Geoscience Reference
In-Depth Information
4. Critique of some simplified seismic displacement methods
4.1. GENERAL
Comprehensive discussions of seismic displacement procedures for evaluating the seis-
mic performance of earth/waste structures have been presented previously by several
investigators (e.g., Makdisi and Seed, 1978; Seed, 1979; Lin and Whitman, 1983;
AmbraseysandMenu,1988;Yegianetal.,1991a,b;Marcusonetal.,1992;Jibson,1993;
Ambraseys and Srbulov, 1994; Bray et al., 1995; Ghahraman and Yegian, 1996; Kramer
andSmith,1997;BrayandRathje,1998;Finn,1998;Jibsonetal.,1998;RathjeandBray,
2000; Stewart et al., 2003; RathjeandSaygili, 2006). There isnot sufficient space inthis
paper to summarize and critique all pertinent studies. In this paper, some of the most
commonly used simplified procedures for evaluating seismic displacement of earth and
waste fills will be discussed with a focus on methods that do not assume that potential
sliding mass is rigid.
4.2. SEED (1979) PSEUDOSTATIC SLOPE STABILITY PROCEDURE
First, several simplified pseudostatic slope stability procedures are commonly used in
practice. They include Seed (1979) and the Hynes-Griffin and Franklin (1984). Both
methods involve a number of simplifying assumptions and are both calibrated for eval-
uating earth dams wherein they assumed that
1m of seismic displacement constituted
acceptable performance. They should not be applied to cases where seismically induced
permanent displacements of up to 1m are not acceptable, which is most cases for eval-
uating base sliding of lined solid-waste landfills or houses built atop compacted earth
fill slopes. Additionally, they provide a limited capability to assess seismic performance,
because they do not directly address the key performance index of calculated seismic
displacement.
<
The Seed (1979) pseudostatic slope stability method was developed for earth dams with
materials that do not undergo severe strength loss that have crest accelerations less
than 0
15 with appropriate dynamic strengths for
the critical earth materials, performance is judged to be acceptable if
FS
.
75
g
. Using a seismic coefficient of 0
.
>
.
15. The
characteristics of the earthquake ground motion and the dynamic response of the poten-
tialslidemasstotheearthquakeshakingarerepresentedbytheseismiccoefficientof0.15
for all cases. Use of
FS
1
15 ensures that the yield coefficient (i.e., dynamic resistance
of the earth dam) will be greater than 0.15 by an unknown amount. Thus, the earthquake
ground motion and dynamic resistance and dynamic response of the earth dam are very
simplycapturedinthisapproach,andtheamountofconservatisminvolvedintheestimate
and the expected seismic performance is uncertain. An earth structure that satisfies the
Seed(1979)recommendedcombinationofseismiccoefficient,
FS
,anddynamicstrengths
maydisplaceupto1m,sosatisfactionofthiscriteriadoesnotmeanthesystemis“safe”
for all levels of performance.
>
1
.
