America’s low-volume roads need help—and they’re getting it!
Low-volume streets and roads make up more than 75 percent of all road miles in the United States, and those low-volume roads are crucial to our nation’s economy. They are where all of our farm, forest, and mine products and most of our manufactured goods begin their journeys to market. They are also where most of our nation’s workers begin and end their daily commutes.
Nearly all of those low-volume roads were constructed in the 1950s and 60s when America was trying to get out of the mud that had bogged down vehicles for hundreds of years. Now those local roads are near or past the ends of their useful lives. This leaves highway officials in counties, townships, and cities all across the nation wondering how they can keep these millions of miles of roads functioning adequately in a time of reduced budgets and increased demand.
Don’t say the ‘U’ word!
One such official is Sue Miller, the county engineer in Freeborn County, Minnesota, in the heart of the Corn Belt. At the 2012 TERRA Pavement Conference, Miller said there’s a “new normal” for county engineers—and for the most part, it’s not a pretty picture: “In our county, we have 224 miles of aggregate-surfaced roads and about 410 miles of asphalt-surfaced roads. Some of them look good, and we’re able to maintain them. But a lot more are in bad condition.”
Miller described the plight of one of her roads: County State Aid Highway (CSAH) 20: “Last year we had a tougher than normal winter, and County Road 20, which was asphalt-surfaced, had a lot of trouble with frost boils.”
“When we tried patching some of the frost boils,” Miller continued, “my truck drivers were coming back into the office saying, ‘We’re just wasting taxpayers’ money on this! As fast as we can fix one spot, our trucks are breaking up the next spot. We’re just making it worse!’”
That left Miller in a bind. “We couldn’t just leave the road as it was,” she said, “but County Road 20 wasn’t scheduled for any type of major maintenance until 2015. So after some discussion, we decided we had no alternative. We had to unpave it—reclaim the useless broken asphalt, and revert to a gravel-surfaced road until we had time to consider alternatives to traditional paving and find the required funding.”
When Miller used that word—unpave—in a county board meeting, she discovered that it was loaded with negative connotations. “For some of my county commissioners, the word unpave means loss of service. It means we’ve failed. No one wants to tell constituents that the level of service they expect can’t be delivered. In fact, I was told not ever to use that word again!”
Frost boils were undoubtedly just the last phase in the deterioration of Freeborn County Road 20. That road, like virtually all of America’s rural roads, was built for a type of vehicle and a volume of traffic that no longer exists. In days gone by, farmers brought their product to market with single-axle trucks. But to compete in today’s economy, farmers need semi-trailers that can hold 800 or more bushels of grain.
Furthermore, the traffic on America’s rural roads is no longer just farm traffic. Ethanol plants, wind farms, soybean processors, and many other enterprises are placing increasingly heavier vehicles on those rural roads. Those businesses need to get their products to market year-round; that includes March and April when, in northern climates, load restrictions are in force because the subgrade and base layers are in their soggiest, most vulnerable condition.
With dwindling budgets, reverting pavements to gravel (“We’re not using the ‘U’ word!”) is a hot topic all across the country. At the recent South Dakota Region Local Roads conference in Rapid City, 220 local road managers from North and South Dakota, Nebraska, Wyoming, Colorado, and Montana attended. Two out of the 15 sessions offered were devoted to the “U” word—but fully half of all comments submitted on evaluation forms were about that topic.
North Dakota’s oil boom
Jack Olson, a planner for the North Dakota Department of Transportation, is dealing with a problem similar to Sue Miller’s—multiplied by about one million! He has attempted to quantify the amount of damage being done to the roads in northwestern North Dakota’s booming oil fields.
The first oil well in North Dakota was drilled in 1951. From that date, for about 30 years, oil production was fairly steady except for a mini-boom in the 1980s. But beginning in 2008, the curve of North Dakota oil production started heading almost straight up. In the year 2000, the state produced about 3 million barrels per month. But by 2011, production had reached about 16 ½ million barrels per month. Olson projects that, within two years, North Dakota will be producing about 30 million barrels of oil per month. “We’ll pass California and Alaska,” he said. “North Dakota will be the number two oil producing state after Texas.”
What’s responsible for the sudden change? In a word, fracking. The geological formation under North Dakota’s oil lands, called the Bakken Formation, is composed of low-permeability, low-porosity shale. Geologists have known there was oil in this formation for more than 100 years. But until fracking was developed, there was no economical way to get the oil out.
