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demonstrated that small deformation estimates based on simplified procedures are not
necessarily an indication of acceptable seismic performance. In light of the resources necessary
to conduct numerical analyses, it is recommended that several simplified methods of analysis be
used to confirm the likely range of slope deformations due to design level ground motions.
A generalized numerical analysis of ground treatment demonstrated the effectiveness of soil
improvement for reducing earthquake-induced embankment deformations to within tolerable
limits.
8.11 LIMITATIONS AND RECOMMENDATIONS FOR FUTURE WORK
This example problem synthesized the results of existing geotechnical investigations and seismic
hazard evaluations in order to perform an in-depth stability analyses of an embankment built of,
and on, potentially liquefiable soils. Although a truly probabilistic, coupled, multi-hazard
evaluation was beyond the scope of this example, the flood and earthquake scenarios have been
selected to cover the broad range of conditions thought to affect the Columbia River levee
adjacent to Hayden Island and interstate highway bridges. Standard-of-practice methods of
analysis have been employed throughout this investigation. However, in light of the complexities
associated with liquefaction hazards and slope stability, several simplifying assumptions have
been made, as discussed in this chapter. It is anticipated that ODOT engineers will apply the
methodology outlined here, as well as in seismic design guidelines, for projects throughout the
state. For this reason, the following primary limitations of this investigation should be noted.
1. The results presented are based on the seismic hazard estimates recommended as of the
date of this investigation. Changes to these estimates at a future date could alter the
conclusions presented. Subsequent analyses may need to be performed using amended
hazard scenarios.
2. Analyses were not performed on the deformed geometry of the levee. For the case of the
river stage at the crest, soil deformations (slumping, sliding, settling) may reduce the
seepage path through the levee, thereby creating high hydraulic gradients. Additional
concerns such as sand boils and piping could occur. As a result, the levee might not be
stable enough to allow the immediate regrading and/or other repairs that could be
required to restabilize the levee. Additional seepage studies should be performed on the
deformed geometry of the levee to evaluate these concerns.
3. Ground treatment by means of densification was the sole method evaluated in this
example. Field applications have included stone columns for drainage and soft ground
replacement, as well as grouting and soil-cement mixing techniques for stability and
seepage cut-off. The seismic performance of these drainage and grouting/cement soil
improvement methods have not been evaluated. It is recommended that these and other
potentially effective liquefaction mitigation measures be evaluated, either independently
or in combinations to assess the most cost effective ground treatment program.
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