The
Swedish soil scientist Albert Atterberg originally defined seven “limits of
consistency” to classify fine-grained soils, but in current engineering
practice only two of the limits, the liquid and plastic limits, are commonly
used. (A third limit, called the
shrinkage limit, is used occasionally.)
The Atterberg limits are based on the moisture content of the soil. The plastic limit is the moisture content
that defines where the soil changes from a semi-solid to a plastic (flexible)
state. The liquid limit is the moisture
content that defines where the soil changes from a plastic to a viscous fluid state. The shrinkage limit is the moisture content
that defines where the soil volume will not reduce further if the moisture
content is reduced. A wide variety of
soil engineering properties have been correlated to the liquid and plastic
limits, and these Atterberg limits are also used to classify a fine-grained
soil according to the Unified Soil Classification system or AASHTO system.
