Chapter 3. Data Gathering

Test Sites

Five test sections were evaluated in this project, all of which are located in Larimer County - Colorado. The treated sections are four in number and are located on the same stretch of road in the Loveland area of the County. The stretch is County Road (CR)12 which changes into County Road (CR)29. The untreated test section is located on CR-40 which is in the Fort Collins area. CR-40 and CR-12/29 are within 20 mile radius of each other. Figure 1 shows part of the map of Larimer County with the locations of the test sites highlighted.

The climatic condition of this region can be described as semiarid with annual precipitation in the test site area being approximately 14 inches/year. The generalized soil/aggregate characteristics of the sites can also be termed glacial till/some silty clay/gravel. The average daily high temperature during the dust production season - late May to October is approximately 78°F with a relative humidity of about 33 percent.

The treated test section CR-12/29 is a five-mile stretch and has an average daily traffic (ADT) of about 400. This unpaved road links two paved roads in the area CR-23 and CR-29. Carter Lake which is a recreational area is also within a 15-mile radius of CR-12/29 thus the treated sections besides serving a rural community of livestock owners, carry a substantial portion of the traffic visiting the lake. The composition of the traffic on this road vary from cars to light trucks to heavy tandem axle vehicles. The untreated test section CR-40 like CR-12/29 links Horsetooth and Harmony roads both carry very high volume traffic. It also serves a historic site - the Strauss Cabin and a small recreation lake in the area. The average daily traffic (ADT) is about 200, and the composition of the traffic can be said to be of all types and sizes of vehicles.

Figure 1. Map of Larimer County

Prior to the start of this research project there was stage construction planning by the Larimer County road officials. The CR-12/29 had been treated twice with Magnesium Chloride, once a year for two years. The treatment evaluated in the research is the third in succession of a road stabilization program that started three years ago. The untreated section did not have any prior treatment.

The construction of the treated test sections followed the procedures recommended in most of the transportation literature and that of the dust suppressants suppliers. Important application techniques for many dust suppressants include; a) road surface scarification, b) adequate grading and smoothing of the road surface, c) application of the dust suppressant in quantities suitable for effective control of dust, and d) proper road finishing procedures that include the forming of the surface crown, optimum compaction of the road surface and proper drainage (Rural Transportation Fact Sheet #84-02, 1984). Preconstruction of these test sections consisted primarily of blade dressing of the road shoulders and ditch line and reclaiming of aggregate pullout. Finally the surface was scarified, mixed with the particular suppressant and compacted to a 6-inch thick wearing course.

The gravel used for the construction of the test sections was from the same gravel pit. Examination of the AASHTO classification indicated a A-2-4 class, rated as good subgrade material. The four dust suppressants evaluated in the research are:

1. Calcium Lignosulfate. A by-product of the paper pulp industry, this form of lignosulfate was supplied by Georgia Pacific Corporation. The application rate was one-half gal/sq yd - supplier's recommendation.
2. Calcium Chloride (35 percent CaCl₂ in a water solution). A deliquescent and hygroscopic by nature. The compound was supplied by Hill Brothers Chemical Company. The application rate was one-quarter gal/sq yd - supplier's recommendation.
3. Magnesium Chloride (32 percent MgCl₂ in a water solution). A deliquescent and hygroscopic by nature. The compound was supplied by Envirotech Services, Inc. The application rate was one-half gal/sq yd - supplier's recommendation.
4. Calcium Chloride Special. A CaCl₂ based compound. The difference between the CaCl₂ Special and CaCl₂ is that according to the suppliers the CaCl₂ Special brand contains no magnesium whereas the CaCl₂ brand contains fractions of magnesium. The compound was supplied by Hill Brothers Chemical Company. The application rate was one-quarter gal/sq yd - supplier's recommendation.

The position of each treatment on the stretch is labeled 1-to-4 as shown in Figure 2. The length of each treated test section is 1.25 miles long. Table 1 describes the features in the terrain of each test section.

Figure 2. Position of each treated section on unpaved road

The number of curves, uphills/downhills and how steep the slopes are as well as the drainage condition of a particular test section not to mention the volume of traffic among other things are very important factors that determine how well a treatment holds up. Table 2 shows the volume of traffic measurement in terms of average daily traffic (ADT) on the test sections.

Untreated section ADT is estimated as 200 (personal communication with Bill Heiden-Director Larimer County Roads and Bridges Department).

Dust Measurement

Although road dust research has been going on for several decades now, it is apparent from reviewing previous works that despite all the money and time invested, quantitative measurements of dust from roads have been practically nil. Without any quantitative dust measurement, it is difficult if not impossible to assess the economics and lasting value of dust palliation methods. With the above mentioned problem in mind, one of the said objectives of this project was therefore to develop a dust monitoring method that would be quantitative, reproducible, portable, cost effective and easy to operate.

From the literature review apparently there have been various attempts by different people to develop devices and procedures that would measure road dust. Notable among them are Schultz (1983), Wellman and Barraclough (1972), Hoover et al. (1973), Langdon (1984) and Irwin et al. (1986) to name a few.

