2. Road Dust Suppression: Effect on Unpaved Road Maintenance

2.1 Introduction

Two-thirds of the road network system in the United States and nearly 90 percent of the roads in the world are unsurfaced or lightly surfaced low-volume roads. In Colorado, about 70 percent of the more than 45,000 miles of local roads are unpaved (Colorado Transp. Info, 1989). Most of the unpaved roads are located in rural and forest areas, although cities and towns also have their fair share.

Most traffic volume is carried by the surfaced (paved) road system and thus much effort in research, construction, and maintenance has centered on paved roads. Nevertheless, to the federal, state, and local authorities in charge of unpaved roads, the problems associated with unpaved roads, maintenance and their costs, are not lost on them. In addition, especially in Colorado, almost all local roads are experiencing ever-increasing traffic volume and vehicle weights. Population growth and tourism are making ever-increasing demands. So are logging trucks and other commercial vehicles carrying much heavier loads. These higher volumes and greater loads are putting a strain on local road maintenance and reconstruction budgets.

2.2 Unpaved Road Definition and Classification

Unpaved road can be defined as engineered or tracks (Paterson, 1987).

1. Engineered Roads: unpaved roads with controlled alignment, defined width, cross-section profile, and drainage.
2. Tracks: unpaved roads that generally evolved from primitive trails - paths of least resistance. First created by wild animals, later used by settlers, and as user needs increased, evolved as part of the low-volume road network. They may have:
   1. topsoil removed
   2. topsoil not removed.

In general, unpaved road classified as part of the low-volume road network are either engineered or partly engineered. Tracks are usually not considered as part of the low-volume road network. Most research studies relating to the effects of deterioration and maintenance of unpaved roads have mainly involved engineered unpaved roads because of available data for analysis (Boresi et. al., 1993).

There are many classifications for unpaved roads. The USDA Forest Service classifies its roads by maintenance levels. State highway departments also have different classification criteria and the criteria might be different from state to state. For the purpose of this report, the classification as provided by Paterson (1987) is considered here. The classification includes:

1. Gravel Surface Roads: usually engineered and the surface material includes sand and gravel or crushed rocks.
2. Earth Surfaced Roads: this terminology sometimes denotes:
   1. a track as opposed to an engineered road.
   2. all unpaved engineered roads with surface material that does not meet the material gradation specification for gravel.
   3. unpaved roads that have a surface material of predominately fine soil with more than 35 percent finer than 0.075 mm (passing No. 200 sieve).
   4. unpaved roads with native soil ("dirt") as the surface material.

2.3 Unpaved Road Characteristics

Figure 2.1 illustrates the characteristics of good unpaved road. The main essential elements are the crown and drainage. A proper crown is required to enhance the roads usefulness, improve drainage, and ease maintenance. The crown is created in the center of the road and in general should be 1/3 inch to 1/2 inch higher than the shoulders for each foot of lane width (Colorado Transp. Info, 1989). For example a 20 ft. wide road (two 10 ft. lanes) will have a 3 inch to 5 inch crown as shown in Figure 2.1. This degree of slope is intended to drain surface water (snow melt and rain water) without washing off the road surface material. The road surface should also slope in a straight, uniform line from the crown to each shoulder edge. The crown may vary within a local jurisdiction because of the quality of gravel material, roadway grades, and weather conditions. The crown may be increased to 3/4 inch per foot, however this is the recommended maximum slope (Colorado Transp Info, 1989).

The other essential element of an unpaved road is drainage. Without proper and adequate drainage the road surface can become flooded during wet weather conditions. Ponding of water on the road surface can lead to structural damage of the base and subbase requiring costly maintenance. Proper drainage is achieved through cross surface drainage and side ditches collecting run-off from the shoulders and conveying it to streams and other natural drainages. The most economical ditch to construct is the V-shape (Figure 2.1). The foreslope of the ditch (next to the shoulder) is required to slope at least as much as the shoulder and usually more to prevent water from flowing back onto the shoulder. A slope of 4 to 1 or flatter is recommended (Colorado Transp Info, 1989). The backslope may be steeper than the foreslope.

