Gravel roads: surface stabilization
Minnesota has 70,654 miles of gravel roads in its 142,913-mile road system, according to MnDOT’s Transportation Management System. Most of these gravel roads, due to funding constraints and traffic volumes, are likely to remain gravel roads. Gravel roads are at once good, bad, and ugly.
The Good. Gravel roads are essential to transport Greater Minnesota’s products to market and people to destinations. The gravel surface, as opposed to a dirt road, provides all-weather, all-year access.
The Bad. Gravel roads require regular replacement of gravel as the gravel is crushed by traffic, washed off by rain, blown away by wind, and pushed off during snow plowing. The roads also require regular smoothing by a motor grader. They are expensive for the road agency to maintain.
The Ugly. Gravel roads are often dusty, bumpy, and, during spring periods, weight restricted. This generates significant complaints to the road agency’s elected leaders and staff.
Minnesota’s county engineers are actively exploring ways to enhance the good and reduce the bad and ugly characteristics of gravel roads. Several counties in recent years have used a Minnesota proprietary product, BASE ONE®, to stabilize the gravel surfacing. Other counties are using calcium chloride or magnesium chloride to treat the gravel surface. A recent innovation being demonstrated by one county is mixing shredded asphalt shingle scraps to stabilize the gravel.
This article explores use of BASE ONE, and future articles will look at the other strategies. BASE ONE is also used by some road agencies to stabilize gravel bases prior to paving. This discussion applies only to gravel-surfaced roads.
What is BASE ONE?
BASE ONE is a patented, proprietary product. As such, its composition and mechanism have not been released for competitive reasons. However, an LRRB-funded research report, Preliminary Laboratory Investigation of Enzyme Solutions as a Soil Stabilizer (2005-25), prepared by University of Minnesota researchers, included a chemical analysis. According to the report, BASE ONE consists primarily of Na and Si but also contains several other elements. It is highly basic with a ph of 11.34.
According to the vendor (Team Lab), BASE ONE is a “water-based solution containing dissolved amorphous silicon dioxide and sodium oxide and other elements.” The vendor states BASE ONE is environmentally friendly and contains no heavy metals, organics, or corrosives and shows no flammability.
How does it work?
The LRRB report did not determine the stabilizing mechanism or measure the stabilizing strength added by BASE ONE. The report did state the calcium and silicates in the product may form silicate hydrate, a cement-like material.
The vendor states BASE ONE acts as a detergent and bonding agent. Further, it states the dissolved BASE ONE polymers have a net negative charge and crosslink with the various metals to form a permanent bond exhibiting an ability to bind soil, aggregate, and other materials. As a detergent, it may act as a surfactant and lubricant and result in higher densities with the same compaction effort. The vendor concurs with the LRRB report that the sodium and silica may form hydrated calcium silicate, a cementing agent, when added to soil.
Independent testing lab data provided by the vendor show BASE ONE can result in increased aggregate base strength for pavements. However, MnDOT does not currently assign an increased GE for aggregate base material treated with the product.
Experience supports the importance of the electrochemical attraction of clay particles. A freshly laid gravel base is referred to as “green” by an experienced base inspector and is often too soft for a good paving platform. With time, perhaps a week or two, especially with heat and sunshine, the gravel base hardens into a strong paving platform. Presumably this is due to an electrochemical attraction between clay particles, which is enhanced by the BASE ONE action.
BASE ONE comes in 275-gallon totes. The BASE ONE can be incorporated into the gravel with a water truck and motor grader or with a reclaimer machine (more accurate and better mixing). The material is then shaped by a motor grader and compacted with a rubber tire roller. The cost of BASE ONE material is about $6,325 for a 4-inch-thick treatment 24 feet wide and one mile long, or $0.11 per square yard and inch of thickness. Labor and equipment and any gravel that may be added are additional. Application may be by contract or county forces.
Polk, Wilkin, and Wadena Counties have experience using BASE ONE to stabilize gravel roads and believe it results in lower maintenance costs and a better road for citizens. As reported by County Engineer Richard Sanders, P.E., Polk County is budgeting for 20 miles of BASE ONE stabilized gravel road per year for the next four years. The county’s project includes 4 inches of additional gravel stabilized with BASE ONE at a total cost of about $35,000 per mile.
As reported by Brian Noetzelman, P.E., county engineer and former Team Lab employee, Wilkin County has a program of stabilizing low-volume gravel roads that receive heavy sugar beet truck loads. The county places 3 inches of additional class 5 aggregate base, windrows this along with an existing 1 inch of gravel, and blade-mixes BASE ONE into the top 4-inch section in two 2-inch lifts. The material is then shaped and compacted. The county reports good performance in carrying heavy loads and less blading required to smooth the road. It may add calcium chloride for dust control. Typically, in seven or eight years BASE ONE is mixed in to the top 1 inch of gravel as top dressing. The year previous to treatment, the county installs centerline drain tile to improve subsurface drainage.
Some considerations if you choose to try it
Studies show BASE ONE can increase strength. However, there is no approved performance specification or trial mix process to determine suitability of the material and amount to use. According to the LRRB report, the mechanism is not well understood. Try it on a small scale.
The stabilizing action depends primarily on the fine, minus #200 particles. It works better with clay than silts. Quality aggregate with adequate clay fines is needed. It has less strength in the spring when saturated, so weight restrictions, an adequate thickness of gravel, maintaining a good crown, and good drainage are still needed. It treats the full gravel section rather than the surface, so dust control is still needed under some conditions.
It is very difficult to objectively measure amount of replacement gravel needed since it depends so much on gravel quality, traffic, rainfall, terrain, etc., and varies year to year. Likewise, the need for blading to smooth a road is largely subjective. Determination of cost-effectiveness will be largely subjective.
—Alan Forsberg, P.E., retired Blue Earth County Engineer