5. Conclusions

5.1 Introduction

The role of dust suppression in the context of unpaved road maintenance and associated benefits and problems has been examined and presented in the previous chapters. The three specified additives MgCl₂, lignin, and MgCl₂/lignin blend, were applied on two different sources of road surface material. The dust emission capacity of the resulting test sections were measured and from the results, conclusions can be drawn about the performance of each soil type-additive combination.

The body of knowledge about soil-additive interaction as it pertains to the specified dust suppressants has been reviewed. Conclusions can be drawn on the expected behavior change between the different soil types and the dust suppressant additives. Because the specified chemicals for dust suppression are the same compounds used for road surface deicing, the documented water quality and other environmental impact studies reported by other researchers have been reviewed and conclusions can be drawn about the potential contamination that can be expected for the long-term use of these chemical compounds.

5.2 Road Dust Suppression - Effect on Maintenance

A good quantifiable maintenance scheme is essential to the successful operation of an unpaved road network. In Larimer County, Colo., the Road and Bridge Department with jurisdiction over the county's unpaved road maintenance program has a comprehensive unpaved road maintenance program in place. The program includes: 1) routine maintenance of surface reblading, shaping and compaction to remove ruts, washboarding and potholes and 2) periodic maintenance which includes - shoulder and ditchline reclamation, road realignment if necessary, aggregate replacement, and judicious use of chemical dust suppressants (MgCl₂ and lignin).

From the dust sampling measurements it can be concluded that:

• the Strang Pit gravel possessed less fines (particle size passing No. 40 sieve) than the Horton Pit gravel and this was exhibited by the amount of dust measured from the two sets of test sections. On aggregate, the sample amounts of dust were less from the Strang test sections compared to the Horton test sections.
• Analysis of the Strang test sections (Figure 2.11) indicates that the MgCl₂ treatment produced equal or more dust than the untreated control Strang test section. This could be attributed to the fact that the Strang Pit gravel has less fines and thus the MgCl₂ molecules have less surface area which to attach themselves and to become effective. Therefore, to treat the Strang material with MgCl₂ implies that more than one treatment would be required to maintain the treatment effectiveness.
• Although it was a requirement for the dust measurement testing to be done on an unpaved road section with a reasonable amount of traffic count (between 100 to 250 ADT), for reasons beyond the researchers control the test roads had a much lower than expected traffic count of 25 ADT. At such a low traffic volume the test sections were not subjected to enough traffic pounding to accurately measure the performance of each road soil type and additive combination.
• The plots of the dust sample measurements (Figure 2.11 and 2.12 ) did not show any consistent increase in dust emission with treatment age as expected because with time the treatments lose their effectiveness and the test sections become more dusty as a result. This lack of pattern in the dust emission can be attributed to the low traffic volume on the test roads.

5.3 Road Dust Suppression - Effect on Stabilization

Soil stabilizing agents are used to improve and maintain soil moisture content, increase soil particle cohesion, and serve as cementing and waterproofing agents for certain soils. The specified dust suppressants or additives used in this test produced changes in the soil properties that have influenced the test road surface stabilization.

The in-depth application of the lignin, for example, produced a road surface that was firm, smooth, dust free, and comfortable to drive for most of the test period of nearly a year. Field observation of the lignin-treated test sections indicated that the lignin acted like cement, binding the soil particles together into a hard surface that show strength gains over time. The MgCl₂ and MgCl₂/Lignin blend treated tests sections likewise showed physical changes indicating a stabilized road surface. Although no field-based testing was done to measure the soil strength increases due to the use of the dust suppressants, there was enough field-observed evidence to indicate surface stabilization. The measured dust amounts presented earlier are also an indirect measure of the stabilization achieved by each different treatment.

5.4 Road Dust Suppression - Effect on Safety and the Environment

Although the application of dust suppressants significantly reduce the emission of dust which impairs motorist visibility and leads to other road hazards, the suppressants are not without their negative effects. The use of the suppressants, especially chloride compounds, on soils with high clay content such as the Horton Pit gravel can produce a slippery road surface under wet conditions. Even under dry conditions they produce driving surfaces that are perceive by motorists as slippery.

In terms of environmental concerns, the use of the suppressants reduces the unpaved road particulate matter loading into the atmosphere by controlling the fugitive dust generated from the unpaved roads. Because dust suppressants are water soluble and contain contaminants regulated by the EPA, their use should be monitored for any environmental degradation. No environmental degradation from the use of the specified dust suppressants were measured in this study. However, results presented by other researchers indicate that only very small amounts of these contaminants enter the environment and therefore their impact is presently not an issue.

5.5 Recommendation for Future Studies

Although unpaved road dust control has been ongoing for several decades and some research studies have been done to measure the effect of dust suppressants on unpaved roads, there is still the need for more information about unpaved road maintenance in general and the strategic use of dust suppressants in the maintenance scheme.

As a start, a research study to review all studies done in this country and elsewhere on unpaved road dust control is recommended. Such a study should identify and categorized each study - whether laboratory, field-based or both; the objective of the studies; and the specific subject area addressed - environmental concern, suppressants effectiveness, road surface stabilization, soil-suppressant interaction, cost analysis, and etc. The study should also identify all areas of interest relating to unpaved road maintenance and the use of dust suppressants. A gap analysis should be performed to identify areas that need research. Such research should serve as a basis for a holistic appraisal of unpaved roads and the use of dust suppressants.