Warmer winters and more frequent severe rainfall events in Minnesota threaten the integrity and longevity of pavement foundations. Understanding the impacts of saturated pavement layers can help MnDOT design resilient roads. An exploration into the relationship between climate conditions over time and pavement foundation stiffness on test road sections led to the development of a pavement foundation saturation prediction model. The model supports the production of a preliminary geographic information systems (GIS) based pavement vulnerability framework that can guide road engineers in prioritizing maintenance in at-risk areas to ensure strong, durable pavements.
What Was the Need?
Winters are warming in Minnesota faster than in other states in the country, resulting in more frequent heavy rainfall. The U.S. Global Change Research Program found that the heaviest 1% of precipitation events increased by 42% between 1986 and 2016 in Minnesota. Previous research led by MnDOT’s Office of Environmental Stewardship assessed Minnesota air temperatures, precipitation and freeze-thaw events over time. Among other findings, the study showed a 0.5- to 1-inch increase in precipitation before the end of the cold weather season.
When the ground becomes saturated, excess water flows beneath the pavement surface and can significantly affect pavement foundations. Too much moisture in a pavement foundation affects its strength, stiffness and durability. A second phase of research was needed to understand the implications of increasing heavy rainfall events on road foundations and pavement resilience.
What Did We Do?
A comprehensive evaluation involving historical data analysis, field measurements and predictive modeling illustrated the relationship between heavy precipitation events and pavement foundation moisture levels. Weather stations and sensors at MnDOT’s MnROAD facility and a climate trends tool from the Minnesota Department of Natural Resources allowed researchers to focus on heavy precipitation trends from the beginning of the 20th century.
“The modeling tool created in this project to predict pavement foundation susceptibility to heavy precipitation has the potential to support transportation asset management activities across Minnesota,” said Raul Velasquez, geomechanics research engineer, MnDOT Office of Materials and Road Research.
To investigate the effects of moisture variability on the performance of pavement foundations, researchers analyzed the correlation between significant precipitation and moisture fluxes in pavement layers. Material properties and moisture data from test cells at the MnROAD facility were investigated.
Hydraulic modeling was used to simulate moisture flux in pavement foundation layers due to heavy rainfall. A finite element analysis methodology, which employs mathematical and engineering calculations to predict physical behavior of materials under various conditions, was used to create a pavement saturation prediction model. Researchers then modeled the saturation and resulting change in resilient modulus—the material stiffness that determines susceptibility to damage—of the pavement base layers examined at MnROAD using rainfall events of different intensities and durations.
What Was the Result?
The assessment of historical precipitation data confirmed increasing trends in heavy precipitation in Minnesota during the 21st century. The new modeling tool simulates moisture fluxes that occur in pavement foundations due to heavy rain events and predicts changes in saturation levels and resulting reductions in pavement stiffness. The model validation from the data collected at MnROAD enabled the creation of a preliminary GIS-based pavement vulnerability framework of the MnROAD facility.
Pavement sections showed reduced resilient modulus from different levels of saturation, depending on the aggregate type and thickness of pavement layers. In response, researchers recommended several asset management practices, including:
- Investing in advanced weather-monitoring and forecasting systems to provide real-time data on significant precipitation events.
- Further assessing climate change impacts on the state, including projections for increased heavy precipitation events and quantifying the potential effects to transportation infrastructure.
- Prioritizing vulnerable road networks for maintenance, especially in heavy rainfall areas.
- Updating infrastructure design standards to ensure resiliency against heavy precipitation.
What’s Next?
MnDOT engineers can use the pavement saturation prediction model, currently calibrated to the MnROAD test cells, to explore the impact of different rainfall scenarios and how pavements with varying characteristics would perform under different saturation levels.
The GIS mapping system framework could potentially be applied across the state to assess pavement foundation vulnerability due to heavy rainfall. MnDOT is considering further research to incorporate diverse subgrade soils, road base and subbase types in the context of regional variations in climate and weather into pavement saturation prediction models to support assessing the vulnerability of Minnesota’s road network.
Crossroads 