The consistency limit or Atterberg’s limit is defined as, “The water content at which the soil changes from one state to the other are known as the consistency limit or Atterberg’s limit”.
In 1911, a Swedish engineer named Atterberg mentioned that fine-grained soil could exist in four states: Liquid, Plastic, Semi-Solid, and Solid State.
The Liquid State– In this state fine-grained soil can flow by its own weight and in this stage, the soil is like a soup.
Plastic State- At this state soil can be remoulded or reshaped without any cracks and in this stage, the soil is like a soft butter.
Semi-Solid State- In this state soil can be changed to any shape but cracks come during remoulding and in this stage, the soil is like a cheese.
Solid State- At the solid state soil can not be remoulded and in this stage, the soil is like a hard candy.
The consistency limit is a significant index property of fine-grained soil.
The Atterberg limit or consistency limit basically refers to the liquid limit, plastic limit, and shrinkage limit of soil, which is very important for soil identification, classification, and strong correlation.
Liquid limit is defined as, “the water content at which the soil changes from liquid state to plastic state is known as the Liquid Limit”.
Soil which contains high water in it, then it is basically in a liquid state. When the water content is high then it flows like liquid and does not offer any resistance to shear deformation but when the water content reduces, the soil starts developing resistance to shear deformation and at some particular water content, the soil becomes plastic (see in the figure).
There are three methods to determine the liquid limit of soil.
- One Point Method.
- Cone Penetrometer Method.
- Casagrande’s Method.
The plastic limit is defined as,” the minimum moisture or water content at which the soil changes its state to plastic state to semi-solid state”.
When the soil is present in the plastic state, it can be moulded into different shapes without any cracking in the shape, but when the water or moisture content decreases then the plasticity also decreases and ultimately soil passes through the plastic state to a semi-solid state.
The shrinkage limit is defined as ” The moisture content at which the soil changes its state from the semi-solid to solid state is called shrinkage limit”.
When due to the reduction of water there is no change in the volume of the soil, then the soil reaches a solid state.
Here one point to be noted is that below the shrinkage limit the soil does not remain saturated.
In other words, we can define the shrinkage limit as, “the lowest water content at which the soil is fully saturated”.
The shrinkage limit of the lowest water content at which the soil stop shrinking further and achieves a constant volume.
Do you know the difference between the liquid limit and plastic limit is known as the plasticity index? The plasticity index is an essential index property of fine-grained soil.
Plasticity Index= (Liquid Limit) – (Plastic Limit)