In fracking, Olson explained, “They drill down about 5,000 feet. Then they put differential pressure on the bore so it starts curving. By the time it reaches 10,000 feet, it’s on the horizontal. Then they continue for another 10,000 feet. Next, they pump fluid down into that 10,000-foot horizontal run. That fractures the rock, which releases the oil so it can be pumped out. It’s transforming the oil industry.”
Boom spells doom and gloom for rural roads
But the oil industry’s success is having the same effect on the rural roads in North Dakota (and surrounding areas in Montana, Saskatchewan, and Manitoba) that Sue Miller is experiencing in Minnesota—but to a much greater extent.
“In the early 1990s,” Olson said, “the typical oil drilling rig weighed about 90,000 pounds. Now the rigs are about 110,000 pounds. But the legal load limit in North Dakota, with the right number of axles, is 105,500 pounds. Everything’s getting bigger all the time.”
And the drilling rigs are just the tip of the iceberg. Drilling one well requires many other heavy vehicles. For example, each well requires two mud pumps that each weigh 164,000 pounds. And three generator houses that each weigh 111,180 pounds. And a hydraulic unit that weighs 127,640 pounds. The list goes on. And that’s for just one well. Olson estimates that, eventually, 33,000 to 40,000 wells will have been drilled in North Dakota.
Furthermore, heavy equipment is not the whole story. Olson said, “It takes about 400 truck loads to drill a vertical well, which is still about 10 percent of our wells. But to drill one of the horizontal wells, it takes about 1,150 truckloads. And of course, all of those trucks then need to go back for more, so it’s actually 800 trips and 2,300 trips! Those are all legal loads, but they’re all going on county roads that were designed for much lower volumes of single-axle farm trucks.”
Working with the North Dakota State Patrol, Olson has calculated the number of equivalent single-axle loads (ESALs) being placed on those rural roads. “The impact on our asphalt-surfaced roads from a single well is 2,017 ESALs. The impact from the same well on our concrete-surfaced roads is 3,175 ESALs.” The photo at right shows the result of oil-drilling-related traffic on one North Dakota road—four-inch ruts.
“In the last legislative session,” Olson said, “state highways in the oil field received $228.6 million. County and township roads received $142 million. Another $60 million was allocated for roads outside that area that are impacted by the oil industry. So that’s a total of about $430.6 million last year—but we’re not keeping up with the damage.” Currently, there is no North Dakota statute that requires any of the taxes paid by oil companies to be ear-marked for road maintenance. Olson said officials are discussing the situation.
Good information about low-volume roads
With so many miles of low-volume roads in jeopardy—and with miles of previously paved roads reverting to gravel surfaces—it would be good if there were some reliable sources of advice on how to keep those roads in service.
Ken Skorseth, of the South Dakota Local Technical Assistance Program (LTAP), is an expert on gravel road maintenance. He hosted the South Dakota Region Local Roads conference referenced earlier in this article. Like Sue Miller, Skorseth abhors that “U” word. “I try to put a positive slant on this topic,” he said. “I’d rather refer to ‘alternatives to paving.’ Maybe that softens it a bit. But the fact is—it’s a bitter pill to swallow.”
Decision tree for unpaving roads
MnDOT and the Minnesota Local Road Research Board created a transportation research synthesis in 2010 that highlights relevant research and information related to road surfacing decisions, pavement preservation techniques, cost analysis, and current road agency experiences. Download Decision Tree for Unpaving Roads.
Based on a study of 120 low-volume road segments, Skorseth recommends three ways to go with low-volume roads:
- Gravel surface—up to 170 ADT
- Chip seal on an aggregate base—up to 650 ADT
- Hot-mixed asphalt—above 650 ADT
He warned that removing asphalt and reverting to a gravel surface may only lead to different types of problems: “If the truck traffic is 25 to 50 ADT and there is low subgrade support, you will need 14.5 inches of gravel. That’s hard to do when trucks are knocking it off constantly and the blade needs to be out there every other day. Furthermore, the quality and availability of gravel varies greatly from place to place.”
The good news, Skorseth said, is that chemically stabilized gravel surfaces can perform very well. “One section that the South Dakota LTAP has been observing has high-quality gravel stabilized with annual chloride treatments. It’s used by up to 80 concrete trucks every day and has held up very well since 1998. Between then and 2011, it has required less than 200 tons of additional gravel.”
