Tire chips reduce road ‘heaving’ in winter
With the approach of winter, familiar signs begin popping up all over the northern climates. Wherever frost-susceptible soils mix with freezing water, road subsurfaces push upward, or “heave,” moving boulders, displacing road stakes and pushing through pavement.
Engineers at the U.S. Army Cold Regions Research & Engineering Laboratories (CRREL) in Hanover, N.H., have found that a layer of tire chips spread beneath the road surface will provide insulation that reduces frost penetration into the ground. The process minimizes frost heave formation and offers a low-cost way to help solve the problem of discarded tires since it uses approximately 150,000 tires per mile of road.
Robert Eaton, research civil engineer at CRREL and developer of the concept, worked with Dana Humphrey, associate professor of civil and environmental engineering at the University of Maine, to test the process during the last two winters. They divided a 750-foot stretch of gravel road in Richmond, Maine, into six test sections, then chopped tires, including steel-belted radials, into 2- to 3-inch pieces and spread the chips across five sections, each having a different thickness from 6 to 12 inches. The sixth section was used as a control and left without chips. Each layer was covered with 12 to 24 inches of gravel.
“In the insulated sections, the frost just penetrated to the underlying soil during the winter,” Eaton says, “while in the section with no tire chips, frost penetrated 5 feet.”
During frost heaving, soil exposed to water as it freezes expands 70 percent. As frost penetrates downward into the soil, moisture at the frost boundary is frozen into thin “lenses.” Capillary action draws more and more water toward the boundary, effectively supersaturating it.
In the spring the process reverses itself. A melting layer of soil with no structural strength forms at the frost/thaw interface, and, when traffic rolls over it, the surface cracks and breaks, causing spring potholes.
The degree of heaving depends on the type of soil, whether it can readily absorb water and the availability of water from underground sources or rain. Silt absorbs water rapidly and causes the most heaving, while “fat” clays are extremely fine and show little propensity for it. By preventing frost from penetrating to base soil, the tire chip method inhibits the capillary action that produces supersaturation of soil with frozen water and thereby inhibits heaving.
During the tests, Eaton tracked the air temperature, soil temperature and the location of the frost boundary. Data were collected automatically and recorded hourly on a data logger, and computers at CRREL retrieved the data through the telephone every morning. At the end of the winter the insulated sections had heaved 1 to 2 inches while the uninsulated section next to them had heaved 6 inches.
“Very clearly the tire chips were doing their job to limit frost penetration and frost heave,” Humphrey says. “The chips also provided excellent drainage for any excess water, which is a big plus. The insulated sections held up very well during the spring thaw, while the uninsulated section rutted.”
CRREL will continue monitoring the Richmond site for several years to determine the optimum thickness of the gravel layer based on different traffic conditions. The lab also plans to test the process on paved roads.