Ripples in the RoadStefanie R. Fishman, Project Assistant

Why Does It Happen?

Properly constructed and maintained gravelroads are adequate for low volume traffic. As mostroad managers know, the most common mainte­nance problems are loss of aggregate, drainage,and corrugations, or "washboarding." This articlefocuses on preventing and repairing corrugatedroads.

Washboarding causes wear and tear on vehi­cles. It causes vehicles to vibrate, annoying driversand riders. Less understood, but more importantly,drivers have less control of their vehicles. Thedeeper the corrugations, the less safe the road.

CrestPitch

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Valley

What Is Washboarding?

Washboarding is a series of high and lowspots in a wavelike pattern. As shown below, thewaves tend to form from shoulder to shoulder. Theroad resembles an old-fashion washboard. Also,driving on a corrugated road often feels like ridingon a very large washboard.

A corrugated, washboard-like gravel road

Corrugation severity is defined by its depth,the height of a ridge from its crest to the bottom ofthe adjacent valley (see diagram). Low severity isa depth less than 1 inch, moderate is 1 to 3 inches,and high severity is greater than 3 inches. Thepitch is the length of a corrugation measured fromcrest to crest, typically 7 to 8 inches.

Road Business, Summer 1999, Vol. 14, No.2

Similar to the asphalt top course on pavedroads, gravel is the "wearing surface" of anunpaved road. The large stones, in contact witheach other, distribute vehicle loads to the roadbase beneath it. The stones are held by smallerparticles, especially "fines" which pass a 200 sieve(a screen with 200 wires per inch). With insuffi­cient fines, vehicle tires more easily move thestones. As their particle-to-particle contactdecreases, so also does their ability to distributeloads.

In addition, continuing traffic begins to movethe stones into the pattern of ridges and valleys.Traffic causes corrugations in two ways:• The force of the tires and the road surface,• The motion of vehicles bouncing ups and

down on their springs and tires.

The degree of damage depends on tire hard­ness and vehicle acceleration, deceleration, turn­ing, and speed. Because hard tires excerpt greaterforce per square inch on the road surface, theybreak and scatter aggregate more than do softtires. Acceleration, deceleration, and turning alsoincrease 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 andbouncing. Fast moving vehicles, therefore,quicken the formation of corrugations and increasetheir depth.

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These factors have the greatest effect on roadswith a weak base, poor drainage, or both. A weakbase deflects under loads, and becomes deformedinto the same wave-like pattern as the surface.Poor drainage leaves water in the road, or allows itto percolate in from the sides. Too much waterlubricates the base and surface particles. Thus,water increases particle movement, and corruga­tion formation and depth.

Preventing Washboarding

Municipalities can reduce washboarding bytaking the following actions:• Encourage drivers to slow down• Set and enforce speed limits• Use well-graded materials for wearing surface

and base courses• Maintain road and roadway drainage• Use synthetic binders

Lower vehicle speeds will help prevent wash­boarding. They will also reduce acceleration anddeceleration, and their adverse affects.

Most drivers on a gravel road live on it. Daily,a corrugated road adversely affects their safety,riding comfort, and vehicle condition. Municipali­ties can encourage some residents to slow downby informing them of the effect that speed has onwashboarding. (Keeping roads narrow and withsharp curves slows traffic, but such roads areunsafe. Safe roads should be the first priority.) Inaddition, some residents and non-residents willrespond to posted and enforced speed limits.

Visible maintenance actions will encouragedrivers to reduce speed and police to enforcespeed limits. Routine maintenance, such as grad­ing roads and filling potholes, will also reduce theoccurrence of washboarding.

A properly constructed road will itself helpprevent corrugations. The wearing course and baseshould be of well-graded materials; that is, aggre­gate and fines in particular proportions by weight.(Call the UNH T2 Center for recommended pro­portions and thickness.)

Road Business, Summer 1999, Vol. 14, No.2

Surface and base courses should be com­pacted and maintained to drain water out of theroad. Highway departments should maintain 'lditches and culvert to carry water away from the )1road.

Synthetic binders, such as calcium chlorideand magnesium chloride, retain fines and providea hard road surface. The hard surface alsoimproves drainage.

How Can It Be Fixed?

If corrugation severity is low (less than one­inch depth), equipment operators can correctwashboarding by routine blading or by draggingwith a rack. If severity is moderate (1 to 3 inchdepth) the highway department should add mate­rial. High severity corrugations (more than 3inches deep) usually require the following, moreexpensive, steps:1. Scarify the corrugated surface.2. Cut down 3 to 4 in.3. Add more gravel as needed.4. Add a synthetic binder.5. Mix the materials in place and reshape the sur-

face. ~6. Compact the surface to be sure there is no

loose aggregate.

It is a good practice to correct corrugation afterit rains since the moisture needed to compact andreshape the surface will be present. Therefore, thebest seasons for repair are the spring and fall.

When dealing with washboarding,preventionis cheaper than repair. The road must be properlyconstructed and maintained. If washboarding stilloccurs because of heavy traffic (greater than 200cars per day), the municipality should considerpaving the road.

Sources:

Problems Associated With Gravel Roads, Federal Highway Administration ,May 1998.

Maine Local Roads Center, Maintaining Gravel Roads Training Manual1998, Maine Department of Transportation & Federal HighwayAdministration. 1998.

Cate, D.W., R.A. Eato l , & S. Gerard, Rating Unsurfaced Roads A FieldManual for Measuring Maintenanc e Problems, US Army Corps of Engi­

neers Cold Region Research & Engineering Laboratory, pp. 14-15,August 1987.

Skorseth, Ken, "Dealing With Washboarding", South Dakota Local Trans- ~portation Assistance Program. )

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