Components of Roads and their Functions

The highway or the road structure must be designed carefully for the traffic load to be carried, physical and geological features, climatic conditions of the locality, and various other factors that would affect the stability and utility of the highway roads.

For design, the road structure consists of the following four component parts:

1. Subsoil
2. Subgrade
3. Base
4. Surfacing.

Components parts of road structure

1. Subsoil: The subsoil is the natural or prepared soil on which the road must be formed. It should be stable and robust enough to safely support roadway construction's traffic load and weight.

2. Subgrade: The subgrade, or the formation, supports the road surface and its foundation. The road's life duration primarily depends on a stable and dry subgrade, which may be the same as or above the natural ground level. The subgrade's support for the road structure is crucial, so considerable attention should be paid to properly preparing it before it is laid out.

3. Base: The base or foundation may consist of two layers, the bottom layer being known as the sub-base, soling, or bottoming. The sub-base should be stable and capable of resisting distortion under traffic loads to a great extent. The function of a road base is to transmit the load from the surfacing to the subgrade. The base should possess structural stability and sufficient thickness to develop a good bond with the surfacing.

4. Surfacing: The topmost layer of traffic travels is road surfacing, wearing layer, or wearing course. The primary function of road surfacing is to provide a smooth and stable running surfacing that is suitable for the type of vehicle and intensity of traffic anticipated on the road. The surfacing should be impervious and should protect the base and the subgrade from the action of weather and rainwater. The desirable qualities of road surfacing are durability, stability, non-slipperiness, economics, and dauntlessness. It should also be able to resist traffic displacement and, therefore, be well-keyed onto the base.

The governing factors that would affect the design of the above road structure components are the initial cost, availability of local raw materials, volume and class of traffic, and the climatic conditions of the locality.