Geotechnical Engineering

DHARM

INDEX PROPERTIES AND CLASSIFICATION TESTS 61

At the liquid limit the soil grains are separated by water just enough to deprive the soil

mass of shear strength. At the plastic limit the soil moisture does not separate the soil grains,

and has enough surface tension to effect contact between the soil grains, causing the soil mass

to behave as a semi-solid.

For proper evaluation of the plasticity properties of a soil, it has been found desirable to

use both the liquid limit and the plasticity index values. As will be seen in the next chapter,

engineering soil classification systems use these values as a basis for classifying the fine-

grained soils.

Shrinkage Index

‘Shrinkage index’ (SI or Is) is defined as the difference between the plastic and shrinkage

limits of a soil; in other words, it is the range of water content within which a soil is in a semi-

solid state of consistency.

SI(or Is) = (PL – SL) = (wp – ws) ...(Eq. 3.38)

Consistency Index

‘Consistency index’ or ‘Relative consistency’ (CI or Ic) is defined as the ratio of the difference
between liquid limit and the natural water content to the plasticity index of a soil:

CI(or Ic) =

()()LL w

PI

ww

I

L

p

− = −

...(Eq. 3.39)

where w = natural water content of the soil (water content of a soil in the undisturbed condi-
tion in the ground).

If Ic = 0, w = LL

Ic = 1, w = PL

Ic > 1, the soil is in semi-solid state and is stiff.

Ic < 0, the natural water content is greater than LL, and the soil behaves like a liquid.

Liquidity Index

‘Liquidity index (LI or IL)’ or ‘Water-plasticity ratio’ is the ratio of the difference between the

natural water content and the plastic limit to the plasticity index:

LI(or IL) =

()

()

wPL

PI I

ww

p I

p

p

− = −

or

...(Eq. 3.40)

If IL = 0, w = PL

IL = 1, w = LL

IL > 1, the soil is in liquid state.

IL < 0, the soil is in semi-solid state and is stiff.

Obviously,

CI + LI = 1 ...(Eq. 3.41)

For the purpose of convenience, the consistency of a soil in the field may be stated or

classified as follows on the basis of CI or LI values: