Report - Oregon State Library: State Employee Information Center ...

engineers; (2) requisite input includes standard geotechnical parameters that are obtained during
routine foundation investigations; and (3) the methods have been coded in straightforward,
efficient computer programs that allow for sensitivity studies of various design options.
For use in determining the seismic stability of slopes, limit equilibrium analyses are modified
slightly with the addition of a permanent lateral body force that is the product of a seismic
coefficient and the mass of the soil bounded by the potential slip. The seismic coefficient
(usually designated as k h or N h ) is specified as a fraction of the peak horizontal acceleration, due
to the fact that the lateral inertial force is applied for only a short time interval during transient
earthquake loading. Seismic coefficients are commonly specified as roughly 1 / 3 to 1 / 2 of the peak
horizontal acceleration value (Seed 1979; Marcuson et al. 1992; CDMG 1997).
In most cases involving soils that do not exhibit strength loss after the peak strength has been
mobilized, common pseudostatic, rigid body methods of evaluation will generally suffice for
evaluating the stability of slopes. These methods are well established in the literature (Kramer
1996). Although they are useful for indicating an approximate level of seismic stability in terms
of a factor of safety against failure, pseudostatic methods suffer from several potentially
important limitations. These are: (1) they do not indicate the range of slope deformations that
may be associated with various factors of safety; (2) the influence of excess pore pressure
generation on the strength of the soils is incorporated in only a very simplified, “decoupled”
manner; (3) progressive deformations that may result due to cyclic loading at stresses less than
those required to reduce specific factors of safety to unity are not modeled; (4) strain softening
behavior for liquefiable soils or sensitive clays is not directly accounted for, and (5) the dynamic
behavior of the slide mass is not accounted for (Kramer and Smith 1997).
7.2.3 Analysis of the “Post-Earthquake” Factor of Safety for Slopes
Traditional slope stability analyses for seismic conditions depend on the overall pseudo-static
factor of safety to indicate whether the slope will remain in equilibrium. While these stability
analyses do not provide explicit information on the magnitude of seismically-induced
deformations, one would expect that slopes with greater pseudo-static stability would be less
prone to movement. Therefore, the factor of safety based on this procedure might provide a
useful index. As previously mentioned, the pseudostatic methods are particularly useful for cases
involving competent soils that do not lose significant strength during shaking. These methods are
not, however, well suited for analysis involving liquefiable sandy soils and sensitive fine-grained
soils that experience a reduction of strength during seismic loading.
In order to evaluate the seismic stability of slopes involving liquefiable soils using standard limit
equilibrium methods, the residual undrained shear strength of the soil can be estimated based on
standard geotechnical parameters such as the penetration resistance of the soil based on in situ
tests (Figures 4-3 and 4-4). The shear strengths provided in these two figures are applicable only
if the seismic loading was sufficient to liquefy sandy soil. Once the residual shear strength of the
sandy soil has been estimated, the layer is now modeled in a manner similar to that of an
undrained cohesive soil. The dynamic stability (sometimes termed the post-earthquake stability)
of the slope can now be assessed. While these equilibrium analyses are relatively simple to
perform, this technique for evaluating seismic stability is still based on rigid body mechanics and
thus suffers from the same shortcomings as the pseudostatic method.
119