 <1 0 3
Case 3 2H to 1V 2.0 0.32 0 0 <1
Lower Bound Till ( = 36 degrees)
Case 1 1H to 1V 0.8 NA NA NA NA
Case 2 1.5H to 1V 1.2 0.07 18-27 11-17 36-44
Case 3 2H to 1V 1.5 0.17 3-5 1-2 8-11
Typically, if the site is nonliquefiable (that is, significant minimum C/D ratio is 1.5 or more, and for natural slopes the
loss in strength does not occur during seismic loading), a seis- acceptable C/D ratio ranges from 1.3 to 1.5, depending on the
mic coefficient of 50 percent of the site-adjusted PGA (after potential consequences of slope instability.
adjustments for site soil effects and wave scattering) will re- The following results were developed to define combina-
sult in ground displacements of less than 1 to 2 inches, as long tions of slope angles and the site-adjusted PGA values below
as the resulting C/D ratio (that is, factor of safety) is greater which a seismic stability analysis did not appear warranted.
than 1.0. In view of the simplifications associated with this This guidance must be used with some care. It works best
method, common practice is to use a C/D ratio > 1.1 to de- when the slope is relatively homogeneous in consistency and
fine acceptable slope conditions. It is a fairly simple task to there is no water table within the slope. As the slope becomes
calibrate the reduction based on the typical site-adjusted PGA more complicated, particularly if there are thin, low-strength
and PGV for the area, the shape of the normalized response bedding planes, then this screening criteria identified in
spectrum, and the displacement that is acceptable. Newmark Table 8-3 should not be used and a detailed slope stability
curves in Chapter 5 then can be used to "back out" the ky analysis performed, in which the strength in each soil layer is
value. If the ky value is used in the slope stability computer modeled.
program as the seismic coefficient, and the resulting factor of
safety is greater than 1.0, acceptable slope displacements are
8.5.3 Liquefaction Potential
predicted.
No effort has been made within this Project to introduce
liquefaction effects into the seismic stability analysis. This
8.5.2 No Analysis Cut-off
topic has been specifically avoided due to the complexity of
The same concept as described in the preceding subsection the issues involved and the on-going debate regarding the
can be used to define a "no analysis" area. In this case, if the best approach for addressing liquefaction.
C/D ratio for gravity loading is greater than a predetermined Several approaches are currently being used or proposed.
value, then the slope will be inherently safe during seismic
loading, as long as liquefaction does not occur. For engi- · The simplest are the empirical relationships suggested by
neered slopes, most transportation agencies require that the Youd et al. (2002) for estimating displacement during lat-
Table 8-3. Proposed screening levels for no-analysis cut-off.
Slope Angle Fpga PGA
3H:1V 0.3
2H:1V 0.2
