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Name: Joel
Status: Other
Age: 60s
Location: N/A
Country: N/A
Date: N/A

Can you tell me what causes the phenomenon know as "washboarding" on an unconsolidated (gravel, aggregate, some soil, etc.) road surface? The most credible answer I have gotten so far is that loose material is pushed ahead of the rolling tire until the irregular shapes of the loose material build up resistance, obliging the tire to roll over it and the cycle is immediately repeated. Is speed a factor?

Thank you very much.

If you consult a good basic civil engineering text on soils and highway/airport design, you will learn that most often the washboarding effect you speak of is generally due to poor or improper compaction of the subpavement layers. A roadway surface works on the principle of the under layers distributing the roadway load to the subsurface. If the various layers that comprise the subpavement are not properly placed in the correct thickness, and then compacted to the proper density (with an eye to the appropriate moisture content depending on construction material used), the subpavement cannot take the load from the road surface that is being propagated into it and it begins to develop soft spots. Washboarding, and pot holes for that matter, are surface manifestations of these soft spots. The roadway surface that cars, trucks, etc. travel on is generally only for weatherproofing the subpavement that takes the load of the traffic. With the exception of a reinforced concrete road surface or runway (at an airport) that has the capability to take some of the structural load of the vehicles passing over it, the surface in contact with the vehicle or plane wheels is merely there to prevent the subpavement from absorbing too much moisture and then breaking down under the constant cyclical loading that traffic imposes on the roadway. Roadways and runways are crowned to facilitate the runoff of rain and snow so the moisture does not create a safety hazard on the road (i.e., hydroplaning) or stand for a long time and eventually get down into the subpavement.

On the gravel road surface the questioner speaks of, the observer is only witnessing the surface effect of the gravel. If the road surface is not sealed, then surface water can be absorbed into both the surface layer and the subsurface layers. When a heavy load passes over the spot it can sink in partially and then the gravel is displaced from the soft spot. You can not tell from merely looking at the surface whether it is the unsealed surface layer being displaced or if the movement is occurring all the way down into the subpavement.

J. Suermann

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