Acknowledgements

Support for this project was provided by the Mountain-Plains Consortium (MPC) as part of the University Transportation Centers Program (UTCP). MPC member universities include North Dakota State University, Colorado State University, University of Wyoming, and Utah State University. The UTCP is funded by the U.S. Department of Transportation.

Special recognition is given to the Larimer County Department of Roads and Bridges for its full participation in this project.

Disclaimer

The contents of this report reflect the views of the authors, who are solely responsible for the facts and the accuracy of the information presented. No researchers from the other MPC member institutions participated in the work. This document is disseminated under the sponsorship of the Department of Transportation and UTCP in the interest of information exchange. The U.S. Government assumes no liability for the contents or use thereof.

Preface

This report describes research conducted at Colorado State University to evaluate the effect of road dust suppression on unpaved road maintenance schemes. A field-based method was used to measure the effect of road soil physical characteristics on the effectiveness of some of the commonly used dust suppressants. The study also evaluated the stabilization of unpaved road base material because of the use of dust suppression. The effect of dust suppression on safety and driving conditions on unpaved roads was examined. The chloride compounds and ligninsulfonate commonly used as dust suppressants are water soluble and can be leached into the environment. They contain chlorides, heavy metals, and organic compounds that are regulated. Their potential to have adverse environmental impact was examined.

Executive Summary

This research was undertaken at Colorado State University in cooperation with the Larimer County Department of Roads and Bridges.

A large portion of U.S. road network is made up of unpaved roads that usually carry a very small volume of the nation's vehicular traffic. Unpaved roads are mostly rural farm-to-market roads, forest service roads, and timber haul roads. They are easily and economically constructed using locally available soil usually with poor engineering properties or imported nearby soil with much better engineering properties. The use of unpaved roads causes dust emission into the atmosphere, loss of the road surface material over time, and frequent road surface deterioration in the form of ruts, washboarding, and potholes. Influenced by the traffic volume, these problems can lead to high economic cost.

To reduce the loss of road surface fines in the form of dust, chemical additives (dust suppressants) are applied to the unpaved road surface to control dust generation and to improve the road surface stability. Commonly used dust suppressants are chloride compounds MgCl₂, CaCl₂, and ligninsulfonate. The key to effective dust control and surface stabilization depends on the resulting interaction between the soil type and the additive. Chloride compounds (salts) and lignin exhibit different characteristics in controlling dust. The salts alter the moisture-holding characteristics of the soil by attracting and holding moisture from the atmosphere to keep the road surface fines moist so they bind to the coarser particles through capillary-dependent apparent cohesion. Lignin acts like a cementing material and binds all the soil particles together into a homogeneous mass. Both of these additive types modify the soil structure, thus affecting the long-term strength characteristics of the soil. The different road soil types also employ different mechanisms to ensure stability. Granular-type soils depend on the angularities of the particles to interlock particles during compaction to resist volume change and the lateral flow of the particles. Conversely, fine-grained or clayey soils depend on the moisture content of the soil to resist volume change and the lateral flow of particles.

Laboratory studies investigated the strength and density variation caused by different additive concentrations in different types of road soil (Palmer, et al., 1995). This study however, used field-based methods to measure the effect of road soil characteristics on the effectiveness of some commonly used dust suppressants in the context of unpaved road maintenance. The effect of dust suppression on the stabilization of the road soils in general was also examined. Safety and environmental concerns arising from the use of dust suppressants were reviewed as well.

Virgin road soil material from Larimer County's two main gravel borrow pits (Strang Pit and Horton Pit) were used in constructing test sections on County Roads (CR) 11 and 68 in the northeast part (Weaverly area) of the county. Each road soil material was treated with MgCl₂, lignin, and MgCl₂/lignin blend on half-mile test sections. In all, four pairs of test sections with one pair serving as a control untreated test sections were evaluated. The Colorado State University Dustometer was used to quantify the dust emission capability of each road soil type/suppressant combination. A few of the conclusions and summaries of the study are listed below:

• The Horton Pit material has more fines than the Strang Pit material. The fines portion of the Horton material is clayey, while the fines portion of the Strang material is cohesionless.
• Regardless of the road soil type, the use of the dust suppressant reduced the dust emission capacity of the unpaved road.
• Evaluation of the performance of the Strang MgCl₂ treated test section indicated the dust suppression capacity of the MgCl₂ had completely been depleted within a year of application such that the Strang MgCl₂ test section produced equal or more dust than the Strang untreated control section towards the end of the testing period. This also indicated that more than one treatment is required per year to maintain the effectiveness of the MgCl₂ when used with the Strang material.
• The lignin and the MgCl₂/lignin blend treatments performed nearly the same with both road soil materials.
• The Strang test sections produced less dust overall than the Horton test sections.
• The use of dust suppressants effects soil structure and can therefore influence soil stabilization but the degree of stabilization is still mixed.
• Although dust suppressants contain contaminants regulated by the EPA, their effect on water quality and other environmental impact is very small.