Why do dirt roads end up with hard ripples?
In: 17
It rains and gets muddy. Then cars go through with tire treads and leave their marks all over the mud. This causes the bumps in the mud. Then it dries and the mud hardens. Voila, dirt road complete with bumps.
As far as I know it’s kinda up in the air as to the cause of the problem and lots of research being done on it over the years has resulted in a lot of different answers.
Probably points to it being a lot of different things stacking up to cause it.
Here’s the wiki article:
https://en.m.wikipedia.org/wiki/Washboarding
> [According to the Federal Highway Administration (FHWA), the four primary causes of washboarding are driving habits, lack of moisture, poor gravel quality, and lack of crown on the road surface.](https://blog.midwestind.com/how-gravel-roads-get-washboarding/)
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.
Notice waves on water? It is a similar effect on the road. Yes the road surface is harder but it is flexible and you have the constant motion of traffic on the surface that results look and act like waves.
As told to the class long long ago by an old time engineering professor. Have not thought on that in years. Perhaps they have more up to date information on the cause now.
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