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1.0 INTRODUCTION
1.1 BACKGROUND
Experience worldwide has demonstrated that bridges and ancillary components (abutments,
approach fills and embankments, pile foundations) located at sites of shallow groundwater and/or
adjacent to bodies of water are highly susceptible to earthquake-induced damage. Liquefaction of
adjacent soils causes a significant amount of the damage. Susceptible soils consist of loose,
saturated, non-cohesive soils that are frequently found in marine and river environments.
Earthquake damage to bridge abutments and embankments is commonly manifested as ground
failures, excessive lateral displacements, and/or settlements. There are many cases of widespread
damage to bridge foundations and approach structures resulting from the lateral displacements
and settlements of surrounding soil.
Earthquake damage to bridges severely impedes response and recovery efforts following the
event. Highways serve as primary lifelines following natural disasters and communities rely on
their access. From a practical perspective, the seismic performance of a bridge is related to its
serviceability following an earthquake. Numerous cases have been documented in postearthquake
reconnaissance reports of bridges that performed well from a structural perspective,
yet were inaccessible due to excessive deformations of approach fills and adjacent foundation
soils. Additionally, the magnitude and pattern of soil deformation around bridges often results in
damage to structural elements.
Bridge abutments and deep foundations are particularly vulnerable to seismic damage. Damage
to bridges has been well documented (see the appendix of this report). For most bridges at river
crossings subjected to medium- to high-intensity earthquake motions, liquefaction occurred and
was likely the primary cause of the reported damage. Contributing factors include reduced
stability of earth structures due to the transient inertial loads, increased active pressures on
abutments due to the loss of soil strength and the seismic inertia of the backfill, and the loss of
passive soil resistance adjacent to the toe of abutments and slopes. All of these factors are
exacerbated by the presence of liquefiable soils. The substantial reduction of strength and
stiffness of the soil leads to possible geotechnical failures including catastrophic ground failures,
limited, yet damaging lateral ground deformations, and/or excessive vertical deformations that
result in uneven and often impassable grades.
Limiting soil deformations adjacent to bridges is a primary seismic design issue throughout
much of the western United States. Several transportation departments are in the initial stages of
adopting deformation-based seismic performance requirements. This method of design also is
becoming more routine in the marine transportation and port communities. The criteria are often
specified in general terms of an allowable limit state (i.e., deformation, load, moment, curvature)
and the exposure time, as follows.
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