Serving rocky roads

June 19, 2003

Many people who live on gravel surfaced roads are simply waiting for the day when those roads will be "oiled" or paved with an asphalt surface. In regions where the population is growing, this will probably be done. But, in much of the Great Plains as well as portions of the mountain West and desert Southwest regions of the U.S., thousands of miles of gravel roads will continue to serve the rural population. These roads will remain gravel surfaced because of sparse population and low volumes of traffic. The cost of paving can't be justified.

Many people who live on gravel surfaced roads are simply waiting for the day when those roads will be "oiled" or paved with an asphalt surface. In regions where the population is growing, this will probably be done. But, in much of the Great Plains as well as portions of the mountain West and desert Southwest regions of the U.S., thousands of miles of gravel roads will continue to serve the rural population. These roads will remain gravel surfaced because of sparse population and low volumes of traffic. The cost of paving can't be justified. Yet these roads are the only link to towns, schools, hospitals and agricultural markets for many rural residents. It is estimated that there are 1.6 million miles of unpaved roads in the U.S. alone and millions of additional miles in the rest of the world. Unfortunately, unpaved roads tend to be a forgotten portion of the transportation grid in the U.S. There is little technical information or support to assist local agencies in managing and maintaining these roads. Consequently, it becomes a "seat-of-the-pants" system of management in many cases.

Stay in shape

Gravel road maintenance and management is changing as is management on other surface types. For example, heavier trucks with higher tire pressures and heavier and larger agricultural, mining and logging equipment causes increased stress to gravel surfaces as well as pavement. While pavements are being designed with greater thickness and better quality base aggregates and surfacing, gravel roads often exist virtually the same as they were 30, 40 and even 50 years ago. This creates real management problems. It becomes more important than ever to understand the two important components of managing gravel roads--the use of the motor grader and selecting quality surface gravel. Each component is equally important. However, the correct use of a motor grader to establish and maintain correct roadway shape must be understood first. Thereafter, good surface must be placed for a gravel road to perform well.

Maintaining the shape on the gravel road surface is a constant challenge to the grader operator. While paved roads generally maintain their shape for an indefinite period of time, gravel surfaces change constantly.

Traffic continually displaces the surface gravel. Loose material tends to collect between the wheel paths and also shifts to the shoulder of the roadway. Regular blade maintenance is required to recover and reshape the surface. The quality of the surface gravel, volume and type of traffic and weather conditions will dictate how frequently blade maintenance needs to be done, but deferred blade maintenance will cost in the long run.

The two greatest challenges to the grader operator are to avoid creating a high shoulder or "secondary ditch" at the edge of the roadway and to maintain the correct crown on the surface.

A high shoulder can develop in several ways. The most common cause is gravel that is displaced by traffic to the shoulder and it lodges in or near shoulder vegetation. This can be hard to recover with the grader. In many regions, this is best done in the spring when shoulder vegetation is dormant. This is referred to as "pulling the shoulder" and is a good maintenance technique. Vegetative cover will quickly replace itself in most regions. Care must be taken not to bring too much vegetation onto the roadway thus creating a large windrow.

Another common cause of the high shoulder is too little angle and too little pitch, or tilt, of the grader's moldboard. This allows material to flow around the leading edge of the moldboard and build up a high shoulder. Yet another cause of high shoulder is gouging with the moldboard at the shoulder line and not matching the edge of the gravel surface to the shoulder.

Perhaps the greatest controversy in maintaining and reshaping gravel roads is establishing proper crown. While nearly all states have standard specifications for pavement crown-- generally 2%--there is seldom a specification for crown on gravel surfaces. A sound recommendation for gravel roads is 4% crown. This can vary a bit, but too much or too little crown is never good.

In addition to the percent of crown, another issue is the surface profile. Many gravel surfaces have a parabolic (or rounded) crown. This can come from the wrong sequence of blading passes by the grader; especially making the final pass down the middle of the roadway.

However, a greater cause is the center wear that develops in the moldboard's cutting edge. Center wear in the cutting edge will occur in any gravel maintenance operation, but if not controlled, it will leave a rounded or parabolic shape on the roadway. This will leave little or no crown in the center, but excessive crown on the outer edges of the road surface.

This leads to two problems. Motorists will tend to drive down the middle of the road, no matter how wide it is since that area will be relatively flat. Potholes and excessive corrugation, or "washboarding," will appear in the middle of the road as well since drainage will be restricted. Good motor grader operators pay attention to this problem and will make a great effort to keep their cutting edges straight. In recent years, cutting edges with carbide inserts or carbide-tipped bits have appeared on the market. This is a helpful tool because the carbide resists abrasive wear and greatly reduces center wear in the cutting edge. Although expensive, the extra cost is often offset by extra wear life.

The correct shape of the crowned surface is like the letter "A" or the roof of a house with a straight line for drainage from the centerline to shoulder. This is always a challenge--and skilled, knowledgeable motor grader operators are a priceless asset to their company or agency.

Still in need of a quality mix

However, with the best use of a grader, gravel roads still do not perform well unless good surface gravel is placed on them. What is good surface gravel? The answer is a bit difficult. Because of low traffic and low budget allocations, agencies generally have to work with materials that are affordable and available locally.

Local materials vary greatly across the country--ranging from high-quality quarry aggregates to seashells. Some states do not even have a standard specification for surface gravel. Even if the state DOT has a surface gravel (or aggregate) spec, a local agency is generally not required to use it when purchasing material for routine road maintenance. Unfortunately, this leads to many problems on the road, and many grader operators are blamed for problems that are actually material-related.

The primary problems are excess "float," or loose gravel on the surface, excess material loss to the shoulder and ditch and excess corrugation. Despite budget constraints, managers should specify the best material they can afford. It is generally good management to purchase a lesser quantity of good quality gravel than vice versa.

Quality surface gravel is a good blend of stone sizes, sand sizes and fine material. Too often, material that may be good base or cushion gravel is assumed to be good for gravel surfacing. This is not true. Quality base aggregate contains a higher percentage of crushed stone with larger top size and generally few fines. This makes a strong, drainable foundation for pavement.

Quality surface gravel has different characteristics. It needs to have smaller top-size stone and a higher percentage of plastic fines which will allow the gravel to take on a tight, or "bound," characteristic. The road surface should form a crust. This type of material will shed water and drastically reduce corrugation and material loss from the surface.

A real bonus is reduced frequency of blade maintenance which offsets part of the material cost. Gravel that is processed by crushing is almost always superior to screened material. If your state does not have a gravel surfacing specification, a useful guide is the 2001 edition of the AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing, section 147.

The quality of surface gravel becomes especially critical where dust control treatments are applied. Dust control products seldom perform well when applied to poor surface gravel. In summary, it pays to study material available in your region and learn to specify the best material you can afford.

Further help is available by requesting FHWA's Gravel Roads--Maintenance and Design Manual. It is a comprehensive, nontechnical resource and is available through your state LTAP Center or by contacting the FHWA Reports Center, 9701 Philadelphia Court, Unit Q, Lanham, MD 20706. The manual can be viewed on the web at www.ltapt2.org/gravelroad.htm.

About The Author: Skorseth is field services manager for the LTAP at South Dakota State University.

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