It is the resistance to flow of a material, Similar to viscosity of a liquid. Consistency of a soil refers to the resistance offered by it against forces that tend to deform or rupture the soil aggregate. It may also be defined as the degree of firmness. This is applicable specifically to clay soils and is generally related to water content.

It is described as soft, medium stiff, stiff or hard. In the remolded state, it varies with water content, which tends to destroy the cohesion of the particles of such a soil. When the water content is reduced, the soil passes though various state of consistency, liquid, plastic semi-solid and solid. The water content at which the soil passes from one of this states to the next are designated as consistency limits- liquid limit, Plastic limit and shrinkage limit. These are called Atterberg limits.

The concept is adapted for classification of soils. Plasticity of a soil is defined as the property which allows it to be deformed, without rupture and without a noticeable volume change.

Consistency Limits

Liquid Limit

LL is defined as the arbitrary limit of water content at which the soil is about to pass from plastic state to liquid state, or in other words the minimum water content at which the soil tends to flow as a liquid.

Plastic Limit

PL is the limit of water content at which the soil tends to pass from the plastic state to the semi-solid state. This is the minimum water content at which the change in shape of soil is occurred with visible cracks.

Shrinkage Limit

SL is the limit of water content at which the soil tends to pass fro semi-solid state to solid state. It remains in constant volume, at that water content.

Consistency Indices 

Plasticity Index

  PI = WL-WP or LL-PL

Shrinkage Index

SI = PL-SL

Consistency Index

(LL-w) / PI

Liquidity Index

(w-PL) / PI 

Where w is natural water content