Physical Modeling of the Washboard Effect on Unpaved Roads

Abstract

Unpaved roads are the predominant type of road in most countries around the world. The construction practices for this type of roads rely on empirical rules. However, sometimes, the emerging transversal ripples produce a phenomenon known as washboard, which affects these roads. The washboard effect appears when a car is rolling on roads made of sand, gravel or mud. It creates waves that are uncomfortable for the driver and can be dangerous due to the possibility of losing contact between the wheel and the road. It starts initially with small ripples that grow bigger and bigger. The amplitude of the ripples in terms of height can reach 20 cm, and the wavelength can be between 30 cm and 1 m. This wavelength is a function of the car’s speed, car’s weight, car’s characteristics (springs, tire pressure, etc.) and climatic conditions. Ripples can move with time, changing its wavelength. It can move in the same direction of the car, or opposite to it. This work presents results from an experimental setup with a rotating machine dragging a wheel arm on sandy soil. Several variables were changed to understand which variable develops or erases the washboard effect and in which rate of growing. The variables that had been studied are the compaction and mass of the wheel arm. These variables were compared at different velocities, in terms of waves’ height, waves’ wavelength and axial forces of the wheel, to set the critical velocity where washboard can be observed.