In the United States today there exists approximately 1.5 million miles of unpaved roads surfaced with stone or a soil/stone mixture. Also, beneath most pavements there is a “hidden unbound aggregate road” in the form of a base or subbase. Therefore, most roads traveled today rely on the support function of crushed stone or gravel to allow traffic to traverse the subgrade soil. It has been shown (Jonesby and Hicks) that as little as 6% fines contamination into a unbound aggregate layer can dramatically reduce the structural support of layer and thereby destroy its ability to spread the traffic load over a subgrade. Therefore, the greatest challenge for road managers is to maintain the integrity of their unbound aggregate support layers by minimizing the potential for contamination form upward migration.
In 1987 to 1989 the US Federal Highway Administration, FHWA, in cooperation with the Oklahoma Center for Local Government Technology tested the effectiveness of separation/stabilization geotextiles in preventing the contamination of unbound aggregate roads form subgrade soils. Nineteen unpaved rural roads sites in six countries were chosen for testing across the state of Oklahoma (Figure 1.) The nineteen sites represented a wide variety of subgrade conditions, aggregate surfacing types, climates, and construction methodologies. A final report, FHWA-RT-89-050, was published as a Technology Sharing Report. The report, Geotextiles Selection and Installation Manual for Rural Unpaved Roads, is a genuine manual for the use of separation/stabilization geotextiles based on the results of the statewide testing of the concept. The geotextiles were found to be an extremely effective, low cost solution to the contamination problem. This is a brief summary of the testing which resulted in the recommendations of the report.