Ripples in the sand
Ripples in the sand

Unpaved roads around the world are plagued by surface ripples -- called washboards -- that are several centimetres high and formed under the rolling wheels of cars, buses and lorries. Unfortunately, numerous attempts by road engineers to find out how washboards could be avoided have met with little success. Indeed, the only practical way of dealing with these ripples is using heavy machinery to smooth-out road surfaces on a regular basis, which can be a very costly exercise in the remote regions where unpaved roads are usually found.

Now Jim McElwaine of Cambridge University, Nicolas Taberlet of the École Normale Supérieure de Lyon and Stephen Morris of the University of Toronto have devised a very simple experiment to study ripple formation. The researchers placed a flat bed of sand on a round table, which could be rotated at a constant rate of about 0.6 revolutions per second. A rubber wheel could be lowered onto the moving sand, where it could move up and down according to the level of the sand (see Ripples in the sand).

The researchers found that the wheel typically caused a small single ripple to form at one location in the sand after about ten or so rotations of the table. New ripples then grew rapidly from the single ripple and spread until the entire path of the wheel was covered in ripples.

The experiment was repeated using a different type of sand and again using long-grain rice. To their surprise, the researchers discovered that changing materials had little effect on ripple formation – suggesting that washboards cannot be avoided by using a specific type of material in road construction.

Indeed, the experiments and related computer simulations revealed that ripple formation is governed only by the speed of the wheel, its weight per unit width and the density of the granular material. Ripples were not seen when the wheel was kept below a critical speed – about 8 km/h for a car – leading the team to conclude that at higher vehicular speeds a flat road is unstable and will quickly become rippled. The study also suggests that heavier wheels will produce smaller ripples because their greater mass inhibits the vertical motion required to make a washboard.

According to McElwaine, ripple formation begins because no road surface is perfectly flat. When a wheel encounters a random hump in the road, it rises up and falls back down, pushing a bit of material out of its way to create a trough and a second hump. Subsequent wheels continue this process and a pattern of regular humps and troughs emerges.

McElwaine believes that the emergence of ripples could be delayed by making make the road as smooth as possible. Indeed, in their experiments, it took hours for ripples to emerge if the sand bed started off being extremely smooth.

Another way of avoiding ripples, according to McElwaine, is the development of active vehicle suspension systems that prevent the wheels from going up and down at frequencies that correspond to ripple creation – something that the team are currently investigating.

Several movies of the experiment can be viewed online.