1. Introduction

1.1 Background and Problem Statement

Dust generation from unpaved roads because of vehicular traffic and wind means loss of road surface material (aggregate or native soil). Subsequently, the loss of surface material fines in the form of dust leads to the formation of ruts, potholes, and corrugation. The amount of dust generated and the consequent fines loss is determined primarily by the volume of traffic using the unpaved road as well as the speed, weight, and number of wheels of the vehicle. The abrasive resistance of the road surface material and the amount of fines in the initial road surface material mix are also important contributing factors (U.S. EPA, 1988). The climatic condition of a region is also a contributing factor affecting the dust generation equation. Long dry spells that often occur in semiarid and arid regions can aggravate unpaved road dustiness (Colorado Transp. Info Btg #3, 1989).

Fugitive dust from unpaved roads is noted as a major non-point source contributor to the particulate loading in atmospheric air pollution (AQMCP, 1985). To residents living along unpaved roads, the traffic-generated dust penetrates their homes causing a nuisance and health problems such as hay fever and allergies. Crops and vegetation near unpaved roads can be covered with the airborne dust stunting their growth due to the shading effect and clogging of the plant's pores. Fine particles resulting from traffic actions can also be washed off during precipitation events and carried into nearby creeks, streams, and lakes increasing their respective particulate loading. For motorists using the unpaved roads the traffic-generated dust can reduce visibility and cause driving hazards.

These conditions represent a significant material and economic loss. As a result, road departments that maintain unpaved roads spend a substantial part of their total budgets on aggregate replacement and maintenance. In Larimer County, Colo., for example, 12 percent of a total budget of $10 million in 1994 was spent on aggregate replacement cost alone. Another 18 percent of the total budget was spent on periodic maintenance of the county's unpaved roads. The county had nearly 1,100 miles of road under its jurisdiction. More than 700 miles (65%) were unpaved (Addo et al., 1995). Hoover, et al., (1981) reported that in 1978 the secondary road departments of Iowa's 99 counties spent about $32 million for aggregate replacement. The U.S. Forest Service, which is in charge of more than 325,000 miles of aggregate and earth-surface roads spent more than $64 million on unpaved road maintenance, which included more than $25 million on dust control alone in 1985 (Irwin, et al., 1986).

The age-old solution to the control of dust and the resulting road surface material loss has been dust suppression. Unpaved road dust suppression methods range from reduction of vehicle speed and application of water to the application of organic and non-organic chemical compounds. Not many studies have been done to test the relative effectiveness of the many proprietary dust suppressants. However, mixed effectiveness results have been reported by a few researchers (Hoover, et al., 1981; Lane, et al., 1984; Addo and Sanders, 1995). Review of these earlier studies indicate that more research is needed to compare the various dust suppressants' effectiveness, determine optimum application doses, and determine the most cost-effective application techniques. Due to the varied sources and different types of unpaved road surface material (aggregate) available, research to study the relationship between the aggregate distribution and the effectiveness of the dust suppressants need to be pursued. The value of dust suppression as part of an integrated unpaved road maintenance and road surface upgrade schemes is unknown. Likewise, the environment impact of the use/nonuse of dust suppressants is unknown.

1.2 Objectives

The objective of this research is to continue the research began at Colorado State University several years ago to establish the cost effectiveness of using chemical dust suppressants to increase the time between routine road maintenance on low-volume unpaved roads (Sanders, et al., 1997). This research study will investigate the quantitative contribution of road dust suppressants on the maintenance of unpaved roads. Since aggregate replacement in unpaved road maintenance is one of the main cost elements, the relationship between different aggregate characteristics and the quality and effectiveness of different dust suppressants or soil additives will be measured. A project measuring the effect of multiple dust treatments in stabilizing the road base prior to surface upgrade such as chip and seal in extending the life of the road will be initiated. The particle size distribution of the road dust with and without treatment will also be determined as will other environmental impacts. The effect of dust suppressants on the driving comfort and safety of unpaved roads will also be examined.

1.3 Scope of Work

To achieve the project objectives, the work breakdown, including but not limited to the following, shall be implemented:

• Consult with Larimer County road officials to determine a suitable unpaved county road as a candidate for testing. The selected road shall: 1) have reasonably high average daily traffic (ADT) count, 2) need aggregate replacement, 3) be long enough to accommodate eight 1/2-mile test sections, and 4) in the near future be a candidate for higher surface upgrade such as chipped and sealed.
• Prepare unpaved road and construct test sections using additional fresh aggregate material from the two different Larimer County gravel pits. Four of the test sections will use aggregate material A and the other four will use aggregate material B. Lignin-sulfonate-treated, magnesium chloride-treated, lignin- and magnesium-chloride blend-treated, and a control untreated test section will be provided for each of the two aggregate base materials being evaluated.
• Conduct a traffic survey to determine the volume and type of traffic using the test sections.
• Measure dust emission from each test section using the Colorado State University Dustometer.
• Perform vehicle braking (stopping) distance test on test sections under dry and wet driving conditions.
• Record all accidents on the test sections and investigate Larimer County records for traffic accidents on unpaved roads to determine if there is a relationship between the number of accidents and the type of road surface treatment (or lack thereof) on the unpaved roads.