Water becomes highly viscous near the particles, but as the distance increases, the viscosity of water decreases until at some distance ordinary water exists. Water molecules are bi-polar and orient themselves like tiny magnets in the magnetic field next to the surface of the soil particles. Plasticity is defined as that property of a soil which allows it to be deformed rapidly, without rupture, without elastic rebound, and without volume change.Īccording to Goldschmidt theory, the plasticity is due to the presence of thin scale like particles which carry on their surfaces electro-magnetic charges. When the plastic limit is equal to or greater than the liquid limit, the plasticity index is reported as zero. When plastic limit cannot be determined, the plasticity index is reported as NP (non-plastic). In the case of sandy soils, the plastic limit should be determined first.

The plasticity index is defined as, the numerical difference between the liquid limit and the plastic limit of a soil: P.I = LL – PL. The range of consistency within which a soil exhibits plastic properties is called plastic range and is indicated by plasticity index. It is lowest water content at which soil can still be completely saturated. Shrinkage limit is defined as, the maximum water content at which a reduction in water content will not cause a decrease in the volume of a soil mass. Plastic Limit is defined as the minimum water content at percent on dry bases at which a soil enters from Semi-Solid State to Plastic State. With reference to the standard liquid limit device, it is defined as the minimum water content at which a part of soil cut by a groove of standard dimensions, will flow together for a distance of 12 mm under an impact of 25 blows in the device. Liquid Limit is defined as the minimum water content at percent on dry bases at which the soil changes its state from Liquid to Plastic State. These limits are expressed as percent water content. The Atterberg limits which are most useful for engineering purposes are the liquid limit, plastic limit, and shrinkage limit.

The Figure below shows the four states of consistency, with the appropriate consistency limits.

Thus, the consistency limits are the water contents at which the soil mass passes from one state to the next. He set arbitrary limits, known as consistency limits or Atterberg limits, for these divisions in terms of water content.

Plastic limit test