THE PRACTICALITIES OF HIGHWAY SUBSURFACE DRAINAGE COURSES

Many highway subjects are related to water in pavements: pavement faulting, settlement at bridges, pavement failures, slope failures, stripping of asphalt, and bridge deck deterioration. This paper comments on the practicalities of some of these problems and the author's experiences in striving to solve the problems in Georgia. Water in pavement systems can be divided into two broad categories: groundwater and infiltrated water. Less pavement distress and fewer problems after construction result from groundwater than from infiltrated water. For wet cuts, keeping the drainage channels well below the level of the excavation is recommended. In extremely wet cuts, a stone blanket will often be necessary. It is important that groundwater conditions be known prior to construction. Concrete pavements are relatively impervious to infiltrated water except at joints. On a 15-mi (24-km) interstate project in Georgia, the seal material, joint configuration, and joint spacing were varied in order to determine the best joint design. The closed cell joint provided the best seal, but all joint designs leaked to some extent. Georgia has also studied erodible material under pavement in some detail. Subbase types found from best to worst, as related to faulting only, were topsoil, soil cement, graded aggregate, sand bituminous, and soil. Plain concrete pavement design in the state calls for 20-ft (6-m) spacing on the joints, the joints doweled, a subbase consisting of soil cement capped with asphaltic concrete, and an asphalt drainage course in the shoulder. Joints are sealed with preformed open-cell or closed-cell neoprene joint material. The paper concludes with three challenges for consideration: what are the criteria for the use of underdrain systems; when underdrain systems are justified, where should they be placed in the pavement system; and what shall be the design of underdrain systems when used.