Ultimately the plan would provide five miles of new paving, rebuilding of 80 miles of gravel roads, and additional rock surfacing for 40 miles of roads. “By improving these 125 miles of roadway, maintenance resources will be available to make improvements on the other 800 miles of our county roads,” Patterson said. “The new gravel roads would be 30- to 50-percent cheaper to maintain, trap less drifting snow, have proper drainage, and have an appropriate surface course for the traffic they are carrying.”

GPR and Gravel Roads

The use of motorized vans carrying ground penetrating radar (GPR) is well-established in characterizing the subsurface conditions of road networks as road agencies update their pavement condition inventories, but in 2012 researchers at the University of Illinois at Urbana-Champaign described use of GPR for unpaved roads.

In Denver, this unpaved alley was topped with a mix of straight RAP and neat rejuvenator, and then compacted; the result is an inexpensive unpaved driving surface.

In their TRB paper Assessment of Subsurface Deformation in Unsurfaced Pavements Using Ground Penetrating Radar, U of I graduate research assistants Debakanta Mishra and Zhen Leng, and professors Erol Tutumluer and Imad L. Al-Qadi describe the findings of full-scale accelerated pavement testing to evaluate aggregate quality in two unsurfaced pavement sections.

“An innovative application of the nondestructive GPR technology was established for assessing subsurface deformations and distinguishing between the different rut mechanisms that contributed to the failure of unsurfaced pavements,” they write.

They observed that aggregate type and angularity clearly governed the mechanisms responsible for rut accumulation in unsurfaced pavements. “An uncrushed gravel layer with high fines underwent internal rutting through shear flow and exhibited significant heaving adjacent to the wheel path,” they say. “The pavement test section with the crushed aggregate base, on the other hand, showed significantly higher load resistance and primarily exhibited subgrade shear failure at load repetitions much higher than the gravel section.”

GPR scanning of unsurfaced pavement sections shows promise for identifying different rutting modes and assessing subsurface layer deformations and damage potentials, they conclude, adding this concept can be further expanded to quantify the accumulation of rutting in individual layers by estimating the dielectric constants of layer materials.

RAP Stabilizes Unpaved Surfaces

With a boom in natural gas extraction – as well as high production from its low-sulfur coal surface mines – Wyoming unpaved roads are suffering from very heavy truck traffic, as well as an increase in automobiles.

But increased use of reclaimed asphalt pavement (RAP) to preserve riding surfaces is an option that offers a low-cost, but substantial, way to suppress dust loss and enhance aggregate retention, say Burt Andreen and Harry Rocheville, graduate research students at the University of Wyoming, and the university’s Khaled Ksaibati, Ph.D., P.E., in their 2012 TRB paper, A Methodology for Cost/Benefit Analysis of Recycled Asphalt Pavement (RAP) in Various Highway Applications.

In recent years, RAP has been used as a surface additive on Wyoming’s unpaved roads, streets and alleys. This has been boosted by recent state legislation, which compensates Wyoming DOT for RAP donated to Wyoming counties. That being said, the DOT and local agencies needed to evaluate the cost effectiveness of blended RAP and virgin aggregate as a surfacing material for unpaved roads, and that was the goal of the research.

For the test, two half-mile sections were constructed using RAP, which was distributed using bottom discharge dump trucks and then spread with a motor grader. The RAP and existing base were then mixed by a mobile stabilizer using a toothed drum, which mixed the RAP with 6 inches of base and released the mixture back onto the road surface. This mix of RAP and existing base then was sprayed with water and compacted to achieve maximum density.

“RAP in gravel roads resulted in a savings of $17.07 per ton of RAP in this case … due to the savings from virgin aggregate and the savings through dust reduction, which will keep the road in better condition by retaining the fine particles embedded in the road,” the authors write. “The air quality will also be improved as a result of the reduction in dust.”