Washboard effect of unpaved roads

In a granulometric material, there can be three different types of instabilities: by vibrations, by shearing or by gravity action. Washboard effect appears when a car is rolling on an unpaved road (made of sand, gravel or mud). It creates waves that are uncomfortable for the driver, and moreover, can be really dangerous due to the possibility of losing contact between the wheel and the road. It is a very tough problem because of its robustness. It starts initially with small ripples that grows bigger and bigger. “The amplitude of the ripples in terms of height can reach 20 cm, and the wavelength can be between 30 cm to 1m. This wavelength is function of the car’s speed, car’s weight, car’s characteristics (springs, tire pressure, etc.) and climate conditions. According to the field observation, the ripples are bigger in the turns, cross-section, slopes, where cars are adding stresses on the road. Ripples can move with time, changing its wavelength. It can move in the same direction of the car, or opposite to it.”[Taberlet, 2012] There are three options nowadays to erase this phenomenon: Build concrete or asphalt roads Add additives on the road to raise the material cohesion (like hydrocarbon), which leads to environmental problems Flat the road, which is the common method, good on short term. However, it has to be done every 2 or 3 weeks because the ripples are emerging pretty fast if the road is often used The final aim of the work is to have coherent results from an experimental set-up with a rotating machine dragging a wheel arm on a sandy soil. And, in a second part, to change some variables one by one in order to understand which variable develops or erases the washboard effect and in which rate of growing. The variables that had been developed are: Compaction / Without compaction of the soil Different masses added to the wheel arm Changing the water-table level, moistening the soil All those variables will be compared at different velocities, in terms of waves’ height, waves’ wavelength and axial forces of the wheel, in order to set the critical velocity where washboard can be seen.