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

This paper outlines state-of-the-practice in Colorado with regard to pavement drainage systems. When the need for underdrain is known during preliminary design, the project plans specify the size of the perforated pipe and location of the underdrain. Colorado does not use drainage layers under pavements. All subsurface paving materials require at least 5% -22 material, so they can not be classified as free-draining material. These materials will hold the water over the subgrade, and this becomes a problem when these aggregates hold the water over a swelling soil. In order to try to cope with the problem, the state uses catalytically blown asphalt membranes through cut areas in expansive clays and shales. Gradation of filter materials is set up in standard specifications. Three specified materials range from rather permeable to a coarse concrete sand with a lower permeability. A typical pipe installation has 3 in. (7.6 cm) of filter material beneath the pipe, the perforations are down, and there is 3 ft (0.9 m) of material above the pipe. This filter material is then covered with building paper, plastic sheeting, loose straw, or an acceptable equivalent. Several methods are used to seal water out of subgrade materials, including the use of full-depth asphalt pavement and asphalt shoulders. Colorado has severe frost-heave problems in the mountainous areas, but no special aggregate is used to prevent this problem. Two styrofoam test sections have been installed. The state is also experimenting with a bituminous base, instead of an aggregate base, to partially prevent this frost-heave problem. Colorado experiences significant infiltration of snowmelt water into the pavement structure and subgrade. In most cases, ditch sections are not deep enough to drain the water away from the pavement structure.