Traveling around in the countryside on dirt roads and sections of road that are minimally maintained, you often find sections of road that has that washboard texture that can rattle you and the vehicle to pieces. I normally found these sections on grade changes in the road or approaching stop signs or intersections. My initial thought was this annoying surface was simply from the physics of vehicle traffic slowing down (or speeding up) and since the dirt is elastic to a degree, it simply stretched out or bunched up.
So when we had a section of road in front of our office redone I noticed something similar that had me question this theory. The roadway is a dead end street/road that is in a rural/residential area. We have some heavy trucks using the road but mostly small trucks and passenger cars to the homes and offices. All traffic is low speed, 30 MPH or less. The section that was refinished is 1/4 mile or 400 meters and is completely flat, no grade change. The section was graded and rolled, and quite smooth. The next day they added a layer of gravel but I don't know what other preparations were done. The road is wide enough for two lanes plus some extra.
Within a week the entire stretch became a torturous washboard that rattled your teeth loose. What intrigued me was the uniformity. The washboard lines went straight across the width of the two lanes. The spacing was fairly uniform at 15- 18" (38-35 cm)
So is the spacing due to the average circumference of the tires used on the road? The speed of the vehicle? and why a straight line across the whole roadway. Is the gravel setting up in that fashion because of resonance created by vehicle vibration on the road surface, (similar to when salt or sand resonates on a plate vibrated by a speaker)?
Or is it something else all together, or a combination of these and many other factors.
This is in the southwest US. There was and has been very little rain (I'd say less than an 1", and none during the time this incident occurred)
Apologies, I couldn't get this photo to display properly and I couldn't get good resolution with my camera, but you can see a bit of the ridges and pattern as you approach the end of the repaired section

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