Analysis and Synthesis of Highway Pavement Design

To better understand the art and science of pavement design, an examination of three flexible and three rigid highway design techniques was executed to determine the relative effects of various design parameters on pavement thickness and to synthesize simplified design methods for the two pavement types.The analysis phase of this investi- gation consists of organizing the design-process logic into a hierarchy structure and formulating each technique into a comprehensive mathematical or graphical thickness model.An evaluation of the influences on thickness of the major design factors by a sensitivity analysis with a theoretical and two practical measures of parameter importance, a study of the parameter variations tolerated by selected thickness change constraints and a least squares fitting of appropriate equations to parameter-thickness data preceded the development of simplified flexible and rigid pavement design methods.The hierarchy structure of design-process logic and the theoretical measure of parameter importance reveal considerable differences among techniques as to the process of resolving xiv design thicknesses and to the relative theoretical influence of various parameters that estimate the same major design factors.However, the practical parameter importance.measures, which account for variations in parameter values as well as the formulated parameter-thickness relationship, demonstrate a greater similarity in the importance of generic factors among design methods.For flexible pavement design, traffic load and subgrade support are the more influencial factors in the determination of pavement thickness.Traffic load and pavement structural properties exert the greater influence on rigid pavement thickness requirements.Design precision is presently not commensurate with design or construction thickness tolerances.Of the various model approximations of the parameter -thickness relationships, the linear model is preferable because it best expresses the thickness design relation- ships and is statistically efficient.The simplified design methods, which were developed from a synthesis of data gener- ated for the three flexible and for the three rigid techniques, relate thickness to the two more important independent design variables for each type of pavement.load, pavement material properties, environmental factors and performance criteria and to develop a simplified pavement design method.Three flexible and three rigid pavement design methods, which are representative of present design practices, were structured into a hierarchy of design-process logic and were analyzed to define the important set of pavement design variables considered in each design process.The relative importance of these design factors was determined in a sensitivity analysis which was developed to investigate the impact of changes in parameter values on the pavement system.Pavement thickness limitations were selected to illustrate the variabilities of design factors allowed before major changes in the final design are required.Tolerable change, when compared to actual variability encountered in some design factors, resulted in a verification of or suggested changes in present design techniques and construction and inspection practices.The feasibility of a simplified design method which contained only the more significant design factors was evident from the results of the sensitivity analysis.A better understanding of the state of the art and science of pavement design has resulted from this research.Suggested modifications of current design and construction practices are permitted by indicating deficient phases of the pavement design process and related activities.The following specific benefits were a consequence of this research investigation.