Soil in Road Construction:
Road Embankment and Sub-grade
Soil is mainly used as the material for constructing a road embankment. The road embankment supports the road pavement which receives the heavy loads from the traffic. The soil forming the embankment should have sufficient bearing power to withstand the loads transmitted to it. The embankment again rests on natural soil, and the natural soil should have sufficient bearing power to withstand the weight of the embankment and the pavement and the traffic loads.
The 300-500 mm layer in the soil embankment immediately below the pavement is known by the name sub-grade (Figure). Since this layer bears the traffic loads transmitted through the pavement, it is generally made with a selected soil and compacted to a density higher than the rest of the embankment. For all important highways, the sub-grade is 500 mm thick. For Rural Roads, a thickness of 300 mm is adequate.
Figure: Road Embankment showing Sub-grade