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3. Compressional forces generated by lateral ground displacement generally cause one of
the following reactions: (a) the superstructure may act as a strut, bracing the tops of
abutments and piers and holding them relatively in place while the bases of these
elements shift streamward with the spreading ground; (b) the connections between the
foundation and the superstructure may fail, allowing piers and abutments to shift or tilt
toward the river with little restraint; or (c) the deck may buckle laterally or vertically,
causing severe damage to the superstructure.
4. Subsidence and increased lateral earth pressures can also lead to deleterious
consequences for bridge foundations. Waterfront retaining structures, especially in areas
of reclaimed land, can experience large settlements and lateral earth pressures adjacent to
bridge foundations. These movements lead to the rotation and translation of bridge
abutments and increased lateral forces on pile foundations.
5. A number of failure modes may occur in pile foundations, depending on the conditions of
fixity, pile reinforcement and ductility. Generally, if concrete piles were well embedded
in the pile caps, shear or flexural cracks occurred at pile heads, often leading to failure; if
steel pipe piles were fixed tightly in the pile caps, failure was at the connection or pile
cap; or if the pile heads were loosely connected to the pile caps, they either rotated or
were detached.
To better understand these phenomena and the potential ramifications, several case histories are
examined in greater detail in the subsequent section.
2.3 OVERVIEW OF HISTORIC DAMAGE TO BRIDGE
FOUNDATIONS
The modes and extent of seismic damage to bridges can be related to the movement of
abutments, lateral spreading and settlement of abutment fills, horizontal displacement and tilting
of piers, severe differential settlement of abutments and piers, and failure of foundation
members. The ability to predict ground failures and associated structural damage are requisites
for seismic resistant design and evaluation of existing structures.
2.3.1 1964 Alaska Earthquake
The Great Alaska Earthquake of 1964 (M w 9.2) caused some of the most devastating and
widespread damage to highway bridges in United States history. The peak ground accelerations
were estimated to be in the 0.10 g to 0.20 g range. Although these values seem quite small
considering the amount of damage, the duration and frequency of the ground motions are equally
important in describing the damage potential of an earthquake. The duration of strong shaking
was estimated to be anywhere between 1.5 to 3.0 minutes, and because of the large epicentral
distances to many bridges, the ground motions were likely robust at longer periods (closer to
fundamental periods of the structures). The seismic performance of the transportation system in
Alaska during this event is particularly germane for Oregon because of the potential for large
subduction zone earthquakes in the Pacific Northwest (this seismic hazard is addressed in
Chapter 8).
Lateral spreads and their deleterious effects on highway and railway bridges were seen as far
away as 130 km from the zone of energy release (Figure 2.2). In particular, a series of pile-
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