Per the Douglas County website: the Department of Public Works Operations is responsible for the maintenance of approximately 300 center lane miles of gravel roadways within Douglas County. These roadways range from rural collectors with several hundred vehicles per day from neighborhoods to local roads that carry less than 20 vehicles per day. Due to the nature of gravel roads, periodic routine maintenance is performed to retain a surface that is safe to travel. There are several factors that determine the level of maintenance required on a gravel road:
- Quality of gravel surface
- Speed of traffic
- Volume of traffic
- Weather conditions
A motorgrader is used to maintain the surface of a gravel road. It can cut out surface irregularities such as washboards, potholes and ruts to improve driver safety. Washboards and potholes typically form on hills, curves, flat areas and near stop signs where vehicle braking occurs. Traffic and speed impact the frequency and severity of washboards formation.
In order to cut the compacted gravel to a sufficient depth to remove surface irregularities moisture is required. To get adequate moisture for proper compaction and a tight surface, water is frequently applied using tankers with spray bars. Roads are frequently bladed after a rain.
Over time, the aggregate surface on gravel roads is lost from deterioration due to vehicle traffic and the effects of weather. When the stability of a gravel surface has substantially deteriorated, new gravel is applied to the road. The average interval between new gravel applications varies based upon traffic volume and other factors, but is normally performed every 5-7 years.
Douglas County performs dust mitigation on approximately 80 center lane miles of gravel roads each year. Some of the reasons for the application of dust mitigation products are:
- To comply with State of Colorado Regulation 1 that requires the mitigation of dust on Douglas County gravel roads with average daily traffic volumes exceeding 200 vehicles per day
- Mitigate dust due to resident health concerns
- Decrease the amount of routine maintenance required
- Reduction of dust to improve sight distance
Processes and products used for dust suppression
Douglas County currently uses two application processes to mitigate dust. Both processes involve the application of a suppression material containing magnesium chloride. The two processes are described below:
Soil Stabilization Method
The majority of dust suppression performed in Douglas County is done through the use of a soil stabilizer. A soil stabilizer is used to evenly distribute the liquid dust suppression material into the top 3 inches of the gravel surface. This method requires additional equipment to grade the road and compact the 3 inches of disturbed gravel.
The dust suppression material currently being used in conjunction with this process consists of a 70/30 blend of magnesium chloride and lignin sultanate. Lignin sulfonate is typically derived from the processing of wood and paper products. This method is typically used on roads that carry higher volumes of traffic because the 3 inch depth of stabilization will typically retain a longer-lasting surface integrity.
Topical Application Method
A topical surface treatment is placed on gravel roads that have lower volumes of traffic. The topical method is faster than the soil stabilizer method, however, the liquid dust suppressant only penetrates approximately ¼ inch below the gravel surface.
Douglas County is currently testing a topical product called Durablend as manufactured by Envirotech Services, Inc. who states that this proprietary product contains magnesium chloride with polymers that has been formulated to reduce the amount of magnesium chloride that leaches into the environment and that can affect the health of roadside vegetation. Douglas County is currently conducting chloride testing on roadside vegetation to determine the product effectiveness.