Why do dirt roads end up with hard ripples?
In: 17
As well as all the other factors mentioned already, any washboarding that forms will naturally get worse due to how tyres move through them. It’s a similar process to how wind forms waves and sand dunes.
As the tyre moves through the washboards, it will thump down hard at the bottom of each valley, which will (particularly when wet) break up the top layer and push dirt/mud ahead of the tyre. As it moves up the next washboard hill, that dirt/mud is pushed up and to the side, depositing it towards the top. Over hundreds of cycles, dirt is slowly migrated from the bottom of the washboards to the top making them deeper and worse. This is similar to the mechanics that make potholes worse over time.
The *period* of each washboard (the length of each up/down cycle as you drive through them) tends to align with the wheel sizes of the of the cars driving through them. Basically, the tyre has to be able to fit into the bottom of the valley, and it ends up just a little bit bigger than that.
Out on dirt farm roads where most traffic is from semi trucks the washboards tend to be bigger and longer than on roads where cars are more frequent.
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