Ghabchi’s research seeks to improve gravel roads

The concrete and asphalt ribbons that interlace South Dakota communities from Buffalo to Beresford and everywhere in between carry the vast majority of the traffic in the Mount Rushmore State. But when it comes to total miles of roadway, gravel roads are No. 1.

There are roughly 82,000 miles of road in South Dakota and 75% of them are unpaved, primarily maintained by counties and townships, where budgets run shallow as a stock pond during a dry summer.

Improved, up-to-date material selection guidelines for building and maintaining gravel roads can help counties and townships extend the service life of every mile they manage.

Rouzbeh Ghabchi, the John M. Hanson Associate Professor in Structural and Construction Engineering in the Department of Civil and Environmental Engineering at South Dakota State University, is leading a South Dakota Department of Transportation-funded project focused on developing specifications for crushed quarry aggregate surfacing for unpaved rural roads.

This summer, SDSU’s research team led by Ghabchi, and in close coordination with the South Dakota DOT’s Research Office, conducted field evaluations on gravel roads in Lawrence, Custer and Pennington counties in and near the Black Hills. The team assessed roadway conditions and collected field samples for laboratory testing to better understand the characteristics and performance of road surfacing materials.

At each site, the team inspected the road for loose aggregate and stiffness, checked whether it had maintained its original geometry and looked for corrugation, rutting and potholes, Ghabchi said. 

Samples were taken down to the road base to examine the aggregate’s type, size, plasticity and strength. Lab testing is being done in the materials laboratory in Crothers Engineering Hall.

The long-term goal is to provide research findings to support DOT’s efforts to develop specifications for the use of quarry aggregate, which is crushed and processed, in gravel road surfacing. Specifications for gravel pit aggregate already exist, but that material isn’t always available, especially in the western half of the state, said Ghabchi, who has been at SDSU since 2016.

“Eastern South Dakota has an abundance of natural gravel pits, largely because glaciers left behind deposits of sand and gravel. West River counties, particularly around the Black Hills, rely more on crushed rock from quarries,” Ghabchi said. “Therefore, as reliable natural gravel sources become harder to find, there is a growing need to use quarry aggregates for road surfacing,” he said.

The South Dakota Local Technical Assistance Program (LTAP), which is housed within the Lohr College of Engineering, is working with Ghabchi on the project. Future field visits and sample collection are planned in early October as well as in early spring to see what the freeze-thaw cycles did to the roads, Ghabchi said.

“By the end of next year we should be ready to summarize and present our findings to the Department of Transportation,” he said.

“Counties regrade these roads and add new surface material to keep them in good condition,” said Ghabchi. “The key question is what specification requirements should the quarry materials meet to be used in gravel road surfacing,” Ghabchi continued.  

“Aggregate sources vary from one part of the state to another, but once the specifications are approved, they can be used anywhere in South Dakota,” Ghabchi said. “By weighing performance and practical factors like material availability, construction feasibility and cost, the South Dakota Department of Transportation will ultimately decide how to implement the findings and technical recommendations.” 

Much research has gone into gravel road construction and maintenance. In fact, Ken Skorseth, former program manager of South Dakota LTAP, was lead author of the Gravel Roads Maintenance and Design Manual, which the Federal Highway Administration published in 2000 and later updated as a national guide in 2015.

Ghabchi’s work focuses on developing quarry aggregate specifications used in construction of gravel surfacing rather than the how-to of construction. 

Greg Vavra, program manager for South Dakota LTAP, called the project “something that has been needed for quite some time.”