Recent research projects in Virginia and several national projects have focused on the design, construction, and evaluation of the drainage layers. The conclusions of these projects are sometimes controversial. One of the challenges to evaluate the effectiveness of drainage layers (in terms of their structural contribution to the overall pavement structure) lies in the difficulty in determining the causes of observed phenomenological distresses to the drainage layers. Typically a core should be acquired to determine whether the distress to the pavement is caused, or in some part contributed to, by the drainage layer. Indirect methods such as the Falling Weight Deflectometer (FWD) tests cannot often directly determine whether the failures come from the drainage layer. In the 1993 AASHTO pavement design guide, the contribution of the drainage layer to pavement structure is not considered. Some states follow this approach and do not give the drainage layer a structural layer coefficient while others do. In the MEPDG, while every layer can be separately evaluated, there is no procedure developed to characterize the fundamental properties of these drainage layers, which are usually composed of high porosity asphalt concrete or granular materials with porosities ranging from 20% to 35%. With these high porosities, the elastic modulus and the strength of these materials are relatively much lower than those of the traditional dense-graded materials. Also, it is quite difficult in preparing lab specimens at such high porosities. A need exists to develop methods or modify existing methods to characterize the elastic modulus and strength of these materials at high porosity, to perform analysis of the stability and failure of the drainage layer in the pavement structure, and to specify the required minimum porosity for effective drainage.

The objectives of the pooled fund study are to develop methods to characterize the elastic modulus and strength of drainage layers for MEPDG, to perform analysis of the stability and failure of the drainage layer in the pavement structure, and to develop specifications for required minimum porosity for effective drainage.

The scope of the work includes the following. 1. Develop testing methods or modify/adapt existing methods to characterize the elastic modulus and strength of asphalt concrete, other bounded materials or granular materials at high porosity. It is anticipated that triaxial tests and a portable ultrasound test device will be used. 2. Perform pavement analysis to evaluate how the drainage layers will perform/fail in a typical pavement structure under typical traffic loading. 3. Conduct a national survey on the requirements of the percent porosity of the drainage layer and its effectiveness in providing adequate drainage, and on general practice of drainage design, construction and evaluation. 4. Based on 1, 2 and 3, formulate recommendations of material properties and methods for failure predictions of drainage layers for MEPDG. It is anticipated that default values, empirical correlations, and testing procedures will be recommended for the material properties. The testing procedures will be formulated into AASHTO format for review and adoption. The pooled fund study will solicit the participation of about ten states and/or agencies. The typical pavement structure, drainage layer designs of these states will be collected and analyzed. The materials from these states will be characterized.

This study is of high significance. First, drainage is critical to the long life of pavements. Second, if contribution to pavement structure by drainage layer is not considered, it may result in an over design. Third, if drainage layer is not well constructed it may cause premature failures of the pavement structure. It is anticipated that the pooled fund study will take three years to accomplish. For each year, each participating state/agency is suggested to contribute $30,000