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A third type of failure is shown in Figure 4.15(c). It involves excessive body rotation of the pile,
which is characteristic of larger diameter piles and piers. This response to lateral soil movement
is primarily due to insufficient restraint at the bottom of the pile because of inadequate
embedment length or a low resistance of the foundation material against lateral deformation.
Two additional modes of loading observed following recent earthquakes include lateral loads due
to soil flow around the piles, and concentration of load on piles at the interface of weak and
dense soil layers at depth. The former mode is characterized by the flow of liquefied soil around
the pile and is usually associated with stiff foundations, such as large diameter piles, piers, and
groups of closely spaced piles. Under these conditions, a relatively stiff pile will flex until the
soil has mobilized its full resistance against the pile. Additional soil movement occurs as a flow
relative to the pile. The latter mode resulted in extensive damage to piles during the Kobe
Earthquake (Iai 1998). Moment concentration at the interface between a loose- to medium-dense
sand and an underlying dense sand and gravel deposit resulted in the formation of plastic hinges
at depths greater than those normally associated with the “depth of fixity” used in analyses of
pile response to lateral loading. This demonstrates a significant limitation in current analysis and
design of piles for dynamic conditions. The potential for load concentration at layer interfaces
should be evaluated for sites exhibiting pronounced variations of soil stiffness (due to soil
liquefaction or existence of weak soils such as marine clays, deltaic-estuarine silt deposits, etc.).
The seismic performance of pile foundations in liquefiable soils remains a topic of intensive
research. The performance of deep foundations is a complex function of the following sitespecific
parameters:
1. intensity and duration of the strong ground motions;
2. extent of the liquefiable soil deposit;
3. site topography;
4. pile type(s), group size and configuration relative to the direction of soil movement, pile
spacing;
5. pile restraint (i.e., fixed-head, free-head condition); and
6. pile design and fabrication, which dictates allowable curvature and moments.
In addition, the following topics are poorly understood: (1) possible slope reinforcement
provided by piles in embankments and reduction in seismically-induced deformations, (2)
performance of piles in improved ground, and (3) seismic performance of batter piles in
competent ground. It is currently recommended that numerical dynamic, effective stress soilstructure
interaction analyses be performed for evaluating the seismic performance of
embankments and foundations of important bridges (categorized on the basis of the Liquefaction
Mitigation Policy). For other bridges, it is recommended that the conventional liquefaction
hazard analyses described in this report be used to estimate seismically-induced deformations of
embankments and foundation soils. If excessive ground deformations are indicated, soil
improvement options should be implemented and the optimal volume of ground treatment
identified, given the tolerable displacement limits and ground motion parameters. When using
standard limit equilibrium methods, the potential embankment or slope reinforcement provided
by piles should not be relied upon. Instead, it is recommended that the ground deformations be
evaluated in the absence of piles, and the piles are assumed to move with the surrounding soil.
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