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Q: Why are there ruts in dirt roads? ( Answered,   0 Comments )
Subject: Why are there ruts in dirt roads?
Category: Science > Earth Sciences
Asked by: jfish55-ga
List Price: $50.00
Posted: 28 Nov 2004 13:47 PST
Expires: 28 Dec 2004 13:47 PST
Question ID: 435184
Why are there ruts in dirt roads?  There seems to be a correlation
between the spacing and depth of ruts. Cars seem to produce less
spacing between ruts and less depth compared to horse carriages.

 Is there a correlation between the wheel speed and/or contact patch and/or weight
 and the spacing and depth of ruts?
Subject: Re: Why are there ruts in dirt roads?
Answered By: jdb-ga on 28 Nov 2004 15:55 PST
Hello, I am following up on your question. I found an article that
addresses your questions as why are there ruts in dirt roads, and what
is the correlation of contact, weight and speed.

Straight Dope Staff Report: What causes the "washboard" effect on unpaved roads?

"According to Tom Pettigrew, a Forest Service engineer, the cause is
an unlikely source: your car's suspension. (Well, maybe not yours
specifically, but it's not innocent in this matter, either.) A
vehicle's suspension system distributes the shock and energy of road
irregularities with a bouncing rhythm called harmonic oscillation. At
each downstroke, the wheels exert extra force on the road, causing the
particles in the road to either pack or displace at regular intervals.
Once a pattern of ruts starts to establish itself, it becomes
self-reinforcing due to what engineers call forced oscillation.  The
next car hits the same irregularities in the road and bounces at the
same rate, causing the pattern to become more and more defined. Forced
oscillation overcomes minor variations in oscillation rate that might
otherwise arise due to differences in car weight."
(See full article for more information)

Here is another article on this subject: 

"Ripples in the Road"
U New Hamphire Technology Transfer Center
"technical and management information about road and bridge repair" 
"The degree of damage depends on tire hardness and vehicle
acceleration, deceleration, turning, and speed. Because hard tires
excerpt greater force per square inch on the road surface, they break
and scatter aggregate more than do soft tires. Acceleration,
deceleration, and turning also increase tire-to-road surface forces.
Therefore, corrugations usually occur first at road and driveway
intersections, and at tops and bottoms of hills.
High speeds increase both tire force and bouncing. Fast moving
vehicles, therefore, quicken the formation of corrugations and
increase their depth."

And an artice in a road engineering journal:

"Preventing Washboarding"
Road Management & Engineering Journal

I hope that you find this information useful. Please let me know if I
can be of furthur assistance. jdb-ga

Request for Answer Clarification by jfish55-ga on 28 Nov 2004 19:00 PST
First two articles very useful in describing ruts or washboarding and
the maintainence of the spacing after they had been made. But question
about correlation of speed and weight with depth and spacing not
answered yet. Understand that after the first set of ruts is created
there is a reinforcement to keep the same pattern- but what
establishes the characteristics of the initial washboard?

Clarification of Answer by jdb-ga on 28 Nov 2004 21:04 PST

  Your further questions led me to some very useful additional
material. The previous articles include passages that describe the
correlation of speed and force with depth. The second article mentions
spacing or "pitch", as typically 7-8". I found additional material
below that includes information about why spacing tends to be typical,
as well as a study describing how and why corrugations initially form.
Weight is a combination of mass and gravity, and force is a
combination of mass and speed. Increased speed means increased force.
The articles make the point that speed is the more significant factor
rather than weight, though I do include additional material below on
the factor of the weight of vehicle or load. Google Answers guidelines
to respect copyright are that only limited passages can be quoted from
sources, so the full articles can be read for further information.

The articles describe how increased speed means increased force,
resulting in greater suspension and tire bounce and depth of ruts. The
Straight Dope article discusses this in terms of the tire and
suspension "downstroke" and the speed of the driver and vehicle:

"Wouldn't variations in speed affect the washboard pattern?
Sure...Drive too fast on a washboard road and the downstroke exerted
by the car wheels may meet the road at a point where a bump is ramping
upwards. You know what that means: You bounce off the ceiling. 
Instinctively most drivers slow to a speed at which the downstrokes
coincide with the troughs between bumps, reinforcing the
pattern...**Forced oscillation overcomes minor variations in
oscillation rate that might otherwise arise due to differences in car

The "Ripples in the Road" article says about speed, force and depth:

"**High speeds increase both tire force and bouncing. Fast moving
vehicles, therefore, quicken the formation of corrugations and
increase their depth**. [Which is why a solution is that]
Municipalities can encourage some residents to slow down by informing
them of the effect that speed has on washboarding."

And this about spacing, or "pitch":

"The **pitch is the length of a corrugation measured from crest to
crest**, typically 7 to 8 inches."

Additional material:

US Dept of Energy
Ask A Scientist©
Engineering Archive  
"When a **heavy load** passes over the spot it can sink in partially
and then the gravel is displaced from the soft spot."

Geophysical Institute, University of Alaska Fairbanks
Alaska Science Forum
Why Do Roads Corrugate?

