5-13. Liquefaction Susceptibility.

5-13. Liquefaction Susceptibility. Where walls are underlain by sands below
the water table in seismically active areas, an analysis should be made of the
safety against foundation liquefaction. Flood walls in alluvial valleys are
particularly likely to be situated over loose, saturated sands that may be
liquefiable. A preliminary assessment of liquefaction susceptibility can be
made using Seed's simplified method (Seed 1976, Seed and Idriss 1982) which is
based on the standard penetration test. If the foundation is found to be
non-liquefiable, no further analysis need be made. If liquefaction may occur,
an assessment should be made of the risks and consequences of liquefaction
failure and the benefits and costs of alleviating the risks. The occurrence
of an earthquake during a flood is a case of the joint occurrence of independent
rare events. For flood walls, the probability (risk) of an earthquake
during a flood will be much smaller than the probability during a non-flood
period, but the associated consequences may be much higher. For certain walls,
(e.g., a low retaining wall remote from other structures) the probability of
liquefaction failure and the related consequences may translate into such a
small risk that accepting the risk may be the preferred alternative. Possible
alternatives to dealing with potentially liquefiable foundations include:
a. Changing the proposed location (usually the best alternative, where
feasible) .
EM 1110-2-2502
29 Sep 89
b. Removing and replacing the liquefiable materials.
c. Improving the liquefiable materials in place, by densification or
grouting .
d. Accepting the risks and consequences of liquefaction