Michigan researchers looking at ways to lessen spring thaw impact on roads

Updated Apr 15, 2016
A logging truck drives down an Upper Peninsula road. Seasonal weight restrictions are designed to protect state roads during the spring thaw when they’re most vulnerable to damage. (MDOT Photo)A logging truck drives down an Upper Peninsula road. Seasonal weight restrictions are designed to protect state roads during the spring thaw when they’re most vulnerable to damage. (MDOT Photo)

WeightRestrictionsGraphic_483954_7Researchers with the Michigan Department of Transportation (MDOT) and Michigan State University (MSU) have created a statistical model in an attempt to predict soil frost conditions to better time when the state’s “frost laws” go into affect. These load restrictions help reduce damage to roadways.

MDOT requires truck weights be reduced 25 percent for concrete pavements and 35 percent for asphalt pavements during the frost law cycle. Permits for vehicles or loads more than 14 feet wide are not issued at this time to help protect roadway shoulders.

Why is the timing of the frost law critical? If they’re set in place too early, it can cost the trucking industry time and money.

“Putting them in late costs the taxpayer through increased damage to the roads,” says Principal Investigator Gilbert Baladi of MSU’s Department of Civil and Environmental Engineering. “The more accurate you are, the more money is saved.”

Michigan estimates that by using a statistical model in the same way helps it increase the service life of low-volume asphalt roads by 10 percent and saves the state roughly $14 million each year in maintenance costs.

“Research shows that our pavements are most vulnerable during the spring thaw,” says Richard Endres, head of MDOT’s geotechnical services section. “The accuracy and timing of our seasonal load restrictions is critical—even if it is a few days off, it could lead to significant damage.”

Endres oversaw the three-year research project to develop a statistical model to predict soil frost conditions.

The agency has been determining frost law timing by measuring frost depths by state highways, looking at road conditions and checking weather forecasts. The restrictions are lifted when moisture has escaped and roadbeds are stable. But accurately predicting the movement of the frost line is the key step in determining timing of the restrictions.

An MDOT research project showed that thermal conductivity, which is easy to measure, could be used to accurately model soil’s freezing behavior. This could be the first step to a better, less expensive way of determining when “frost laws” are implemented. (MDOT Photo)An MDOT research project showed that thermal conductivity, which is easy to measure, could be used to accurately model soil’s freezing behavior. This could be the first step to a better, less expensive way of determining when “frost laws” are implemented. (MDOT Photo)

MDOT’s steps have involved manually measuring soil freezing using frost tubes at various locations across the state. The tubes contain a liquid that changes color upon freezing and surveyors must check these frequently, which is a time consuming task.

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The department also uses Road Weather Information System (RWIS) stations to automatically measure ground temperatures. So a key goal of the research project was to help MDOT use data collected by the RWIS sites, in combination with rates the area’s soils conduct heat, to create the statistical model to determine frost depth.

“In many winters, freeze-thaw cycles don’t neatly follow the calendar year,” Endres says. “Having the ability to model frost below pavements using climate data and soil type can help to reduce the pavement distress caused during thaws.”

“The statistical model uses the maximum and minimum daily temperature data from the RWIS system along with the thermodynamic properties of the soil,” Baladi says. “The model can be further developed to give us an alert that it’s time for posting or removing weight restrictions for the Lower Peninsula, the northern Lower Peninsula and the Upper Peninsula.”

Researchers say the statistical model has shown to be accurate throughout testing and recommend it be used with a to-be-developed thaw model to set up “procedures and policies” to help MDOT set the frost law dates.

It may take several years for new procedures to be initiated, and the department won’t completely remove the frost tubes or stop visual inspections. But, officials believe the “predictive modeling approach” could lead to better-protected roadways and maintenance and repairs savings for MDOT, as well as less revenue lost by the trucking industry.