Ripples in the Road Stefanie R. Fishman, Project Assistant

Properly constructed and maintained gravel Pitch roads are adequate for low volume traffic. As most Crest road managers know, the most common mainte­ nance problems are loss of aggregate, drainage, t and corrugations, or "washboarding." This article focuses on preventing and repairing corrugated roads. Washboarding causes wear and tear on vehi­ cles. It causes vehicles to vibrate, annoying drivers and riders. Less understood, but more importantly, drivers have less control of their vehicles. The Valley deeper the corrugations, the less safe the road. Why Does It Happen?

What Is Washboarding? Similar to the asphalt top course on paved roads, gravel is the "wearing surface" of an Washboarding is a series of high and low unpaved road. The large stones, in contact with spots in a wavelike pattern. As shown below, the each other, distribute vehicle loads to the road waves tend to form from shoulder to shoulder. The base beneath it. The stones are held by smaller road resembles an old-fashion washboard. Also, particles, especially "fines" which pass a 200 sieve driving on a corrugated road often feels like riding (a screen with 200 wires per inch). With insuffi­ on a very large washboard. cient 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 force ofthe 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 hard­ ness and vehicle acceleration, deceleration, turn­ ing, and speed. Because hard tires excerpt greater A corrugated, washboard-like gravel road force per square inch on the road surface, they Corrugation severity is defined by its depth, break and scatter aggregate more than do soft the height of a ridge from its crest to the bottom of tires. Acceleration, deceleration, and turning also the adjacent valley (see diagram). Low severity is increase tire-to-road surface forces. Therefore, a depth less than 1 inch, moderate is 1 to 3 inches, corrugations usually occur first at road and drive­ and high severity is greater than 3 inches. The way intersections, and at tops and bottoms ofhills. pitch is the length of a corrugation measured from High speeds increase both tire force and crest to crest, typically 7 to 8 inches. bouncing. Fast moving vehicles, therefore, quicken the formation ofcorrugations and increase their depth. Continued on page 8 Road Business, Summer 1999, Vol. 14, No.2 page 3 Continued from page 3 Surface and base courses should be com­ pacted and maintained to drain water out of the These factors have the greatest effect on roads road. Highway departments should maintain 'l with a weak base, poor drainage, or both. A weak ditches and culvert to carry water away from the )1 base deflects under loads, and becomes deformed road. into the same wave-like pattern as the surface. Synthetic binders, such as calcium chloride Poor drainage leaves water in the road, or allows it and magnesium chloride, retain fines and provide to percolate in from the sides. Too much water a hard road surface. The hard surface also lubricates the base and surface particles. Thus, improves drainage. water increases particle movement, and corruga­ tion formation and depth. How Can It Be Fixed?

Preventing Washboarding If corrugation severity is low (less than one­ inch depth), equipment operators can correct Municipalities can reduce washboarding by washboarding by routine blading or by dragging taking the following actions: with a rack. If severity is moderate (1 to 3 inch • Encourage drivers to slow down depth) the highway department should add mate­ • Set and enforce speed limits rial. High severity corrugations (more than 3 • Use well-graded materials for wearing surface inches deep) usually require the following, more and base courses expensive, steps: • Maintain road and roadway drainage 1. Scarify the corrugated surface. • Use synthetic binders 2. Cut down 3 to 4 in. 3. Add more gravel as needed. Lower vehicle speeds will help prevent wash­ 4. Add a synthetic binder. boarding. They will also reduce acceleration and 5. Mix the materials in place and reshape the sur- deceleration, and their adverse affects. face. ~ Most drivers on a gravel road live on it. Daily, 6. Compact the surface to be sure there is no a corrugated road adversely affects their safety, loose aggregate. riding comfort, and vehicle condition. Municipali­ ties can encourage some residents to slow down It is a good practice to correct corrugation after by informing them of the effect that speed has on it rains since the moisture needed to compact and washboarding. (Keeping roads narrow and with reshape the surface will be present. Therefore, the sharp curves slows traffic, but such roads are best seasons for repair are the spring and fall. unsafe. Safe roads should be the first priority.) In When dealing with washboarding,prevention addition, some residents and non-residents will is cheaper than repair. The road must be properly respond to posted and enforced speed limits. constructed and maintained. If washboarding still Visible maintenance actions will encourage occurs because of heavy traffic (greater than 200 drivers to reduce speed and police to enforce cars per day), the municipality should consider speed limits. Routine maintenance, such as grad­ paving the road. ing roads and filling potholes, will also reduce the Sources: occurrence of washboarding. Problems Associated With Gravel Roads, Federal Highway Administration, A properly constructed road will itself help May 1998. prevent corrugations. The wearing course and base Maine Local Roads Center, Maintaining Gravel Roads Training Manual 1998, Maine Department of Transportation & Federal Highway should be of well-graded materials; that is, aggre­ Administration. 1998. gate and fines in particular proportions by weight. Cate, D.W., R.A. Eato l , & S. Gerard, Rating Unsurfaced Roads A Field Manual for Measuring Maintenanc e Problems, US Army Corps of Engi­ (Call the UNH T2 Center for recommended pro­ neers Cold Region Research & Engineering Laboratory, pp. 14-15, portions and thickness.) August 1987. Skorseth, Ken, "Dealing With Washboarding", South Dakota Local Trans- ~ portation Assistance Program. )

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