Pavements are among the costliest items associated with highway construction and maintenance, and are largely responsible for making the U.S. highway system the most expensive public works project undertaken by any society. Because the pavement and associated shoulder structures are the most expensive items to construct and maintain, it is important for highway engineers to have a basic understanding of pavement design principles.
In general, there are two types of pavement structures: flexible pavements and rigid pavements. There are, however, many variations of these pavement types. Composite pavements (which are made of both rigid and flexible pavement layers), continuously reinforced pavements, and post-tensioned pavements are other types.
As with any structure, the underlying soil must ultimately carry the load that is placed on it. A pavement’s function is to distribute the traffic load stresses to the soil (subgrade) at a magnitude that will not shear or distort the soil. Typical soil-bearing capacities can be less than 50 lb/in2 and in some cases as low as 2 to 3 lb/in2. When soil is saturated with water, the bearing capacity can be very low, and in these cases it is very important for pavement to distribute tire loads to the soil in such a way as to prevent failure of the pavement structure.
typical automobile weighs approximately 3500 lb, with tire pressures around 35 lb/in2. These loads are small compared with a typical tractor–semi-trailer truck, which can weigh up to 80,000 lb—the legal limit, in many states, on five axles with tire pressures of 100 lb/in2 or higher. Truck loads such as these represent the standard type of loading used in pavement design. In this chapter, attention is directed toward an accepted procedure that can be used to design pavement structures for high– traffic-volume highway facilities subjected to heavy truck traffic. The design of lower-volume facilities, which may have stabilized-soil and gravel-surfaced pavements, is discussed elsewhere