Consistency limits in soil - Liquid Limit, Plastic Limit, Shrinkage Limit

The gradual increase of water content in soil, makes the soil change its form from solid to semi solid to plastic to liquid. The water contents at which the phase change of soil occurs, are called the consistency limits.

Consistency is the relative ease with which soil can be deformed. It is used mostly for clays. These are soil aggregate properties. 

Depending upon the water content the soil mass has 4 stages.

  1. Solid

  2. Semi solid

  3. Plastic stage

  4. Liquid stage

Liquid limit (LL) or Water limit (WL)

Minimum water content at which the soil has the tendency to flow upon disturbance is the liquid limit. It can also be defined as, water content at which a groove of 2 mm closes by 25 blows in the casagrande apparatus. All the soils have similar shear strength at liquid limit.

Plastic limit (PL or WP)

Minimum water content at which the soil is just in the plastic stage is the plastic limit.  Water content at which a soil would just begin to crumble if rolled into the threads of 3 mm diameter.

At the plastic limit the shear strength is about 100 times than at the LL.

Shrinkage limit (SL)

It is the minimum water content at which the soil is fully saturated. Maximum water content below which there is no volume change is also the SL.

Plasticity Index (PI)

This is the range of water content in which the soil remains in plastic state. It depends upon the clay amount present. Clay having more specific surface area will have higher PI.

Soils with large PI and Liquid Limit are called fat clays and soils with low PI and Liquid limit are called lean clays.

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