Page 257 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Consistency and Engineering Classification
252 Geotechnical Engineering

death. Quick clays that appear stable can turn into a soup that can be poured
like pancake batter.

12.4.3 Liquefaction

Another example where a liquid limit may be exceeded is when a saturated sand
or silt suddenly densifies during an earthquake so that all of its weight goes to
pore water pressure. This is liquefaction, which can cause a sudden and complete
loss of shear strength so that landslides develop and buildings may topple. The
consequences, diagnosis, and prevention of liquefaction are discussed in more
detail in a later chapter.

12.5 THE PLASTICITY INDEX

12.5.1 Concept
The plasticity index, or PI, is the numerical difference between the liquid and
plastic limit moisture contents. Whereas the two limits that are used to define a PI
are directly applicable to certain field conditions, the plasticity index is mainly
used to characterize a soil, where it is a measure of cohesive properties. The
plasticity index indicates the degree of surface chemical activity and hence the
bonding properties of clay minerals in a soil. The plasticity index is used along
with the liquid limit and particle size gradation to classify soils according to their
engineering behavior.

An example of a direct application of the plasticity index is as an indicator of the
suitability of the clay binder in a soil mixture used for pavement subgrades, base
courses, or unpaved road surfaces. If the PI of the clay fraction of a sand-clay or
clay-gravel mixture is too high, the exposed soil tends to soften and become
slippery in wet weather, and the road may rut under traffic. On the other hand,
if the plasticity index is too low, the unpaved road will tend to ‘‘washboard’’ in
response to resonate bouncing of wheels of vehicular traffic. Such a road will
abrade under traffic and antagonize the public by producing air-borne dust in
amounts that have been measured as high as one ton per vehicle mile per day per
year. That is, a rural unpaved road carrying an average of 40 vehicles per day can
generate up to 40 tons of dust per mile per year. Most collects in roadside ditches
that periodically must be cleaned out.

12.5.2 A PI of Zero
Measurements of the LL and PL may indicate that a soil has a plasticity
index equal to zero; that is, the numerical values of the plastic limit and the
liquid limit may be the same within the limits of accuracy of measurement.
Soil with a plasticity index of zero therefore still exhibits a slight plasticity, but

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