Analysis of Factors Causing Corrugation of Gravel Roads

Gravel and unpaved roads are viable and important components of the road transportation network throughout the world. In the United States, gravel and unpaved roads still make up close to 50% of road miles. Motorists traveling on a gravel road will encounter the problem of corrugation. Corrugation, or washboarding, is a problem with many causes and no long-term solutions, requiring continual maintenance to control its appearance. Several factors have been researched and found to contribute to corrugation, such as traffic volumes, vehicle speeds, subgrade characteristics, harmonic vibration of the vehicle's suspension system, and gravel properties. This study examines the causes of corrugation by using variables previously considered by other researchers as well as new variables added by the study's research team. The objective of this study is to develop a comprehensive prediction model that can identify the potential occurrence of corrugation at a specific location. The model calculates corrugation height and wavelength. One significant and considered new conclusion is that corrugation is affected by the amount of natural soil placed between the gravel layer and the natural subgrade material. This natural soil is placed on the road surface during the original construction of the road and comes from formation of the side ditches. Another conclusion is that the moisture content of the subgrade, the ratio of the thickness of the surface gravel to the depth to the subgrade, and traffic speeds are the most significant influencing factors in the formation of corrugation.