Geoscience Reference
In-Depth Information
The comparison of the simplified methods' estimates of seismic displacement (columns
8-10)withthemaximumobservedseismicpermanentdisplacement(column3)isshown
inTable14.1.Forthiscomparison,onlythebestestimateoftheslope'syieldcoefficient,
its initial fundamental period, and the spectral acceleration at 1.5 times the initial fun-
damental period of the slope are considered. Hence, the computed displacement range is
due tothevariability intheseismicdisplacement given the values ofthe slope properties
and the seismicload.
There are four cases shown in Table 14.1 in which the observed seismic displacement
was noted as being “None,” or
≤
1cm. For these cases, all of the simplified methods
indicate that negligible displacements are expected (i.e.,
D
≤
1cm), which is consistent
with the good seismic performance observed of these earth/waste structures. Only the
Bray and Travasarou (2007) method provides sufficient resolution to indicate the correct
amount of observed displacement for La Villita Dam for Event S5 and that moderately
more displacement should be expected forEvent S5 instead of S3.
Therearethreecasesofobservedmoderateseismicdisplacementforsolid-wastelandfills
during the 1994 Northridge earthquake (i.e.,
D
max
15-30cm). For these cases, the
Bray and Travasarou (2007) method indicates a very low chance of “zero” displacement
occurring. Moreover, the observed seismic displacements are all within the ranges of
the seismic displacement estimated by this method. The Makdisi and Seed (1978) and
Bray et al. (1998) simplified methods provide reasonable, albeit less precise, estimates
of the observed displacements for two of the cases, and both significantly underestimate
the level of seismic displacement observed at the Sunshine Canyon landfill (i.e., both
estimate 0cmwhen 30cmwas observed).
=
Lastly, there are two cases of moderate seismic displacement of earth dams shaken by
the 1989 Loma Prieta Earthquake. The Bray and Travasarou (2007) method provides
refined and more accurate estimates of the observed seismic displacement due to devi-
atoric straining at these two dams than the other two simplified methods. The Bray and
Travasarou (2007) screening equation clearly indicates that the likelihood of negligible
(i.e., “zero”) displacements is very low, and the 16% to 84% exceedance range for the
nonzero displacement captures theobserved seismicperformance.
In judging these simplified methods, it is important to note that they provide predomi-
nantly consistent assessments of the expected seismic performance. However, the Bray
and Travasarou (2007) method captures the observed performance better than existing
procedures. Moreover, it is superior to the prevalent simplified seismic displacement
methods, because it characterizes the uncertainty involved in the seismic displacement
estimate and can be used in a probabilisticseismic hazard assessment.
5.7. ILLUSTRATIVE SEISMIC EVALUATION EXAMPLE
The anticipated performance of a representative earth embankment in terms of seismi-
cally induced permanent displacements is evaluated to illustrate the use of the Bray and
