To determine pavement thicknesses from design charts and tables, it is necessary to know only the EAL's (equivalent axle loads), the CBR of the subgrade soil, and the modulus of elasticity of the bituminous concrete. Charts permit selection of pavement structures employing alternative proportions of bituminous concrete and crushed stone base. Total thickness varies according to the proportions chosen. It is implicitly intended that the selection of alternative structures be based on engineering considerations, such as 1. estimates of comparative construction costs, 2. compatibility with cross section template and shoulder designs, 3. uniformity or standardization of design practices, 4. highway system classification, 5. engineering precedence, and 6. utilization of indigenous resources.
Designs based on 33- and 67-percent proportions (thickness of pavement structure) of bituminous concrete and crushed rock base, respectively, conform with the current design chart (for high-type pavements) of the Kentucky Department of Transportation, representing conventional or precedential designs. The charts otherwise represent theoretical extensions of conventional designs and, from a theoretical standpoint, provide equally competent structures.
Heretofore, the Kentucky design system was based on EWL's (equivalent wheel loads). The proposed system is based on EAL's. This transformation was made for the sake of unifying design practices and standardizing design terms. EAL's are defined here as the cumulative number of equivalent 18-kip axleloads in the design lane. An approximate conversion is made by dividing EWL's by 32 - that is, divide by 2 to reduce two-directional EWL's to one direction and divide by 16 to convert from a 10-kip axleload (or 5-kip wheel load) to an 18-kip axleload.
Normally, traffic volumes are estimated in connection with needs studies and in the planning stages for all new routes and for major improvements of existing routes. Whereas the anticipated volume of traffic is an important consideration in the geometric design of a roadway, composition of the traffic in terms of axle weights and lane distributions is essential to the structural design of pavements. Traffic volumes used for EAL computations should therefore be reconciled with other planning forecasts of traffic. Historically, actual growths, particularly in EAL's, have exceeded forecasts in the majority of cases. Even though predictions of traffic volumes may be reasonable, estimates of EAL's are also dependent upon predictions of vehicle types and loadings over the design life. Again, previous experience shows an underestimation of EAL's due to inadequate predictions (or even the disregard of known overloads) of vehicle loadings. Thus, the design lives of the pavements may differ from the geometric design period.
Computation of EAL's involves an estimate of the total number of vehicles during the design life and multiplying factors for various vehicle types and loading configurations and magnitudes to convert traffic volumes to EAL's. Ideally, yearly increments of EAL's could be calculated and summed; this approach would permit consideration to be given to anticipated changes in legal weight limits, changes in styles of cargo haulers, and changes in routing.
Digital Object Identifier
Havens, James H.; Deen, Robert C.; and Southgate, Herbert F., "Design Guide for Bituminous Concrete Pavement Structures" (1981). Kentucky Transportation Center Research Report. 1005.