Soils used in road construction need to be thoroughly rolled and compacted. Compaction of soil is needed for the following reasons.
(a) A well compacted soil does not undergo settlement under traffic,
(b) a well compacted soil has greater strength and thus a pavement constructed on such layers requires less thickness, and
(c) a well compacted soil is less susceptible to changes in moisture content, and thus the volumetric changes like swelling and shrinkage are minimised.
The density to which a soil can be compacted depends upon :
(a) the type of the soil
(b) the moisture content
(c) the compactive effort
Soils like clay can be compacted to a lower density than soils like sand and gravel.
The amount of moisture in a soil influences the density to which it can be compacted. It has been found that for a given soil and a given compactive effort there is one moisture content, called the optimum moisture content (OMC) at which the maximum dry density (MDD) is obtained. This is shown in Figure.
Figure: Moisture-Density Relationship