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.
PI and their
meanings
·
(0-3)- Non plastic
·
(3-15) - Slightly plastic
·
(15-30) - Medium
plastic
·
>30 - Highly
plastic.
Plasticity Index
To Calculate the plasticity index as follows: PI = LL ‑ PL where:
LL = liquid limit, and PL = plastic limit.
Liquid limit
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. The precise
definition of the liquid limit is based on standard test procedures described
below.
The original liquid limit test of Atterberg's involved
mixing a pat of clay in a round-bottomed porcelain bowl of 10–12 cm diameter. A
groove was cut through the pat of clay with a spatula, and the bowl was then
struck many times against the palm of one hand. Casagrande subsequently
standardized the apparatus and the procedures to make the measurement more
repeatable. Soil is placed into the metal cup portion of the device and a
groove is made down its center with a standardized tool of 13.5 millimetres
(0.53 in) width. The cup is repeatedly dropped 10 mm onto a hard rubber
base at a rate of 120 blows per minute, during which the groove closes up
gradually as a result of the impact. The number of blows for the groove to
close is recorded. The moisture content at which it takes 25 drops of the cup
to cause the groove to close over a distance of 13.5 millimetres (0.53 in)
is defined as the liquid limit. The test is normally run at several moisture
contents, and the moisture content which requires 25 blows to close the groove
is interpolated from the test results. The Liquid Limit test is defined by ASTM
standard test method D 4318.[3] The test method also allows running the test at one
moisture content where 20 to 30 blows are required to close the groove; then a
correction factor is applied to obtain the liquid limit from the moisture
content.[4]
Another method for measuring the liquid limit is the fall cone test, also called the
cone penetrometer test. It is based on the measurement of penetration into the
soil of a standardized cone of specific mass. Although the Casagrande test is
widely used across North America, the fall cone test is much more prevalent in Europe due to being less
dependent on the operator in determining the Liquid Limit.
Plastic limit
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 a moisture
content where its behavior is plastic, this thread will retain its shape down
to a very narrow diameter. The sample can then be remoulded 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.
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