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Design of a given component shall limit permanent displacement to the following:
1. Less than 10 cm for a Level 1 earthquake (10% probability of exceedance in
50 years).
2. Less than 30 cm for a Level 2 earthquake (5% probability of exceedance in 50
years).
These standards are intended to insure that following a Level 1 earthquake (operating level event
for structures of normal importance), the damages will be negligible, non-structural, and the
bridge will remain serviceable. Following a larger, Level 2 earthquake (operating level event for
structures of high importance and/or collapse prevention), the damage will be non-catastrophic
and repairable in a reasonable amount of time. The deformation limits are bridge and component
specific, and reflect the sensitivity of the structure and appurtenant components to deformation.
Several transportation departments are developing programs to mitigate liquefaction hazards at
major bridge sites. Common ground treatment methods include soil densification, increasing the
strength and stiffness of the soil by grouting, and/or improved soil drainage. These
improvements are accomplished using many methods such as deep dynamic compaction, vibrocompaction,
stone columns, soil mixing, and many others. Although the use of soil improvement
methods is increasing, there are very few tools currently available for establishing the extent of
ground treatment necessary to minimize earthquake damage. The most comprehensive reference
has been prepared by the Japanese Port and Harbour Research Institute (PHRI 1997). Although
this reference is based on experience gained in the port environment, most of the
recommendations are transferable to the highway transportation field. The recommendations are
largely based on limit state analysis and model testing. Additionally, the guidelines do not
address permanent deformations as a function of design-level ground motions, a primary concern
in performance-based design.
Current “standard of practice” seismic design for embankments and bridge abutments involves
using pseudo-static, limit equilibrium mechanics. The design utilizes empirically determined
seismic coefficients, which are functions of the maximum ground accelerations. The coefficients
are used to estimate the seismic inertial body forces. These limit equilibrium methods can be
used to account for the presence of potentially liquefiable soils, but only in a simplistic manner
by decreasing the soil strength. Additionally, the output of these methods is usually the factor of
safety against the exceedance of a given limit state, and therefore, are not directly applicable for
deformation-based analysis.
There have been several recent enhancements to pseudo-static design methods for evaluating
seismic deformations of the earth structures (the term “embankment” will be used for the
remainder of this report to cover the types of earth structures encountered adjacent to bridges).
These methods include the well-known rigid body, sliding block methods for both nonliquefiable
and liquefiable soils, and numerical modeling procedures for evaluating the patterns
of deformations resulting from strong ground motion. In summary, the prevalent issues for the
deformation-based, seismic analysis of highway embankments include the need to estimate
lateral deformations for non-liquefiable soils, potentially liquefiable soils, varying design-level
ground motions, and sites with remedial soil improvement.
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