Design of Pavement
In determining the priority for highway construction, the National Roads Board uses a sufficiency rating which takes into account the various road features, such as width, alignment, grade, pavement strength, surface condition, and shoulder widths. These are compared with desirable roads standards and then grouped to show the sufficiency of the section in relation to safety, structural adequacy, and traffic serviceability. It is said that “in Roman times each legion had an engineer and the roads were well laid in layers with ditches on either side for drainage”. The construction of a modern highway remains much the same; it is still compacted in layers and drained on either side. But the engineer today has a more scientific approach. It is his responsibility to test the soil profile of the various routes and to decide how much use can be made of local materials. In some areas sealing chips, for instance, have to be brought long distances; costs can therefore be reduced if local materials are available. Well-equipped laboratories are available throughout the country to carry out such sampling and testing. The engineer also uses such aids as the California bearing ratio, a device in which performance of the pavements has been related to physical soil constants, or the Benkelman beam, which measures the curvature of the deflection of the pavement. From these tests the engineer is able to calculate the various thicknesses of the selected material and treatment to carry the loading in all weathers. Some materials, like clay, are particularly sensitive to moisture, and their ability to carry loads is greatly reduced when they become wet.