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Gravel
Sand
Figure 3.16: Relationship between Residual Excess Pore Pressure and Factor of Safety Against Liquefaction (FS L )
for Level Ground Sites (Marcuson and Hynes 1990)
Reliable analyses can be performed considering the following guidelines (Seed and Harder
1990; CDMG 1997).
1. Soil elements with low factors of safety against liquefaction (FS L ≤ 1.1) should be treated
as fully liquefied. Undrained residual strengths (S r ) should be assigned to these zones for
further stability and deformation analyses. This evaluation is described in Chapter 4.
2. Soil elements with a high factor of safety (FS L ≥ 1.4) would experience relatively minor
cyclic pore pressure generation, and should be assigned some large fraction of their
(drained) static strength for further stability and deformation analyses.
3. Soil elements with intermediate factors of safety (FS L ≈ 1.1 to 1.4) should be assigned
strength values somewhere between the values appropriate for the conditions previously
addressed (as described in Chapter 4).
Once the factor of safety against the triggering of liquefaction at a site has been determined, the
next step is to predict the type of behavior expected in the soil mass upon loading. If the factor of
safety is low (FS L ≤ 1.1), the potential for liquefaction-induced ground movements must be
considered. This evaluation requires the determination of the post-cyclic loading strength of the
soil with consideration of the excess pore pressures generated during shaking. The procedures
available for estimating the dynamic and post-loading strength of sandy soils, as well as methods
for approximating the magnitudes of liquefaction-induced ground deformations, are presented in
Chapter 4.
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