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by Curtis Glasoe,
PE/PLS, NDLTAP Western Energy Technical Support Representative
(the info below is an enhanced version of Curt's presentation at the December 2016 WDEA Killdeer Roundtable)
Gravel roads are dusty. For many roadways, the dust creates unsafe driving conditions. Additionally, loss of fine material degrades the ability of the driving surface aggregate to bond together and shed water. Logically, increased traffic volumes result in increased dust. So what can we do to reduce the dust? As traffic volumes exceed 100 ADT, consideration should be given to the application of dust control.
Spot applications in front of residences near roadways may suffice for some roadway segments. As traffic volumes increase, dust control for full roadway sections should be considered. According to various studies, the volume of gravel material that is blown away or lost from the surface can be estimated at 1 ton of material per mile/vehicle/year. To put it another way, consider a sample roadway with a traffic volume of 250 vehicles per day, we can estimate a loss of 250 tons of gravel per mile/year. That is 10 semi loads of gravel on a mile section of roadway! In addition to the obvious safety problem of sight distance with dust, the big picture is existing gravel sources are getting used up and not many new sites are being found to provide adequate aggregate to surface roads in western ND. McKenzie County, like some other counties in the state, is already going outside the county boundary to secure aggregate surfacing. This is a serious situation that has to be dealt with now through changes in the way we do business.
To better understand the dust control problem,
I gathered information from past studies and looked at the existing situation with help from consultants, industry people, and contractors, along with force account county crews from the oil producing counties. I found that many different methods have all been tried and most of them work to a certain degree when used as the manufacturer recommends.
The most prevalent product used for dust abatement is magnesium chloride (MgCl) followed by calcium chloride (CaCl). Production water salt brines, lignin sulfonates, chip seals, Permazyme, and Base One are some of the many dust control options that are available. The success or failure of dust control applications depends on factors such as gravel properties, weather, traffic volumes, and the application process.
To maximize the success of the dust control applications, let's review 4 key factors.
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Dust control starts with a good gravel specification. A
quality gravel surfacing should be tested for gradation with the minus 200 sieve (i.e., a sieve that has 200 wires in 1" means that only very fine material passes through) targeted to be between 7-15% of the material by weight. Material that passes the 200 sieve may be either silt or clay. Silt has poor engineering properties and tends to take on water. As silts take on water, the roadway's structural and water shedding capabilities are reduced. As such, a gravel surfacing with silt will allow water to migrate downward into the subgrade rather than shed off to the edge of the road. A saturated subgrade means that truck carrying capacity is reduced. On the other hand, a clay-based gravel surfacing will in essence cap the subgrade and effectively shed water off of the road surface. Clay helps to keep the subgrade dry and maximize its potential to support truck loadings. How do we test for clay/binder in gravel?
The minus 200 sieve material should be tested for plasticity index, PI, or clay content, and should be targeted to be between 4 and 12. The plasticity index tells us how well the minus 200 material will serve as a binder. In other words, it tells us if we have clay in the gravel fines that will serve as the glue to hold the rocks and sand together and to create a water tight surface that sheds water off of the roadway.
Quite a few counties are specifying gradation and PI in their crushing contracts. Similar to concrete, a mix of sand, rocks and cement (the binder), so does gravel need a binder (clay) to hold the sand and rocks together. Good gravel, made by specifying gradation and PI, allows us to be better stewards of the finite gravel supplies.
At the start of the article I mentioned that gravel loss can be estimated at 1 ton/mile/vehicle/year. That is the case for a poor quality gravel. Good gravel with binder can greatly reduce this loss. Add dust control to good gravel and this loss can be virtually eliminated. Dust control applied to gravel without binder most often yields poor performance. Start with a good product and it will pay off with longer surface replacement cycles and cheaper overall costs for the road agency.
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The road surface has to be shaped and compacted properly. The road should be prepared/shaped with a 4% crown prior to dust control applications. The crown allows water to shed off of the road and preserve the gravel and subgrade materials. With the proper crown, grader maintenance and surface repairs are minimized under normal rain events.
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Next, the shaped roadway will need to be watered. Looking back to my work in Idaho, generally, we watered the surfacing so that it had a 15-20% moisture content prior to dust abatement efforts. Watering is critical for water-loving dust control materials, (e.g., chlorides) to soak down into the gravel section a couple inches. Chlorides follow the water into the gravel and once in place they draw moisture out of the air. The water pulled from the air serves to maintain a tight and bonded road surface. First time chloride application of about 0.5 gal/square yard is a good starting point.
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Last is the application of the dust control solution. The magnesium chloride experts recommend to shoot two lifts of 0.25gal/square yard. The first shot should be applied right behind the watering process and should be allowed to soak in prior to the application of the second shot. Performance of the dust abatement depends on the quality of gravel. Chlorides and many of the other dust control products require clay to perform.
Now let's review a few pointers for specifying and purchasing chlorides. It is real hard to compare the different solutions for some of the dust control materials, however chloride comparisons are pretty easy. Studies show that a mixture of 34.5-35% calcium chloride is equal to 30% magnesium chloride solution. Since chlorides are mixed with water you should verify the chloride concentrations of the material you purchase by testing it with a hydrometer. Do the math to compare chloride options. Be sure to consider the cost of: chloride concentration, availability of product, freight to get it to your site, and local contractor availability.
The guidelines above are a good starting point for any dust abatement efforts.
Quality gravel, treated with a dust control product will help to improve roadway safety and reduce roadway costs. With nearly 60,000 miles of local gravel roads in North Dakota, our efforts to select the right treatment process for the right roadway section is an obligation we owe to the taxpayer.
I look forward to helping you wade through roadway issues and develop a sound gravel road plan. Hope to stop by your shop in the near future.
On-site training reminder:
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Curt Glasoe, PE/PLS NDLTAP Western Energy Technical Support Representative [email protected] 701-425-6384 (M) Dale Heglund, PE/PLS NDLTAP Program Director [email protected] 701-318-6893 (M) |
NDLTAP/UGPTI | 701-328-9855 | 515 1/2 E Broadway Suite 101, Bismarck ND 58501 |
www.ndltap.org
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7708 or Title IX/ADA Coordinator, Old Main 102, 701
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