Report - Oregon State Library: State Employee Information Center ...

7.0 DEFORMATION ANALYSIS OF EMBANKMENTS
7.1 INTRODUCTION
The application of performance-based seismic design specifications is becoming routine for
transportation applications. Requisite input for this method design includes the definition of limit
states for the structure and/or components being evaluated (allowable force-deformation limits).
As applied to embankments and approach fills tolerable permanent deformations are defined.
Therefore, engineers require practice-oriented design tools for evaluating earthquake-induced
deformations of earth structures. Simplified procedures for estimating deformations of slopes in
competent soils have been well developed over the past three decades. However, equivalent
methods are still in development for estimating displacements of earth structures built with or on
soils that lose appreciable strength during earthquake shaking. Also, at many bridge sites the
prevalence of weak soils necessitates the application of soil improvement for mitigating seismic
and geologic hazards.
In order to effectively retrofit a bridge foundation sited on or adjacent to liquefiable soil deposits,
it is necessary to determine the location and lateral extent of the soil that should be treated by
ground improvement techniques. Clearly, the cost and effectiveness of the ground treatment will
depend on the volume of soil to be improved. There are currently few guidelines for relating the
volume of soil treatment to the anticipated seismic performance of earth structures. Predicting
the amount of deformation at a site and the extent and effectiveness of the soil improvement
requires a method for evaluating the deformation produced by the earthquake before and after
ground treatment. Since acceptable performance is usually defined in terms of limiting
deformations below some critical value, assessment of ground improvement strategies depends
on the engineer’s ability to analyze likely deformations of deposits including liquefiable soils.
Guidelines for evaluating both the magnitude and pattern of deformations of earth structures
caused by strong ground motions are limited. Traditionally, the static factor of safety (FS)
computed using limit equilibrium procedures has been used as an indicator of the anticipated
seismic slope stability. These analyses often employ a lateral inertial force intended to represent
the effects of the earthquake motions on the stability of the slope. The pseudostatic methods of
analysis and the associated sliding-block deformation procedures have been described in the
literature (Kramer 1996) and will not be thoroughly addressed here. Primary limitations of the
pseudostatic method of analysis for performance-based design are that: (1) the influence of
excess pore pressure generation on the strength of the soil can only be handled in a de-coupled
manner; (2) the relative motion of the slide mass is confined to a single slip plane therefore the
pattern of deformations cannot be assessed directly; (3) the sliding block method cannot be
applied for cases with static and/or dynamic FS < 1; and (4) the sliding block method can result
in negligible permanent displacements for cases where the yield acceleration has not been
exceeded, despite evidence that soil yielding prior to reaching the limit state can result in
significant deformations. The latter two topics will be addressed in subsequent sections of this
chapter. These limitations can be overcome with the use of numerical modeling procedures.
117