Guidance For Selection Of Unbound Pavement Layer Seasonal Stiffnesses

One of the benefits of using mechanistic-empirical (ME) design methods for pavements is the ability to calculate pavement response to various loading and climate conditions, and then in turn model the entire damage process that is expected to occur over the pavement life time. One property that is currently not accounted for within California’s ME design software (CalME) is the change in stiffness of unbound materials that may occur due to seasonal moisture patterns. Engineering properties of unbound material may change due to a variety of factors, such as fluctuations in water content, changes in suction during wetting or drying periods, changes in overburden stress, and geologic setting. Before implementing more complex relationships to model changes associated with different environmental factors, the University of California Pavement Research Center (UCPRC) wishes to evaluate the extent of variation that is observed in the field. The goals of this research are to evaluate whether or not these speculated seasonal changes in unbound material properties warrant further design optimization, and if so, how research to characterize such optimization should proceed in the future.In this research, an experiment was performed to evaluate if noticeable changes in subgrade stiffness can be identified and explained using available, pertinent, and easy to use pavement monitoring equipment. Testing was performed twice on sections across California, once in the wet season and once in the dry season, to get a broad picture of the types of materials present and their corresponding properties during wet and dry seasons. Monitoring of a test section at UC Davis was also performed to observe changes in stiffness occurring after rainfall events and during wetting and drying cycles. The literature and various laboratory experiments investigating the influence of moisture and suction on the resilient response of unbound materials strongly suggest that a large degree of variability in stiffness should be encountered in different moisture conditions. However, the results of the study revealed that a majority of the unbound material tested experienced minor, if any, changes in stiffness between the two rounds of testing. The most susceptible materials to stiffness variation were not necessarily the compacted subgrade material, but were the stabilized and unstabilized granular materials directly underlying the asphalt surface. While changes in moisture content and penetration resistance were observed between the two rounds of testing, they did not necessarily correspond to significant fluctuation in field-tested stiffness of the unbound materials; rather, other factors such as spatial variability, drainage conditions, soil type, and influences from overlying layers tended to have a much larger influence on the resilient response of these materials than did seasonal moisture change.