Dust, Washboards, and Deep Stabilization

The Copper Creek Forest Service Road is a 20-ft.-wide, unpaved road located in the Helena National Forest in western Montana. Similar to many unpaved roads, it has a history of dusting up in the summer and washboarding. In fact, severe washboarding occurs even though the road is bladed, watered, and recompacted three times each year. The United States Department of Agriculture Forest Service (Region 1 Materials Engineering Section, Missoula, MT) conducted an evaluation of alternate chemical stabilization treatments on the road. This evaluation stemmed from concerns about ongoing requirements for road maintenance needed to maintain acceptable ride quality and to eliminate excessive sediment runoff from the road into nearby streams. The goal was to identify a method to improve ride quality, reduce dusting and sediment runoff, reduce aggregate loss, and minimize ongoing maintenance.

The Copper Creek road comprises good-quality, well-graded, crushed aggregate. Traffic volumes range from 20 to 50 light vehicles per day from mid-May through November. Vehicle speeds are typically faster than 35 mph.

In June 1998, 17 test sections were constructed using seven different maintenance techniques (Table 1). Grades in the test sections were approximately 2%. Bentonite clay and 77% flake calcium chloride were used as stabilizing agents on eight of the test sections. The treatment methods included traditional blading, watering, and compaction, as well as mixing the stabilization agents with the aggregate to a depth of 2.5 in. The stabilization agents were applied and blended into the aggregate with the use of an in-place processor operated by Triple Tree Inc. of Missoula. The processor is designed specifically to grind native road surfaces to a specified depth and is essential to ensure that the product is adequately mixed. Use of dry calcium-chloride flake ensures even blending at high application rates of product with one pass of the processor.

Among the treatment methods evaluated, the best performing was the 77% flake calcium chloride applied at a rate of 4.2 lb./yd.² and blended with the in-place processor to a depth of 2.5 in. This application rate is designed to provide a calcium-chloride concentration (100% basis) in the aggregate of approximately 1.3% by weight. After 54 weeks, the section stabilized with calcium chloride still had not required reblading. The other sections all required grading at least twice because of road-surface deterioration. Calcium chloride, when incorporated as a roadbase stabilizer, has a characteristic ability to attract and hold moisture at low humidity and high temperatures. This property enables calcium chloride to keep unpaved road surfaces damp and minimize the loss of fines under severe summer conditions. With its strong moisture film and high surface tension, calcium chloride helps bind aggregate particles together. Consequently, unpaved road surfaces remain compact and stable.

Cost: The Big Picture

Although the initial cost per mile of applying calcium chloride at the 4.2-lb./yd.² rate is higher than most of the other treatment options tested in the study (Table 2), a cost comparison based on maintenance of comparable road quality shows this to be the most cost-effective option.

From Table 1, the number of poor weeks of road serviceability identified by the Forest Service over a 12-month post-treatment evaluation period was used to calculate the number of additional gradings required to maintain a smooth road-surface quality. For every two weeks of poor road quality, the cost of one grading was added. For example, if a treatment method resulted in 10 weeks of poor road serviceability, then five additional gradings (5 × $600) would be needed to provide a good-quality road surface over the length of the study. These costs were then combined with the total cost per mile from Table 2 to give a more accurate comparison of the cost to provide a smooth road surface with each treatment method. The results of this adjustment are shown in Table 3. When downstream maintenance costs are considered, the 4.2-lb./yd.² flake calcium-chloride treatment clearly becomes the most cost-effective of all options.

Reducing Sedimentation and Airborne Dust

One of the largest sources of sedimentation in rivers, streams, and lakes is surface runoff of fine-grained material from unpaved roads. This sedimentation disrupts fish habitats in nearby waterways. The surface runoff occurs during rainstorms and results from snowmelt. The problem is worsened whenever a road is bladed because fine-grained materials are in a loose, vulnerable state. Rain shortly after a blading washes a significant amount of fines to nearby watercourses. According to Bob Griel, an engineering consultant with Triple Tree Inc., “The stabilization of this fine material has the potential to keep thousands of tons of sediment out of streams where bull and brown trout and salmon live.” As for the elimination of airborne dust from vehicular traffic, a visibility and dust nuisance problem for road users has been virtually eliminated.

Based on the results of this study, the Forest Service Region 1 recommends the stabilization of roads with flake calcium chloride where ongoing annual blading is taking place to minimize maintenance costs and reduce environmental impact associated with sedimentation and airborne dust. In June, the Forest Service used calcium-chloride flake to stabilize two areas that previously required frequent blading maintenance, primarily because of washboards. The Toll Mountain road east of the Continental Divide near Butte, MT, was a short test project similar to Copper Creek. Various calcium-chloride flake application rates were blade mixed about 2 in. deep. This project is located in a low-humidity area. Surfacing material was crushed aggregate, and road grades were from 6% to 10%. Twelve miles of the Selway River Road in northern Idaho were also treated with 5 lb./yd.² of calcium-chloride flake to about 3-in. depths with the in-place processor. This road is located in a high-humidity area adjacent to the Selway River. The surfacing materials consisted mainly of a native decomposed granitic, and road grades were generally less than 2%. Evaluations conducted in November 2000 indicate that both roads’ performance is similar to that of Copper Creek. Additional evaluations will be needed in 2001 to determine cost-effectiveness.