In all pavement structures, one or more support layers separate the pavement from the subgrade. In addition to offering structural support, these layers have several important functions, which include: providing a stable and uniform construction platform, facilitating drainage, mitigating pumping of the subgrade fines, and protecting the pavement from the effects of frost heave. Their performance is critical in achieving the desired pavement smoothness, and in extending the service life of the structure. A range of designs, making use of unbound or stabilized aggregates and in some cases geosynthetics, are employed by different agencies to fulfill these functions.

This project was motivated by constructability and long term performance concerns with the existing base/subbase design employed by INDOT for concrete pavements, as well as the desire to identify state of the art design solutions that could be applicable to both concrete and asphalt pavements. The primary objectives of the study were to: critically reexamine INDOT’s existing design; perform a preliminary evaluation (based on aggregate compaction, hydraulic conductivity, strength and compatibility properties) of select unbound design options identified in collaboration with the Study Advisory Committee (SAC); explore the potential use of geotextiles as separator; and develop recommendations for base/subbase aggregate laboratory testing and evaluation.

Report Number



separator layer, pavement layer compatibility, unbound aggregate, hydraulic conductivity, shear strength, compaction, geotextiles, pavement subbase, drainage layer, strength

SPR Number


Performing Organization

Joint Transportation Research Program

Publisher Place

West Lafayette, IN

Date of this Version