Atterberg limits - Liquid Limit, Plastic Limit, Shrinkage Limit.

Atterberg limits 

It is a basic measure of finding the critical water contents of a fine-grained soil at its liquid limit, plastic limit and  shrinkage limit.

Further the changes on state from solid to liquid state can be defined as

Liquid limit

It is defined as minimum water content at which soil has tendency to change its state from liquid to plastic.

At liquid limit soil passes from liquid stage of consistency to plastic stage of consistency and vice-versa.

Standard liquid limit device is used to determine the moisture content in which the clay soil pass from a plastic state to a liquid state. It consists of removable adjustable crank and cam mechanism, blow counter, brass cup and base.

The standard liquid limit device is used to check the  minimum water content in which a part of soil is cut into groove of standard dimensions, and will flow together for a distance of 12 mm which will be impacted for 25 blows in the device.

All the soils at liquid limit passes same negligible shear strength of 2.7 KN/m^2.

Type of soil	Liquid limit (WL)
Gravel	Non-Plastic
Sand	Non-Plastic
Silt	30-40
Clay (Alluvial)	40-150
Clay(Black soil)	400-500
Clay (Bentonite)	400-800

Note:- Soils having higher value of liquid limit passes high compressibility (Volume change in these soils are more). 

PLASTIC LIMIT(ω_p):-

It is defined as minimum water content at which soil is in plastic stage of consistency or behaves as plastic limit.

At the stage of plastic limit, the soil pass from plastic stage consistency to semi sold stage and vice versa.

From determination point of view plastic limit is defined as minimum water content at which 3 mm diameter thread can be formed without any crack.

The Plastic limit test is performed by repeated rolling of soil mass by hand on a non-porous surface. 

Casagrande defines the plastic limit of a soil having water content from which carefully threads are made of 3 mm diameter. 

• If the thread gets crumbled below 3 mm diameter than consider the soil as too wet
• If the thread crumbles at a diameter more than 3 mm, Than it can be said that the solid is drier than plastic limit.

SHRINKAGE LIMIT:-

The maximum water content after which soil does not show change in volume even after reduction in water content.

Therefore the shrinkage limit can be calculated by

Sample 1- 

Soil volume = Vi

Soil mass = M1

Sample 2 - 

Soil Volume = Vf

Soil Mass = M2

therefore the formula

Plasticity Index.

It is a Range of consistency in which soil posses plastic properties

The plasticity index is defined as, difference of the liquid limit and the plastic limit of a soil: 

P.I (Plasticity Index) = LL(Liquid Limit) – PL(Plastic Limit).

Plastic limits should be determined first in case of sandy soil. 

NP (non-plastic)- When plastic limit can not be determined, its PI be termed as NP.

When the PL(plastic limit) is equal to or greater than the LL (Liquid limit), than the P.I (plasticity index) is said as zero.