Ask A Scientist© Engineering Archive

 
   >    name         Joel
 >    status       other
 >    age          60s

 >    Question -   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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