Criteria, Level 2
The compositional and state criteria are important
for a complete understanding of the liquefaction phenomena and are therefore
explained in more detail below.
As described in Level 1, a uniformly graded
soil is more susceptible to soil liquefaction than a well-graded soil because the
reduced tendency for volumetric strain of a well-graded soil decreases the amount
of excess pore pressure that can develop under undrained conditions.
Historically, sands were considered to be the
only type of soil susceptible to liquefaction, but liquefaction has also been
observed in gravel and silt. Strain-softening of fine grained soils can produce
effects similar to those of liquefaction. Fine-grained soils are susceptible to
this type of behavior if they satisfy the criteria (Wang, 1979) shown in the
Fraction finer than 0.005 mm< 15%
Liquid Limit, LL < 35%
Natural water content > 0.9 LL
Liquidity Index < 0.75
Liquefaction susceptibility also depends on particle
shape. Soil deposits with rounded particles, usually found in the types of deposits
described in geological criteria, are more
susceptible to liquefaction than soils with angular particles.
There are many factors that can be incorporated
in the state of soil deposit. Here are some described that are of importance to
the liquefaction susceptibility. At constant confining pressure, the
liquefaction resistance increases with the relative density, Dr,
and, at constant relative density, the liquefaction resistance increases with
increasing confining pressure. Various investigations (Castro, 1969;
Geotechnical Engineers, Inc.,1982; and Kramer and Seed,1988;) have shown that
pre-existing shear static stress in a soil deposit is critical to a soil's
susceptibility to static liquefaction. The higher the initial shear stresses,
the greater is the liquefaction potential and the smaller disturbance is needed
to liquefy the soil.