Pavement Structural Layers

To ensure all-year-round operation of vehicles traffic on a road, the carriageway is covered with a pavement. The pavement is laid on the surface of the roadbed. It can have rigid or semi-rigid structure. The pavement resists traffic stresses and climatic factors.

The stresses induced in the pavement by motor vehicle wheels attenuate with the depth. This enables the pavement to be designed in the form of a multilayer structure. The pavement consists of the following layers: the surfacing, the pavement base, the sub-base and the subgrade.

2. Below the surfacing base coat is the pavement base, a strong bearing layer of stony material or stone with a binding matrix. This layer is designed to distribute the individual wheel-loads. The pavement base is not subjected to the direct action of automobile wheels.

3. The sub-base is a layer of earth or stone materials, resistant to moisture, inserted when necessary between the pavement base and the roadbed. The sub-base is made of gravel, slag, soil treated with binding agents, sand, etc. Subbase is often the main load-bearing layer of the pavement. The primary functions of the sub-base are to provide structural support and improve drainage. The quality of subbase is very important for the useful life of the road.

4. The subgrade is the native material underneath a pavement. It comprises the thoroughly compacted upper layers of the roadbed upon which the layers of the pavement are laid. It is also called formation level.

Side Ditches

For collecting water from the roadbed side ditches, flumes, interceptor ditches and drain channeling can be used. Present road construction provides side ditches parallel with the roadway. A side ditch is intended to collect the water discharged by the crown. It also collects the water from the roadside. Water from the adjoining land must be collected by the side ditch too. The side ditch discharges into a natural outlet at the first opportunity.

Side ditches in cuttings and next to embankments may be excavated to a depth of up to 0.6 m. These ditches are for the collection of water flowing off the road surface and from adjoining land during rainfall or snow thawing. The side ditch may contribute to the drainage of the subgrade because of the evaporation of moisture from the side ditch inner slopes. However, the major use of the side ditch is to permit the rapid discharge of water. When this water discharge is not ensured and ponding occurs, the side ditch becomes a source from which water may penetrate back under the road, resulting in saturation of the subgrade.