Soil is an accumulation or deposit of earth material, derived naturally from the disintegrated of rocks or decay of vegetation, which can be excavated readily with power equipment in the field or disintegrated by gentle mechanical means in the laboratory. The supporting soil beneath pavement and its special under courses is called sub grade. It is most important part of road structure. It is the soil which provides support to the road from below and therefore, it should possess sufficient strength and stability undermost adverse loading and climate conditions. It is the first layer of any type of road construction and considered as one of the principal highway material.

Undisturbed soil beneath the pavement is called natural sub grade. Compacted sub grade is the soil compacted by controlled movement of heavy compacters.

Effect of poor sub grade conditions

1.Improper compaction and inadequate strength in sub grade soil causes road failure in form of wavy surface, corrugations; rutting and shoving in flexible roads (i.e. normally bituminous road)
2.Result the phenomena of soil pumping, blowing and cracking the cement concrete roads.
3.Plastic deformation of road pavement under the application of excessive wheel road.
So it is very important to make sub grade soil incompressible- To achieve this, the selected superior quality soils are borrowed and used for construction. The soil must be compacted adequately and method stabilization technique must be adopted.