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4.0 POST-LIQUEFACTION SOIL BEHAVIOR
4.1 INTRODUCTION
Common methods for evaluating liquefaction potential were outlined in Chapter 3. Although
these methods can be used to indicate the triggering of liquefaction, they do not describe the
behavior of the soil after this state has been reached. As the cyclically-induced excess pore
pressures build up in the soil, it experiences a dramatic reduction in stiffness and strength. This
occurs as the excess pore pressure increases; therefore, it is not necessary for the soil to reach a
state of liquefaction (r u = 100%) for it to be potentially hazardous. Ground failures associated
with liquefied soil include:
1. flow failures of sloping ground or free-face conditions;
2. limited, yet excessive, deformations of sloping ground and free-face conditions (termed
lateral spreading);
3. bearing capacity failures of shallow foundations;
4. increased lateral earth pressure on walls leading to large displacements;
5. loss of passive soil resistance against walls, anchors, laterally loaded piles; and
6. excessive ground settlement.
Liquefaction-related ground failures have been a primary source of damage to highway
structures during recent earthquakes. Excessive deformations of pavements, approach fills, pile
foundations, and bridge substructures result in the loss of bridge operation. The engineering
parameters utilized in conventional limit equilibrium analyses must be assessed. The substantial
change in engineering properties of soils throughout the cyclic loading and liquefaction process
is complex; however, simplified procedures have been developed for estimating the post-cyclic
loading, or post-liquefaction behavior of cohesionless soils. Recently developed methods for
evaluating the shear strength, post-loading volume change, and magnitude of lateral spreading
are discussed in this chapter.
4.2 POST-CYCLIC STRENGTH OF SANDS AND SILTS
In order to assess the seismic stability of earth structures and foundations, it is necessary to
estimate the shear strength of the soil during and after the seismic loading. The potential loss of
soil shear strength is a function of the excess pore pressures that are developed during shaking.
The following categories delineate the various stages of soil shear strength reduction due to
excess pore pressure generation. The stages are defined by the factor of safety against
liquefaction determined using one or more of the procedures outlined in Chapter 3. While
theoretically, the value of FS L indicating full liquefaction should be 1.0, in practice, it is often
recommended that a value of 1.1 be used to account for the very rapid rise in r u with FS L values
less than 1.1. The use of 1.1 for “liquefied soil” provides appropriate conservatism to the
analysis.
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