DESIGN OF SUBDRAINAGE SYSTEMS

This paper addresses pitfalls in drainage design and outlines current procedures used for the design of subdrainage systems in New York State. Determining field conditions--the range of permeabilities in the construction materials, pavement section, and embankment--is more important than a refinement in the application of Darcy's law. When analyzing the field situation, two sources of water must be considered: control of the groundwater and pavement drainage. Other factors that should be considered in the design of an underdrain pipe installation is the trench width, filter material under the pipe, and the perforation of the pipe. In New York, it was found that the asphaltic concrete (AC) base was the water-bearing layer in the pavement. Thus, they designed an underdrain that is connected to the AC base. They did not extend the AC base out under the shoulder as the guidelines recommended, but made the underdrain 18 in. (46 cm) wide, with a 6 in. (15 cm) diameter perforated pipe in it. This picks up the water coming out of the AC base. This subdrainage system is used at low points on vertical curves and on the low sides of superelevated curves, where the water seems to concentrate while moving through the AC base. In a curb section, or raised mall, or median, they use the same design, bringing the underdrain in contact with the layer to be drained. Although the system is effective, it is quite costly. Therefore, in New York, the emphasis is now on finding an economical way to put in subdrains.