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The comparisons between the standard Newmark-type analyses and the results of more rigorous
numerical modeling suggest that when carefully performed, the simple sliding block method
captures the important aspects of the seismic performance of slopes, and provides similar
conclusions about the influence of embankment geometry and soil treatment configurations. This
observation applies for cases with no embedded structures or piles in the slope. Incorporation of
these bridge and foundation elements limits the applicability of the simple sliding block methods,
and necessitates the use of numerical soil-structure interaction models. The scatter and
uncertainty inherent in all of the simplified approaches must also be noted when evaluating the
seismic performance of earth structures such as approach fills, embankments and abutments.
The results of this comparative study are supplemented by the findings of a similar study
performed to evaluate the effectiveness of liquefaction remediation measures for bridges (Riemer
et al. 1996; Lok and Riemer, 1999). Riemer modeled an embankment and soil profile that was
very similar to that studied here. The 2-D FLAC model and the Makdisi and Seed procedure
were used to estimate lateral embankment deformations for cases with and without soil
improvement by densification. Very good agreement has been found between the results of the
two investigations. One important conclusion made from the Riemer study was that the
predominant frequency of the design earthquake might have a significant effect on the expected
displacements. Low frequency shaking may produce much larger total movements than high
frequency shaking for earthquakes of comparable magnitude and duration. This is predominantly
due to the greater area under acceleration pulses that exceed the yield acceleration thereby
resulting in greater computed I a , I b , and displacement.
7.5 CONCLUSIONS
The seismic performance of a variety of embankment configurations were evaluated in
sensitivity studies involving different soil profiles, ground treatment configurations, and input
ground motions. The effectiveness of the soil improvement was evaluated using standard limit
equilibrium slope stability methods combined with rigid body Newmark-type displacement
analyses. Supplementary analyses were performed using a well-validated 2-D numerical
effective stress model, in order to evaluate the applicability of the more simplified, standard
procedures for cases involving soil liquefaction. Based on this study, the following observations
and conclusions can be made.
Simplified Newmark-based methods of evaluation are valuable tools for identifying
embankments vulnerable to excessive lateral spreading and seismically-induced
deformations, provided that appropriate dynamic soil properties and acceleration time
histories are used. It is recommended that these methods be supplemented with more
rigorous stability analyses in cases where computed deformations approach allowable
limits. Given the inherent variability of the slope displacements estimated using the
simplified approaches, and the ease of use of these procedures, it is recommended that
two or more of the methods be used on projects requiring preliminary estimates of
permanent displacement.
A simplified four-step method for estimating deformations that is similar to the
established method of Makdisi and Seed (1978) has been developed. The proposed
method is based on the Bracketed Intensity. This technique has been found to yield
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