THE STATE-OF-THE-PRACTICE IN IOWA--PART II

This paper presents state-of-the-practice for pavement drainage systems in Iowa. Iowa designs pavement structures with the goal of making the materials strong enough so they will not be susceptible to deterioration from water. The state's pavement drainage systems are primarily to control subsurface groundwater. They use 5-ft (1.5-m) ditches along rural highways and shoulder subdrains in tight right-of-way situations. When granular select soils are used in the subgrade, they are underdrained. For flexible pavement, designers strive for dense, high-asphalt content materials that have a high cohesive strength and a fairly low permeability so that they conduct fairly low amounts of water. The companion subgrades, in general, are designed to take the amounts of water that come to them. Rigid pavement design starts with a plain section of uniform thickness with 20-ft (6-m) joint spacing and no load transfer devices. When the traffic prediction indicates 500 trucks per day, load transfer dowels are added at the contraction joints. When anticipated traffic includes 1,000 trucks per day, a stabilized subbase is used. This reduces the pumping through the reduction in deflection of the thicker section; load transfer devices change the deflection characteristics at the joints.