Fine-Grained Soils:
Water content is a major factor, especially when
the soil contains so-called active clays.
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Each soil type will generally have different
water contents at which it behaves like a
solid, semi-solid, plastic, and liquid.
For a given soil, the water contents that
mark the boundaries between these soil
consistencies are the so-called
Atterberg Limits.
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Consistency
The physical state of a fine-grained soil at a
particular water content is known as its
consistency.
Depending on its water content a soil may exist
in:
• The liquid,
• Plastic
• Semi-solid or
• Solid
state.
The water contents at which the transitions
between states occur vary from soil to soil.
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Atterberg (Consistency) Limits:
• liquid limit, LL
• plastic limit, PL and
• shrinkage limit, SL
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• The dividing line between the liquid and
plastic states is the liquid limit (LL).
• The dividing line between the plastic and
semisolid states is the plastic limit (PL).
• And the dividing line between the
semisolid and solid state is the shrinkage
limit (SL).
The plasticity index (PI) defines the complete range
of plastic state.
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Liquid Limit (LL)
• The liquid limit defines the boundary between
plastic and viscous fluid states.
• It is determined using a standard "Liquid Limit
Device," which drops a shallow cup full of soil 1
cm consistently.
• When a groove cut through the sample closes
1.2 cm, the number of drops is recorded and a
moisture content sample processed.
• Repeating the procedure for a total of four
drop-count ranges provides enough data to plot
on a semi-log scale. From the plot, the
moisture content at 25 drops defines the Liquid
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Limit.
Liquid Limit Device (Cassagrande’s Device)
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Determination of Liquid Limit:
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Plastic Limit (PL)
• The plastic limit is that moisture content of
a soil at which it becomes too dry to be
plastic.
• The plastic limit defines the boundary
between non-plastic and plastic states.
• It is determined simply by rolling a thread
of soil and adjusting the moisture content
until it breaks at 3 mm diameter.
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Determination of Plastic Limit:
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Plastic Limit:
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Determination of Plasticity Index
• Plasticity Index, PI = LL - PL
• The plasticity index (PI) is the range of the
plastic region. These limits are expressed
as a percent of moisture content.
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• 5- Liquidity Index:
Significant values of LI indicating the consistency of
the soil are:
LI < 0
semi-plastic solid or solid
0 < LI < 1 plastic
LI >1
liquid
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