Depending upon the water content soil may appear in one of these four states.

Liquid State

Plastic State

Semi Solid State and

Solid State

Water content at which the soil changes from one of these states to another are called consistency limits. These limits are:

Liquid Limit

Plastic Limit

Shrinkage Limit

In liquid state water content of the soil is such that it behaves like liquid and offers no resistance to flow.

When water content of this soil drops it gains shear strength and develops resistance to flow. After reaching certain water content named as liquid limit soil enters into a state called plastic state of soil.

In plastic state soil can be moulded in any shape due to a property called plasticity.

When water content of the soil is further reduced, after reaching another certain water content named as Plastic limit, soil enters into another state called semi solid state.

Here we define a quantity consistency index which is also called Relative Consistency. It indicates the consistency of soil that is firmness of a soil. We represent consistency index by I_C and it is defined as the ratio of the difference between the liquid limit and the natural water content of the soil to its plasticity index. We write it mathematically as this.

We know plasticity index of a soil is difference between its liquid limit and plastic limit. So expression becomes this.

We can determine the natural water content WN of the soil by any of the water content determination method.

From this expression we can see when soil’s natural water content is equal to its liquid limit then consistency index is zero. That says soil is not at all firm.

Now if soil’s natural water content decreases to its plastic limit then numerator and denominator will be equal and consistency index value increases from zero to 1.

At even lower natural water content values of soil consistency index value increases. As only numerator will increase but denominator will remain constant.

From these observations we can imagine that consistency index of soil indicates the firmness or hardness of soil and it begins from zero at liquid limit where soil’s shearing strength is considered almost zero or infinitely small. Soil is almost liquid like and ready to flow.

Then with decrease in the water content soils hardness increases.

It becomes 1 at plastic limit and soil is slightly harder here.

Now water content is further reduced, the soil becomes even harder and value of consistency index becomes greater than 1 which indicates soil is in semi solid or solid state.

When water content of the soil is higher than its liquid limit, soil has no shear strength and no firmness. Soil behaves like liquid and consistency index of the soil will be negative.

When we go from higher water content to lower water content Consistency Index increases so does the firmness of soil.