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4.3.2 Localized Liquefaction Hazard and Lateral Spreading
The three main types of localized liquefaction hazard include:
1. loss of bearing capacity beneath shallow foundations and around deep foundation piles;
2. excessive ground settlement; and
3. localized lateral displacements and lateral spreading.
The loss of soil strength can result in potential foundation bearing failure and large foundation
settlements. The assessment of these potential hazards requires evaluating the liquefaction
potential and the factor of safety with respect to bearing failure (Sasaki et al. 1996; Naesgaard et
al. 1998). Estimating the post-cyclic strength of the soil should be determined in accordance with
the methods described in Section 4.2. To evaluate the factor of safety with respect to bearing
failure, conventional bearing capacity analyses should be performed using modified strengths of
the foundation soils reflecting the adverse impact of cyclic pore pressure generation. Engineering
judgment should be used when determining the factor of safety with respect to bearing capacity
failure. It has been shown that settlements of individual footings can be highly differential in
nature and can be very damaging to structures.
Hazards also are associated with lenses of liquefiable soil or by potentially liquefiable layers that
underlie resistant, nonliquefiable capping layers. In situations where a few thin lenses of
liquefiable soil are identified, the interlayering of liquefiable and resistant soils may serve to
minimize structural damage to light, ductile structures. It may be determined that life safety
and/or serviceability requirements may be met despite the existence of potentially liquefiable
layers. Ishihara (1985) developed an empirical relation that provides approximate boundaries for
surface damage for soil profiles consisting of a liquefiable layer overlain by a resistant surface
layer (Figure 4.5). This relationship has been validated by Youd and Garris (1995) for
earthquakes with magnitudes between 5.3 and 8.0. In light of the heterogeneous nature of most
soil deposits and the uncertainties inherent in the estimation of ground motion parameters, it is
recommended that this method of evaluation only be considered for noncritical structures.
As the excess pore pressures generated by cyclic loading dissipate by drainage, the soil is
consolidating which results in ground surface settlements. Similarly, in non-saturated
cohesionless soils, cyclic loading can result in densification of loose to medium dense soils, even
though no significant cyclic pore pressure generation may occur. The procedures for assessment
of these settlements are discussed in Section 4.6.
Lateral ground displacements represent one of the most destructive hazards associated with
liquefaction. Liquefaction generally leads to three types of ground failure that produce lateral
ground displacement: flow failure, ground oscillation and lateral spread. Flow failures form on
steep slopes (greater than 6%). Ground oscillation generally occurs on flat ground with
liquefaction at depth decoupling surface soil layers from the underlying unliquefied ground. This
decoupling allows rather large transient ground oscillations or ground waves to develop. The
permanent displacements associated with this movement are usually small and chaotic with
respect to magnitude and direction. Observers of ground oscillation have described largeamplitude
ground waves often accompanied by opening and closing of ground fissures and
ground settlement, which can inflict serious damage to overlying structures and buried facilities.
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