America's Pavements: World's Longest Bathtubs

Pavements are the most unusual structures designed by civil engineers. Water flows through their tops, bottoms and sides but because pavements are relatively flat, flows out again very slowly unless they are well drained under their full width. Without highly permeable drainage layers—as under most pavements in the U.S.—damage from traffic and environment can reduce the life of undrained pavements to a third or less of that for well drained pavements. Yet in the U.S., concern over water in pavements has been largely ignored over the past 30-40 years. Most designers feel that if they make pavements strong there is no need for fast drainage out of structural sections. Key features of well drained pavements include 1) a layer of very high permeability material under the full width of traffic lanes; 2) a suitable filter of subbase on the subgrade to prevent soil from working upward into the open-graded drainage layer; 3) collector pipes along lower edges; 4) well marked outlet pipes. The open-graded layer, which should pass 10,000 to 100,000 ft/day, costs the same or less than compacted aggregates.