Recently, Oregon designed and constructed two types of permeable bases under both flexible and rigid pavements: an asphalttreated permeable material (ATPM) and an open-graded aggregate material. Permeability and resilient modulus of both materials have not been determined. During pavement structural design using the AASHTO Guide for Design of Pavement Structures, 1986, layer and drainage coefficients had to be assumed to establish pavement thickness designs. In addition, construction with the existing open-graded aggregate revealed that the material was less stable and would ravel easily under construction traffic. In 1990, a research project was initiated to determine the desirable material properties for the two types of free-draining base materials and establish a more stable gradation for the opengraded aggregate base. This project consisted primarily of a laboratory investigation. Pavement cores of the asphalt-treated permeable base and samples of aggregate materials were tested in the laboratory for permeability and resilient modulus. The permeability was determined using both constant and falling head test procedures. The laboratory study indicated that the current Oregon A TPM has a sufficient drainage capability, and the resilient modulus of this material is typical of the findings of other states. A modified open-graded aggregate gradation resulted, which has a higher permeability and higher resilient modulus than the existing gradation. Recommendations for implementation include selection of layer and drainage coefficients for pavement structural design and use of the proposed open-graded aggregate gradation in pavement construction.

Inadequate drainage of pavement structures has been identified as one of the primary causes of pavement distress (1-3). For many years, researchers have theorized that improving pavement drainage might combat many pavement problems and extend the pavement service life (4). Subsurface drainage includes the disposal of water that has entered the pavement structure; therefore, a positive drainage layer in a pavement structure is critical for subsurface drainage. The subsurface drainage system can be designed by providing a drainage layer along with transverse and longitudinal drainage pipes to remove the water from the pavement structure. Recently, Oregon initiated the design and construction of permeable bases under both flexible and rigid pavements. Two types of permeable bases used are an asphalt-treated permeable material (ATPM) and an open-graded aggregate material with the existing gradation designed by Oregon. Permeability and resilient modulus of both materials have not been determined. During pavement structural design, using