Properly constructed and maintained gravel roads are adequate for low volume traffic. As most road managers know, the most common maintenance problems are loss of aggregate, drainage, and corrugations, or "washboarding." This article focuses on preventing and repairing corrugated roads.
Washboarding causes wear and tear on vehicles. It causes vehicles to vibrate, annoying drivers and riders. Less understood, but more importantly, drivers have less control of their vehicles. The deeper the corrugations, the less safe the road.
Washboarding is a series of high and low spots in a wavelike pattern. As shown below, the waves tend to form from shoulder to shoulder. The road resembles an old-fashion washboard. Also, driving on a corrugated road often feels like riding on a very large washboard.
Corrugation severity is defined by its depth, the height of a ridge from its crest to the bottom of the adjacent valley (see diagram). Low severity is a depth less than 1 inch, moderate is 1 to 3 inches, and high severity is greater than 3 inches. The pitch is the length of a corrugation measured from crest to crest, typically 7 to 8 inches.
Similar to the asphalt top course on paved roads, gravel is the "wearing surface" of an unpaved road. The large stones, in contact with each other, distribute vehicle loads to the road base beneath it. The stones are held by smaller particles, especially "fines" which pass a 200 sieve (a screen with 200 wires per inch). With insufficient fines, vehicle tires more easily move the stones. As their particle-to-particle contact decreases, so also does their ability to distribute loads.
In addition, continuing traffic begins to move the stones into the pattern of ridges and valleys. Traffic causes corrugations in two ways:
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.
These factors have the greatest effect on roads with a weak base, poor drainage, or both. A weak base deflects under loads, and becomes deformed into the same wave-like pattern as the surface. Poor drainage leaves water in the road, or allows it to percolate in from the sides. Too much water lubricates the base and surface particles. Thus, water increases particle movement, and corrugation formation and depth.
Municipalities can reduce washboarding by taking the following actions:
Lower vehicle speeds will help prevent washboarding. They will also reduce acceleration and deceleration, 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 down by informing them of the effect that speed has on washboarding. (Keeping roads narrow and with sharp curves slows traffic, but such roads are unsafe. Safe roads should be the first priority.) In addition, some residents and non-residents will respond to posted and enforced speed limits.
Visible maintenance actions will encourage drivers to reduce speed and police to enforce speed limits. Routine maintenance, such as grading roads and filling potholes, will also reduce the occurrence of washboarding.
A properly constructed road will itself help prevent corrugations. The wearing course and base should be of well-graded materials; that is, aggregate and fines in particular proportions by weight. (Call the UNH Tē Center for recommended proportions and thickness.)
Surface and base courses should be compacted and maintained to drain water out of the road. Highway departments should maintain ditches and culvert to carry water away from the road.
Synthetic binders, such as calcium chloride and magnesium chloride, retain fines and provide a hard road surface. The hard surface also improves drainage.
If corrugation severity is low (less than one-inch depth), equipment operators can correct washboarding by routine blading or by dragging with a rack. If severity is moderate (1 to 3 inch depth) the highway department should add material. High severity corrugations (more than 3 inches deep) usually require the following, more expensive, steps:
It is a good practice to correct corrugation after it rains since the moisture needed to compact and reshape the surface will be present. Therefore, the best seasons for repair are the spring and fall.
When dealing with washboarding,prevention is cheaper than repair. The road must be properly constructed and maintained. If washboarding still occurs because of heavy traffic (greater than 200 cars per day), the municipality should consider paving the road.
Sources:
Problems Associated With Gravel Roads, Federal Highway Administration, May 1998.
Maine Local Roads Center, Maintaining Gravel Roads Training Manual 1998, Maine Department of Transportation & Federal Highway Administration. 1998.
Cate, D.W., R.A. Eato1, & S. Gerard, Rating Unsurfaced Roads A Field
Manual for Measuring Maintenance Problems, US Army Corps of Engineers Cold Region Research & Engineering Laboratory, pp. 14-15, August 1987.
Skorseth, Ken, "Dealing With Washboarding", South Dakota Local Transportation Assistance Program.
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