"The January 1963 issue of Scientific American contains an article by
Dr. Keith B. Mather, now Vice Chancellor for Research and Advanced
Studies at the University of Alaska, which puts the matter to rest
once and for all."

[See website for description of experiment and further information]

"Among the more significant findings were that: 

-**If the wheel moves slowly, no corrugations were formed**, but a
deep rut instead;
-it did not matter whether the wheel was driving or idling, **at
sufficient speeds, washboarding occurred**;
-**the trough-to-trough distances between ruts increased linearly with
increasing speed**...
-sand was not pushed ahead of the wheel and then overridden to begin
another cycle, as had been commonly believed.

  The most important contribution to understanding washboarding lay in
the observation of how the corrugations are actually formed. When the
wheel reaches a certain critical speed, it begins to move in short
hops, bounding on random irregularities of the surface. Hitting an
obstacle, even a small one, propels the wheel into the air for a
certain distance. When it lands further down the track, it sprays sand
forward and to the side, thus creating the beginning of a crater. Each
time it digs itself in at a crater it has to ride out again and thus
repeats the pattern. **If traffic were to move at widely diversified
speeds, different "hop-lengths" might tend to cancel each other out,
but depending on road conditions, all traffic tends to travel in a
rather closely constrained speed range, thus compounding the problem
with each successive vehicle**."

You might check your library catalog for this January issue of
Scientific American and if not, you can request the specific article
through your library's Interlibrary Loan Service.

Here's an online New Scientist question on this topic with a response
by the same Keith Mather, that includes the "periodicity"  or spacing
of the corrugations:

"Corrugations may form whenever the force at right angles to the
surface is sufficient to cause permanent deformation. In the case of a
wheel on a road, when the wheel encounters any bump or irregularity,
it is projected upward, moves through an arc, and impacts further
along the road, where it tends to dig in and produce a small crater.
Subsequent wheels that ride up from this crater are likewise projected
and again form a little crater beyond. The corrugations already formed
are reinforced by the repeated passage of vehicles, and new
corrugations continue to propagate down the road from the initial
bump. You can watch the pattern develop on a test track. The
periodicity, averaging about 70 centimetres (27 inches) on dirt roads,
depends on the average vehicle speed (which determines the impact
force), the material of the road, and the tyre and springing constants
of the vehicle."
Mather mentions:

For more complete information, a popular account appeared in
Scientific American (January 1963), and a more technical version in
Civil Engineering & Public Works Review (May 1962, p 617 and June
1962, p 781).

I hope this is helpful. Let me know if I can be of further assistance.  jdb-ga

Clarification of Answer by jdb-ga on 28 Nov 2004 22:39 PST
Hello, I find a few more resources on weight of vehicle, load, load
distribution front and rear, and on causes:

"Causes and Cures for Washboarding"
"The causes of washboarding fall into three general catagories.
1. Lack of moisture...
2. Driving habits....Hard acceleration (speed) or hard braking are the
 greatest problems. As a result, washboarding normally appears first
(what establishes initial characteristics) at intersections, around
sharp curves, business entrances and driveways...Light vehicles with
small wheels and light suspensions cause more washboarding than heavy
3. Poor quality of gravel..."
"Washboarding forms on roadways where the
crust on the surface layer is dry and crumbly and is caused by the
loss of the fines (primarily in the form of dust) that are displaced
by passing vehicles. Washboarding is amplified in areas experiencing
high vehicle speeds or areas where vehicles are constantly braking and
accelerating...Empty pickup trucks with light rear ends traveling at
higher speeds appear to increase washboarding more so than other

UC Berkeley ITS Technology Transfer Newsletter
(As the html and pdf versions were not loading when I found this, I
included the Google cached copy url as well, in tinyurl form)
"Light vehicles with small wheels and light suspensions are a bigger
culprit than heavy trucks, which tend to grip the road better.

Simply blading over washboards with a motorgrader - skimming off the
ridges and filling the depressions - is almost useless. The best way
to eliminate washboarding is to cut all of the material loose to a
depth of one inch or more below the bottom of the washboard area, then
work the material to mix in the fines brought up from below, finally
reworking the material back to the proper shape and crown.

Finally, a note to dispel a common myth: motorgraders do not cause
washboarding. While it is true that graders can cut distortions into a
gravel surface, this distortion will never look like washboarding.
When an operator runs a grader too fast, the machine can begin to
"lope" or bounce. The resulting humps and dips will be farther apart
and cut at an angle across the roadway, at the same angle as the
moldboard during the blading. Motorgraders help you control
washboarding, they don't cause it.

This article was adapted from "Dealing with Washboarding," South
Dakota LTAP Special Bulletin #29, by Ken Skorseth, Field Operations
Manager for the South Dakota LTAP. This article was adapted and
reprinted with permission from the South Dakota LTAP."

All of the articles say that the solution is to use good gravel, and
go into this and optimal road construction and maintenance practices.

I hope these are helpful.  jdb-ga
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