Atterberg limits:
Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage
limit, plastic limit, and liquid limit.
Depending on its water content, a soil may appear in one of four states: solid, semi-solid, plastic and
liquid. In each state, the consistency and behavior of a soil is different and consequently so are its
engineering properties. Thus, the boundary between each state can be defined based on a change in the
soil's behavior. The Atterberg limits can be used to distinguish between silt and clay, and to distinguish
between different types of silts and clays.
Plastic limit:
Plastic Limit is defined as the minimum water content at percent on dry bases at which a soil enters
from Semi-Solid State to Plastic State.
The Plastic Limit (PL) is determined by rolling out a thread of the fine portion of a soil on a flat, non-
porous surface. The procedure is defined in ASTM Standard D 4318. If the soil is at moisture content
where its behavior is plastic, this thread will retain its shape down to a very narrow diameter. The sample
can then be remolded and the test repeated. As the moisture content falls due to evaporation, the thread
will begin to break apart at larger diameters.
The plastic limit is defined as the moisture content where the thread breaks apart at a diameter of 3.2 mm
(about 1/8 inch). A soil is considered non-plastic if a thread cannot be rolled out down to 3.2 mm at any
moisture possible.
Liquid limit:
Liquid Limit is defined as the minimum water content at percent on dry bases at which the soil
changes its state from Liquid to Plastic State.
The liquid limit (LL) is conceptually defined as the water content at which the behavior of a clayey soil
changes from plastic to liquid. However, the transition from plastic to liquid behavior is gradual over a
range of water contents, and the shear strength of the soil is not actually zero at the liquid limit.
Plasticity index:
The plasticity index (PI) is a measure of the plasticity of a soil. The plasticity index is the size of the range
of water contents where the soil exhibits plastic properties. The PI is the difference between the liquid
limit and the plastic limit (PI = LL-PL). Soils with a high PI tend to be clay, those with a lower PI tend to
be silt, and those with a PI of 0 (non-plastic) tend to have little or no silt or clay.
Soil descriptions based on PI: