Atterberg limits, named after the Swedish soil scientist A.
Atterberg, are water content values at which notable changes in soil behavior
occur. The liquid limit , denoted LL or wL, marks the transition between liquid
and plastic behavior.
At water contents above the liquid limit the soil behaves as
a viscous liquid; below the liquid limit the soil behaves as a plastic solid.
The liquid limit is determined in the laboratory by partly filling a standard
brass cup with wet soil and cutting a groove of a standard dimension in the
soil.
The liquid limit is taken as the water content at which the
groove closes a specified amount when the cup is lifted and dropped 1 cm
exactly 25 times. The details of the test are given in AASHTO T 89 and ASTM D
4318-93.
The plastic limit, denoted PL or wp, is the transition
between plastic and brittle behavior. It is determined in the laboratory as the
water content at which a 1/8-inch diameter thread of soil begins to crumble
when rolled under the palm of the hand.
Details of the liquid limit and plastic limit tests are
provided by AASHTO T 90 and ASTM D 4318-93. The shrinkage limit, denoted SL or
wS, is the water content below which the soil no longer reduces in volume when
the water content is reduced.
Although Atterberg limits are water contents and are
properly decimals or percentages, they are usually expressed as an integer
percentage without a percent sign. Thus, a liquid limit of 40% is usually reported
as LL = 40.
The plasticity index, denoted PI or IP, is the difference of
the liquid limit and the plastic limit:
PI = LL – PL
The liquidity index, denoted LI or IL , is a measure of the
natural water content (w) relative to the plastic limit and the liquid limit:
