Consistency or Liquidity Index of Soil (LI)

It is defined as the natural moisture content of the soil in excess of the FL expressed as a percentage of the PL,
i.e.,

{ (Natural moisture content—PL )/(Liquid limit—Plastic limit) }x 100 Percent

and is a measure of the consistency of the soil. It merely describes the moisture condition of a soil with respect to its index limits. It shows in what part of its plastic range a given sample of soil lies. A cohesive soil with a natural water content of the same order as its Liquid Limit will, in general, be a very soft material, while with a natural water content of the same order as its plastic limit will, in general, be a stiff material. Soft clays have a LI approximating to 100 per cent, while stiff clays have a LI which approximates to zero and may be even negative.