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Table 7.4: Summary of FLAC Displacements
POST EQ
MAXIMUM FLAC DISPLACEMENTS (m)
F.S. Morgan Hills Capitola El Centro Landers Taft Mexico
1.22 0.02 1.00 0.69 0.92 0.30 0.80
1.41 0.02 0.57 0.49 0.62 0.02 1.34
1.48 1.14 1.40 1.63
1.54 0.39 0.34 0.33
1.60 (a) 0.57 0.66 0.75
2.08 0.42 0.42 0.57
2.21 (a) 0.22 0.13 0.20
(a) configurations with a lowered groundwater elevation
The resources required for advanced numerical modeling of liquefaction and soil structure
interaction are not available for the majority of bridge seismic hazard evaluations. In light of the
benefits of these techniques for bridge engineers, it is worthwhile to compare the seismicallyinduced
deformations computed using FLAC with the post-earthquake FS calculated with a
routine slope stability analysis. A well-defined relationship between the results of these two
methods of analysis would facilitate simplified estimates of embankment deformation from the
results of standard and widely-used limit equilibrium methods of slope stability analysis. The
embankment deformations computed with FLAC are plotted with respect to the minimum “postearthquake”
FS obtained from UTEXAS3 in Figure 7.7. This figure has been developed using
the deformations computed at the toe of the slope, as they are the most relevant. Soil movements
near the middle and lower portions of the slope will likely impact the foundations of bridge
abutments, as well as pier walls and bents located near the embankment slope.
To account for the intensity and duration of the ground motions used in the FLAC analyses a
Ground Motion Intensity (GMI) parameter was developed by dividing the peak horizontal
acceleration of the input motion by the appropriate MSF (Figure 3.4). The MSF values proposed
by Arango (1996), which closely follow the most recent consensus (Youd and Idriss, 1998), were
used in this investigation. The Ground Motion Intensity is given by the expression
GMI = PGA/MSF (7-2)
where:
GMI = Ground Motion Intensity parameter
PGA = Peak Ground Surface Acceleration
MSF = Magnitude Scaling Factor for the earthquake of interest.
The GMI parameter is useful for sites in Oregon where the seismic hazard reflects contributions
from both large subduction earthquakes and moderate, near-surface crustal events. The GMI
value provides a simplified “weighting” factor that demonstrates the effect of ground motion
duration on permanent embankment deformations.
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