Limitations of Using Conventional Unpaved Road Specifications for Understanding Unpaved Road Performance

Abstract

Numerous specifications exist for selecting materials for unpaved roads. The basis for them is often unclear; most do not indicate expected performance across the full range of grading and plasticity index (PI) envelopes. Observations of many U.S. roads that met federal or local material specifications often showed that poor performance, specifically in regard to washboarding and raveling, was common. These distresses are expensive to maintain and can lead to rapid loss of material and result in vehicle damage from loose stones. A simple method of interpreting unpaved road material properties (using grading analysis and Atterberg limit test results) in relation to expected performance was developed by Paige-Green in 1989 and modified by Jones in 2000 to interpret these properties when materials are modified with chemical treatments. Although this approach has been adopted and implemented in many countries, its use in the United States has been limited. With the approach, analysis of various grading and PI envelope combinations that meet commonly used specifications in the United States showed that poorly performing materials can still easily meet the specifications. Many practitioners managing unpaved roads may not have the expertise to refine the specifications for local conditions. Considering the analysis discussed in this paper, federal and county officials who write specifications, as well as unpaved road practitioners, are encouraged to use the approach to interpret material test results and select the most appropriate materials for unpaved roads. If optimal materials are not available, the approach can be used to refine maintenance programs, select appropriate ratios of materials for blending, or select an appropriate chemical treatment.