R. R. proctor showed that there existed a definite relationship between the soil water content and dry density.
Soil sample was compacted at different water contents in a cylinder of volume 1000 cc and dry densities were obtained.
Fig. Dry Density vs Water Content Relationship
Compaction curve is plotted between the water content as abscissa and the dry density as ordinate.
It is observed that the dry density increases with an increase in water content till the maximum density is attained.
With further increase in water content, the dry density decreases.
At low water content, the soil is rather stiff and has lot of void spaces and, therefore, the dry density is low.
As the water content is increased, the soil particles get lubricated and slip over eachother, and move into densely packed positions and the dry density is increased.
However, at a water content more than the optimum, the additional water reduces the dry density.