The Plastic Limit is defined as the moisture content at which a thread of soil passing the 425 micron sieve can be rolled without breaking until it is only 3 mm in diameter.  It is dependent upon both the type and amount of clay present.  At the plastic limit sufficient water is required to wet all the surfaces and reduce cohesion so that the particles can move past one another under stress, but maintain a new moulded position.
 

The Liquid Limit is defined as the moisture content at which the soil passing the 425 micron sieve is sufficiently fluid to flow a specified amount when jarred 25 times in the standard apparatus e.g.  the Casagrande apparatus.  It is dependent upon both the type and the amount of clay present but it is more sensitive to the type of clay than it is the plastic limit.  The soil is water saturated and the distance between particles is such that the forces of interaction between the clay particles is sufficiently weak to allow easy movement of the particles relative to one another. 
 

The Plasticity Index is simply the numerical difference between the liquid limit and the plastic limit for a particular material and indicates the magnitude of the range of moisture content over which the soil remains plastic.  It is a measure of the cohesive qualities of the binder resulting from the clay content.  Also, it gives some indication of the amount of swelling and shrinkage that will result in the wetting and drying of that fraction tested.  If some soils do not have sufficient mechanical interlock they require amounts of cohesive materials to give a satisfactory performance.  A deficiency of clay binder may cause ravelling of gravel wearing courses during dry weather and excessive permeability.  While an excess of clay has previously been regarded as a dilution of strength, the use of Endurazyme allows the use of these high plastic materials without any dilution of pavement performance.
 

Linear Shrinkage  This test is used to measure the percentage decrease in dimension of a fine fraction of a soil when it is dried after having been moulded in a wet condition approximately at its liquid limit.  The plasticity Index would give some indication of the volume change which is likely to occur in the soil when the moisture content changes.  It is a useful test for soils with low clay content in which the liquid and plastic limits and hence the plasticity index are difficult to measure.  An approximate estimate of the plasticity index can be made by measuring the linear shrinkage.  The plasticity Index is then approximately two and a half times the linear shrinkage. 

MAXIMUM DRY DENSITY TEST
Maximum Dry Density tests set out to determine varying densities achieved in a soil as a result of the use of varying amounts of moisture.  It is the objective of this 5 point test to straddle the optimum moisture for that soil to give a curve from which the maximum dry density and the optimum moisture content can be read.  A sample of one such graph is given below.