Page 15 - BULETIN GEOTEKNIK 2023
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The Three Limits SL Shrinkage
Limit
The water content of the soil when the soil
changes its state from semi – solid to a solid
state. At this particular point, the volume of the
Liquid LL soil mass stops changing with further drying of
Limit the material. This parameter is used less than
the other two limits.
Also widely known as the upper plastic
limit, is the amount of water that changes
the soil from liquid state to a plastic state. It PL Platic
is the minimum water content that allows Limit
the soil to flow upon the application of a Also known as the lower plastic
very small amount of shear force. limit, is the amount of water that
changes the soil from plastic state
to a semi – solid state. It can also
With the combination of both PL and LL, the be said that PL is the moisture
Plasticity Index (PI) is calculated by using the content of the soil that cannot be
formula; PI = LL – PL. This is the important remolded without cracking.
value to classify the soil for design.
How to Obtain the Limit? Why is it important?
These limits can easily be obtained by testing The plasticity index of the soil represents the clay
either disturbed or undisturbed samples in content in soil. Higher plasticity index indicates that
the soil testing laboratory. LL can be obtained the soil is clay. For some soil, plasticity index
by two commonly used methods; increases proportionally with the percentage of clay
Casagrande Method and Cone Penetrometer particles as proposed by Skempton, 1953. If the
Method while PL can be determined by particles size decrease, the plasticity index is
rolling repeatedly an ellipsoidal – sized soil expected to increase rapidly. Thus, it can be said that
mass by hand on a non – porous surface. plasticity index is a measure of fineness of particles
Cone Penetrometer Apparatus Casagrande Apparatus
Relationship between plasticity index and clay percentage
13
(Skempton, 1953)