Another section observed by SDLTAP is in an area of Montana where there is heavy traffic from oil and gas development as well as from agribusiness. “Their problem is availability of gravel,” Skorseth said. “They’re running out. They lose a lot of fines from the surface because the gravel they can get is non-plastic. So they mix Bentonite™, a highly plastic clay, into the top three inches of an eight-inch gravel layer. That becomes their wearing course; it’s a poor man’s pavement.” However, Skorseth warned that Bentonite™ can be a dangerous material: “If you blend too much of it into the gravel, it will stick to tires and cake them up to the point that the wheels can’t turn. In one instance like that, the driver held the accelerator down, and it destroyed the transmission.”
100 years of gravel road research
David Jones, of the University of California’s Pavement Research Center, is another source of research-based advice on how to maintain low-volume roads with chemical additives. In harmony with Skorseth and others, Jones said, “The first thing to say about surface treatments for gravel roads is that you can’t make a bad road good. We need to have a good road to start with, and then we can keep that road good with a chemical additive. It makes no sense to chemically treat a gravel road once and then sit back and wait until it fails.” Instead, Jones said, “We try to introduce some form of maintenance that will prolong the road’s life. If we come back after three months and put down about 10 to 20 percent of the original application, we’ll find that it lasts another three months. And if we come back the next year and top it off with another 10 or 15 percent, we can actually preserve most of the materials and keep the ride quality up to an acceptable standard with [fewer] grader visits than if the road wasn’t treated.”
But Jones said the only way to know how to choose among the more than 100 available additives is to understand the properties of the gravel in a road. He adds that the two essential factors to take into account in assessing the performance of gravel are plasticity and gradation. Using simple formulas on inexpensive standard test results, he plots these factors on a graph (see Figure 1).
He said, “Of course, everyone would want their material to fall in the ‘Good’ box in the middle of the graph. But materials outside that box can still work, provided the road manager understands the implications. For example, anything that falls to the left of center is likely to erode. But you could use it in level areas of North Dakota or parts of Arizona where they don’t have too much rain.”
Next he divides the “Good” box into two sub-categories and plots three examples from different gravel sources numbered 1, 2, and 3 on the graph (see Figure 2). Sample 1 near the bottom will washboard and ravel—and therefore will require frequent grader maintenance to maintain an acceptable ride—because it has too little clay to bind the larger aggregate particles. Jones adds that a road manager can use this graph to determine just how much Bentonite™ or other clay to add to bring the material into the “Good” box and prevent the too-much-Bentonite™ problem described by Skorseth.
Jones agreed with Skorseth by pointing out that plasticity and strength are mutually exclusive. Strength on gravel roads is usually measured with the California Bearing Ratio (CBR) test. CBRs need to be above 15 to ensure all-weather passability. Materials falling in the “Good” block on the graph usually meet this criterion. But if the road carries a lot of trucks, the CBR needs to increase. In other words, you need lower plasticity materials closer to the “Good” box. Sample 2 on the graph would probably meet this requirement.
Jones said most chemical treatments are appropriate for treating materials that fall within the “Good” box and for treating materials halfway into the box below, i.e., down to a shrinkage product of about 50. This group, he said, includes chlorides (MgCl and CaCl), lignosulfonates, synthetic polymer emulsions, and mineral oils—but excludes those designed specifically for high-clay content aggregates. “You don’t want to use chlorides with high-clay-content gravel up at the top of the chart because the chlorides retain moisture and will actually aggravate the slippery conditions,” he adds. “But on materials at the bottom of the graph with low plasticity, they act as binders, holding the fines together.”
As an example, Jones cites the road shown below. “Before we treated it, there was lots of dust and material being displaced, and it required grader maintenance every couple of weeks. But with a CaCl treatment, it now gets about 400 vehicles a day—about 30 to 40 percent heavy vehicles—and it’s performing very well. They go back to it about once a year with 20 to 30 percent of the original application. It hasn’t lost any gravel, and it needs about one blading a year to get it back into shape.”
For the Number 3 gravel—with high-plasticity—Jones said electrochemical, sulfonated oil, or enzyme-based additives would be most appropriate. “Those additives require some form of clay mineral to react with, which is what you will have in materials at the top of the chart. These additives can give you better compaction, which will increase your CBR.”
Good information makes good roads
With solid advice from experts like Ken Skorseth and David Jones, road managers like Sue Miller and Jack Olson can hope to provide serviceable pavements in this “new normal” environment in which budgets cannot keep up with demand. A study co-authored by Skorseth, along with several other in-depth sources of information on low-volume road maintenance, is at the Minnesota LTAP low-volume topics page. UC’s David Jones is currently working with other universities and the FHWA’s Central Federal Lands Highway Division on a comprehensive guideline for the use of additives on gravel roads.
—Richard Kronick, LTAP freelancer