Consistency of a clays : Atterberg Limits

This topic is bit on higher side for the readers who are interacting for the first time with soil mechanics. We define consistency by the firmness of soil in qualitative manner. Sounds like very complicated parameter. Don't worry this is so easy to understand.

"Consistency indicates the relative with which a soil can deformed. We use this parameter for fine grained soil."

You can understand this property by the amount of water in the soil. More water you mix in the soil it becomes more easy to work with. You can now easily conclude that consistency must be related with water. We generally define four state of consistency based on the amount of water.

1. Liquid state
2. Plastic state
3. Semi-solid state
4. Solid state

We will discuss this one by one for better understanding. When soil changes its stage that water content is called "consistency limit". In 1911 Atterberg discovered the significance of this limits. Hence in respect to his work these limits are also called "Atterberg limits".

Also read: void ratio, permeability of soil, water content

• Liquid limit - Water content of soil when soil changes from liquid state to plastic state.
• Plastic limit - Water content of the soil when soil changes from plastic state to semi solid state.
• Shrinkage limit - Water content of the soil when soil changes from semi solid state to solid state.

Do you know? All soil posses same value of shearing strength at liquid limit. It is about 27 (g/cm^2).

You can see theoretically this definition is very easy. But practically who will tell you that soil is in which state. Hence to overcome this problem researchers devised many solution. They listed different test procedure to identify the state of soil.

Consistency limit determination

• Casagrande's tool - Liquid limit determination
• Ball formation - Plastic limit determination
• Shrinkage dish experiment - Shrinkage limit determination