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Table 8.21: Comparison of Predicted Maximum Levee Displacements With and Without Soil Improvement
at the 100 yr Flood Stage.
METHOD OF CALCULATION
MAXIMUM HORIZONTAL
RIVERWARD LEVEE DISPLACEMENT (cm)
Without
With Case 1 With Case 2
Soil Improvement Soil Improvement Soil Improvement
Newmark 7 0 0
Makdisi and Seed 100 N/A N/A
Bracketed Intensity 1 0 0
Results of the Parametric Study 30 2 < 1
Numerical Model: N. Levee Crest 16 < 1 < 1
Negative horizontal displacements represent movement toward the river and negative vertical displacements
signify settlement.
N/A denotes peak acceleration at the depth of interest is less than the yield acceleration for deep-seated,
riverward slides (i.e., the deformation is 0 cm).
8.10 SUMMARY AND CONCLUSIONS
The significant consequences associated with the failure of the Columbia River levee in the
vicinity of the Interstate 5 and 205 bridges and Marine Drive provided the impetus for this
thorough multi-hazard stability investigation. Standard of practice methods were used in
conjunction with more sophisticated numerical models to assess the overall stability of the levee
under static and dynamic loading conditions. The primary objectives of this investigation
included: (1) evaluating the static stability of the levee at three different river stages; (2)
assessing the seismic stability of the levee at the aforementioned water levels; (3) evaluating the
potential extent of earthquake-induced deformations of the levee, and (4) indicating, in a general
sense, the effectiveness of soil improvement for reducing seismically-induced deformations of
the levee.
The following conclusions can be drawn based on this investigation.
Under static conditions, the levee appears to be stable at all sections with the river stage at the
100-year flood elevation or lower (elevation 8.8 m). However, landward failures due to high
seepage forces were estimated to occur if the river was to reach the crest of the levee. The
liquefaction hazard was considered to be minimal for a magnitude 6.2 earthquake however the
generation of partial excess pore pressures combined with the higher intensity of ground motions
produced permanent deformations that were similar to, or larger than those estimated for the
larger scenario earthquakes. Conversely, the liquefaction hazard was greater for the distant,
larger earthquakes but the destructiveness is limited somewhat by the low intensity of the
motions. The silt rich soils in the levee and foundation are considered liquefiable based on cyclic
laboratory tests performed on specimens from the site.
Under worst-case conditions, the levee is estimated to experience displacements of
approximately 0.6 to 0.9 m (2-3 ft) based on the numerical modeling.
Simplified chart solutions for estimating seismically-induced deformations provided reasonable
values from the standpoint of a preliminary screening for liquefaction hazards. It was
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