Some of the measuring techniques used stationary devices which characterize road dustiness at a particular point while others used moving devices which provided a dust measurements that describe a section of road. The dust quantification method common among all the previous works was either the weighing of the collected dust or the measurement of air opacity using visible light.

In this research project, "Effectiveness and Environmental Impact of Dust Suppressants", early attempts to use the dust collecting bucket method according to ASTM D-1739 (see Figure 3) in the summer and fall of 1992 proved to be ineffective and inefficient. The reasons were: (1) it was difficult to obtain permission from the land owners to setup the dust collectors, (2) the land along the test sections are grazing pastures, and (3) there was the problem of livestock continually tampering with the dust collectors. The method also requires so many vehicles to drive by the buckets before a substantial amount of dust can be collected for quantification and there was a problem with wind.

Figure 3. Dust Collecting Buckets after ASTM D - 1739.

For the dust monitoring program of the project from spring through fall 1993 a decision was made to develop a device and procedure that measures the dust production from the test sections in-situ and on a real time basis. Considering the inherent variability associated with road dust, the method should measure dust along a section of road rather than at a single point. The method should also be quantitative and reproducible. Modeling a device and procedure after Langdon (1984) who used a portable cyclone dust collector mounted on the back of dust generating vehicle, the Colorado State University Dustometer was developed, field tested and used in this research.

The moving dust sampler is illustrated in Figure 4. The device consists of: (1) a quarter-ton pickup truck, (2) a 3,000 watt electric generator, (3) a standard high volumetric suction pump, (4) a fabricated metal box that contains a 10 in by 8 in (25.40 cm X 20.32 cm) glass fiber filter paper, (5) a metal bracket attached to the bumper of the truck, and (6) a 2 in (5.08 cm) internal diameter flexible tube for connecting the suction pump to the filter box. The filter box is rigidly secured to the bumper by way of the bracket behind the left rear tire of the quarter-ton pickup truck. The generator and the high volumetric sampler are secure in the bed of the truck. Figures 5-7 show the components of the setup. The filter box when mounted behind the truck is horizontally aligned with the left rear tire and the distance from the center of the tire to the front of the box is three feet. There is a vertical clearance of one foot between the bottom the box and road surface.

The design of the box and bracket system is such that although the box is rigidly secured to the bumper of the truck after the setup, the box can be mounted and dismounted with relative ease. The bracket is made out of 1/4 in thick 6 in x 6 in steel angle treated with a rust-proof paint. It is fixed to the bumper by two bolts as shown in Figure 7. The filter box is fabricated out of a steel sheet. It has 12 in x 12 in opening that is covered with a 200 micron mesh sieve which faces the tire. The 200 micron sieve prevents any non-dust particles from being drawn onto the filter during dust measurement. The bottom part of the box contains a sieve that supports the filter paper. The box is designed to allow for easy replacement of the filter paper.

To run a typical dust measurement test the device is setup as shown in Figure 4 with the flexible tube connecting the pump to the filter box. The generator is started and by the aid of a switch the pump is turn on and off from the cab of the truck. One mile long sections have clearly been demarcated on the 1.25 mile sections. With the filter box fitted with preweighed filter paper and generator started, the truck is driven at the stipulated speed of 45 miles per hour (MPH). Right at the beginning of the mile mark the pump is turned on and a portion of the dust kicked up by the left rear tire is drawn onto the filter as the truck traverses down the road, at the end of the mile run the pump is turned off and the truck is brought to a stop. Three runs are made in the same driving lane for each test section. The dust laden filters are taken to the laboratory and weighed. The average of the three measurements for each test section are compared to assess the relative effectiveness of the different treatments.

Figure 4. Colorado State Dustometer (Moving Dust Sampler Method)

Figure 5. Electric Generator and High Volumetric Suction Pump in the Bed of Truck

Figure 6. Filter Box and Connecting Flexible Tube

Figure 7. Metal Bracket Fixed to Bumper of Truck

Runoff Sampling

Similar to the problems associated with road de-icing, there may be possible environmental problems of water quality degradation in both surface and subsurface water due to the long term use of dust suppressants. All of the four compounds evaluated in this research are water soluble. A runoff sampling analysis program was designed to help answer some questions about how much of suppressants are being washed off during rainfall events. The sampling technique employed involved the use of plastic containers installed at the shoulders of the test sections. The locations selected for the installation of the containers were such that during a rainfall event and when there is adequate runoff from the road surface some amount of the runoff would enter the containers. For the non-road runoff sampling, containers were installed near the test sections but at a distance far enough up gradient to insure only non-road surface runoff is collected. Figure 8 shows an installed container on the shoulder of a test section. After a rainfall event all the containers are inspected and emptied. The sampling containers are reinstalled and the samples are sent to the laboratory for analysis.

Figure 8. Runoff Sampling Container Installed at Shoulder of Road