Moisture can significantly affect flexible pavement performance. As such, it is crucial to remove moisture as quickly as possible from the pavements, mainly to avoid allowing moisture into the pavement subgrade. In the 1990s the Indiana Department of Transportation (INDOT) adopted an asphalt pavement drainage system consisting of an open-graded asphalt drainage layer connected to edge drains and collector pipes to remove moisture from the pavement system. However, over the intervening two decades, asphalt pavement materials and designs have dramatically changed in Indiana, and the effectiveness of the pavements drainage system may have changed. Today, in-place field densities achieved during construction make asphalt mixtures less susceptible to moisture intrusion than their 1990s counterparts. Additionally, there are challenges involved in producing and placing open-graded asphalt drainage layers, they can potentially increase costs, and they tend to have lower strength than traditional dense-graded asphalt pavement layers.

Given the potential difficulties, the overall objective of this research was to evaluate the effectiveness of INDOT’s current flexible pavement drainage systems given the changes to pavement cross-sections and materials that have occurred since the open-graded drainage layer was adopted. Additionally, the effectiveness of the filter layer and edge drains were examined. Laboratory experiments were performed to obtain the hydraulic properties of field-produced asphalt mixture specimens meeting INDOT’s current specifications and the results used in finite element modeling of moisture flow through pavement sections. Modeling was also performed to investigate the rutting performance of the drainage layer in flexible pavements under various traffic loads and subgrade moisture conditions in combination with typical Indiana subgrade soils. The results were used to develop design graphs to assist the pavement designer in more accurately assessing the need for a pavement drainage system in any given flexible pavement.

In general, the results indicate that drainage layers do effectively lower the subgrade moisture content and act to maintain subgrade moisture contents at native levels, while flexible pavements without drainage layers result in fully saturated subgrades. Also, while the results show that either a dense-graded aggregate or a dense-graded asphalt mixture can be used as a filter layer between the subgrade and the open-graded drainage layer, the subgrade tends to have lower moisture content when a granular filter is used. Moreover, the results indicate that edge drains have a positive effect on flexible pavement performance, especially those that do not contain a drainage layer. As expected, the modeling results showed an increase in pavement rutting whenever high moisture levels are present in the pavement system.

Report Number



flexible pavement, pavement drainage

SPR Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, Indiana

Date of this Version