Classification and Physical Properties of Soils - Elements of Soil Mechanics - page 10

Fig. 1.3 Shear stress/deformation relationships at different water contents.

As further moisture is driven from the soil it becomes possible for the soil to resist large shearing
stresses. Eventually the soil exhibits no permanent deformation and simply fractures with no plastic defor-
mation, i.e. it acts as a brittle solid. The limit at which plastic failure changes to brittle failure is known as
the plastic limit (w P or PL); see Fig. 1.3a .

Plasticity index (I P )
The plasticity index is the range of water content within which a soil is plastic; the finer the soil the greater
its plasticity index.

Plasticity index = Liquid limit Plastic limit −

I = w − w

P L P

or

PI LL PL

= −

The shearing strength to deformation relationship within the plasticity range is illustrated in Fig. 1.3b .

Note: The use of the symbols w L , w P and I P follows the recommendations by the ISSMFE Lexicon ( 1985) .
However, the symbols LL, PL and PI are still used in many publications.

Liquidity index

The liquidity index enables one to compare a soil's plasticity with its natural water content (w).

w w
I

−

P

I = L

P

If I L = 1.0 the soil is at its liquid limit; if If L = 0 the soil is at its plastic limit.

Shrinkage limit

If the drying process is prolonged after the plastic limit has been reached the soil will continue to decrease
in volume until a certain value of water content is reached. This value is known as the shrinkage limit and