Another important additional element is the use of good quality aggregate mix as the road surface material. The aggregate mix should consist of gravel or crushed rock, sand, and fines (silt and clay) in the right proportions to enable the road to support traffic loads, resist abrasion, shed water, and enhance the surface material's ability to absorb and disperse moisture. The percent of each component in a good aggregate mix can vary depending on the source of material. In general there should be 40-80 percent gravel or crushed stone, uniformly graded from 1/4-inch to 3-inch diameter size; 20-60 percent sand, smaller than 1/4 inch in diameter; and 8-15 percent fines of the total aggregate weight (Woods, 1960).

2.4 Type of Aggregate (Gravel) Mixes)

The type of gravel material available for use in engineered unpaved road depends on the source of material for the area. In general gravel can be described as one of the following:

1. Pit Run Gravel: gravel mined out of natural deposit, very often an old stream or river bed.
2. Washed Gravel: gravel in which excess fines have been removed by washing.
3. Screened Gravel: pit run gravel with oversized stones removed. The maximum size in gravel mix should not exceed two-thirds of the thickness of the layer of road bed being placed (Woods, 1960).
4. Crushed Gravel: this type of gravel is produced by running pit gravel including oversized stones through a mechanical crusher. The resulting material usually has angularities and uniform gradation. Crushed gravel is preferred for the construction of gravel roads because the angularities of the material provide better interlocking between particles. The voids between the large particles are also easily filled by the smaller and finer particles thereby forming a dense mass when compacted.

2.5 Unpaved Road Deterioration Mechanism

A properly engineered and constructed unpaved road comprised of a well-blended aggregate mix, compacted at optimum moisture, and featuring adequate drainage can provide an all-weather road surface capable of supporting high traffic volume over a considerable length of time before deteriorating.

For unpaved roads, unlike paved roads, the mechanism of surface deterioration is a simple progressive process. The fines in the road surface material mix with moisture to act as the binder or glue that holds the coarser aggregate together to provide the firm, smooth riding surface for traffic. The fines under the abrasive action of vehicle tires gets pulverized as surface moisture dries out, especially during dry conditions. The pulverized fines are then swirled off as dust. The persistent loss of fines in the form of dust leads to the coarser aggregate being pried loose by traffic action. Progressively, the road surface begins to unravel, leading to the formation of ruts, potholes, and corrugations.

Small ruts or depressions formed under traffic action accumulate snow melt and rainwater during wet weather conditions. When vehicle wheels roll over these water-filled depressions, the fines are immediately suspended and splashed out, starting the formation of potholes. Once started, potholes grow rather rapidly in the presence of water and additional traffic. Freezing and thawing during the winter months can also accelerate the pothole formation process. The accumulation of loose gravel material on the road surface in a wave-like pattern is called washboard effect or corrugation. This occurs as a natural result of the vehicle tire and loose surface material interaction.

According to many of the researchers that have studied deterioration and maintenance of unpaved roads (Robinson, 1980; Heath et. al., 1980; Visser, 1981; Paterson, 1987 and 1991), the main factor that strongly affects the deterioration of unpaved roads is the behavior of the road surface material under vehicular activities and the environment. Hodge et al., (1983), in a USDA Forest Service unpaved road deterioration study reported that there is a strong correlation between vehicle tire structure and pressure and unpaved road deterioration, particularly with regards to the tendency of unpaved roads to corrugate. Figure 2.2 and 2.3 show unpaved road surface corrugations. For unpaved roads, washingboarding effects or surface corrugations are a measure of the roughness of the road. Road roughness is a characteristic of the longitudinal profile of the road surface. However, as stated by Boresi and Palmer, (1993) "a significant measure of road roughness must include the effects of road surface profile, characteristics of the vehicles that travel over the road, and sensitivity of the vehicle occupants."