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Introduction to Seismic Risk Assessment and Ranking

Seismic stability analysis and retrofit of earth embankments, including site remediation, has been, to date primarily, focused on embankment dams and earth retaining structures [1]. If a bridge embankment on a priority route is at a high failure risk, soil stabilization may be required, depending on the importance of the bridge. The Seismic Retrofit Manual for Highway Bridges [3] stipulates techniques for assessing the seismic vulnerability of bridges with regard to technical and socio-economic issues. The seismic retrofit manual stipulates that for bridges near unstable slopes, detailed geotechnical investigations should be carried out to assess the potential for slope instability under seismic excitations. The required detailed investigations include material testing, borehole examination, and trenching to check for unstable layers and vertical fissures.
However, for the preliminary evaluation of bridges on priority routes the use of detailed geo-technical investigations and sophisticated models are typically limited because of the associated cost and effort.
There is current interest in a careful assessment of the “most critical” embankments along priority routes. In order to achieve this goal, a means of assessing the embankments that qualify as “most critical” is required. Other than the work reported by the authors, almost no complete studies have been reported to identify and prioritize highway embankments that are susceptible to seismic failure. Data regarding soil types and depth of bedrock required for detailed seismic analysis and risk assessment are not available for the majority of bridge embankments. For instance, while the total number of bridges located on both I-24 and the Parkways in western Kentucky is 519 bridges, soil data is only available for few bridge sites. Therefore, the objective of this study is to provide a methodology to conduct seismic evaluations of bridge embankments in order to identify, rank, and prioritize the embankments that are susceptible to seismic failure and are in need of detailed analysis.

This Chapter addresses the technical component of embankment prioritization and is wellsuited to a reliability-based model for seismic risk assessment.
In order to achieve the objective of this study, a flowchart is generated to assess the seismic vulnerability of multiple bridge embankments simultaneously. The embankment geometry, material, type of underlying soil, elevation of natural ground line, upper level of bedrock, and expected seismic event in accordance with associated seismic zone maps constitute the variables for each embankment. This methodology results in calculating the seismic slope stability capacity/demand (C/D) ratio, estimated displacement, and liquefaction potential of each bridge embankment for the respective expected seismic event. Seismic vulnerability ranking and prioritization of embankments are conducted by using the “Kentucky Embankment Stability Rating” (KESR) model. Three categories are identified in the KESR model to represent the failure risk of the embankments. A priority list of the embankments with the highest seismic risk can be generated for any set of embankments.