Consistency is a term used to describe the degree of firmness of soil in a qualitative manner by using descriptions such as soft, medium, stiff or hard. It indicates the relative ease with which a soil can be deformed.
- This term is associated only with fine grained soils, especially clays.
- Atterberg Limits
The physical properties of clays are considerably influenced by the amount of water present in them.
Depending upon the water content, the following four stages or states of consistency are used to describe the consistency of clayey soil –
- Liquid State.
- Plastic State.
- Semisolid State.
- Solid State.
The boundary water contents at which the soil undergoes a change from one state to another are called consistency limits.
In 1911, Mr. Atterberg (A Swedish soil scientist) first demonstrated the significance of these limits, hence these limits are termed as Atterberg limits. These limits are liquid limit, plastic limit and shrinkage limit.
They are of great significance in understanding the behaviour of clays.
- Liquid Limit (wL)
The boundary water content between liquid state and plastic states of consistency of soil is called liquid limit (As shown in above figure.)
It can also be defined as minimum water content at which soil flows by gravity with a little or no shearing resistance.
- As per laboratory concern, w.r.t. standard liquid limit device, it is defined as the minimum water content at which a part of soil cut by a groove of standard dimension will flow together for a distance of 12mm (0.5’’) under an impact of 25 blows in the device.
- Plastic Limit (wp)
The boundary water content between plastic state and semisolid state of consistency of soil is called plastic limit. (As shown in the fig.)
It can also be defined as minimum water content at which a soil will just begin to crumble when rolled into a thread of about 3mm in diameter.
- In the plastic state, the soil can be moulded to different shapes without rupturing it.
- Shrinkage Limit (ws)
The boundary water content between semisolid state and solid state of consistency of soil is called shrinkage limit (As shown in fig.)
It can be defined as the maximum water content at which a reduction in water content will not cause a decrease in the volume of soil mass. It is also the lowest water content at which a soil can still be completely saturated.
- In the semisolid state, the soil does not have plasticity and it will be brittle.
- Shrinkage Ratio (SR)
It is defined as the ratio of a given volume expressed as a percentage of dry volume, to the corresponding change in water content above shrinkage limit.
i.e. SR =
Where, V1 = vol. of soil mass at water content w1 %
V2 = vol. of soil mass at water content w2 %
Vd = vol. of dry soil mass.