Each spring, as roads that have been frozen all winter begin to thaw from the surface downward, melting snow and ice saturate the ground. The roadbed, softened by trapped moisture beneath the pavement, is more susceptible to damage during spring than at any other time of year—when potholes begin forming due to the freeze-thaw cycles. For Construction Pros.com recently reported on Michigan Department of Transportation‘s (MDOT’s) “frost laws” process and predictability of timing. It’s a rite of spring: spring weight restrictions going into effect across the state to protect roads during the thaw. What isn’t so routine is exactly when these restrictions, also known as “frost laws”, go into effect and how long they last. But researchers from Michigan State University (MSU) and the MDOT are trying to make the process more predictable.
“Research shows that our pavements are most vulnerable during the spring thaw,” said 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.” During the frost law period, MDOT takes a number of precautions to protect state roads. On state highways designated “all season”, no overweight loads are permitted. On seasonal state highways, truck weights are reduced by 25% for rigid (concrete) pavements, and 35% for flexible (asphalt) pavements.
One major problem with the weight restrictions: the conditions under the pavement are hard to predict. “In many winters, freeze-thaw cycles don’t neatly follow the calendar year,” said Endres, who oversaw a three-year research project to develop a statistical model to predict soil frost conditions. “Having the ability to model frost below pavements using climate data and soil type can help to reduce the pavement distress caused during thaws.”
MDOT determines when weight restrictions begin each spring by measuring frost depths along state highways, observing road conditions, and monitoring weather forecasts. Weight restrictions remain in effect until moisture has escaped and the roadbeds regain stability. Understanding and accurately predicting the frost line movement is critical in determining when seasonal load restrictions should be enacted. Frost tubes are embedded in the ground and filled with a solution that changes color when it freezes. The color is checked periodically to determine how deep freezing temperatures have penetrated into the soil.
Frequent visits to these locations for manual frost tube readings is a time-consuming task. So, the MDOT has also invested in a number of Road Weather Information System (RWIS) stations that measure ground temperatures automatically—this reduces staff time and expenses. One goal of the research was to help MDOT leverage the RWIS data by developing a statistical model to estimate frost depth from available weather data. The new statistical model accurately predicts the frost depth based on the RWIS data and the rates at which Michigan’s sand and clay soils conduct heat.
“The statistical model uses the maximum and minimum daily temperature data from the RWIS system along with the thermodynamic properties of the soil,” explained Gilbert Baladi, Principal Investigator of MSU’s Department of Civil and Environmental Engineering. “The model can be further developed to give us an alert that it’s time for posting or removing weight restrictions.” The model has proven to be very accurate, and researchers are recommending that it be used along with a thaw model (to be developed) to create procedures and policies to assist MDOT in making accurate decisions regarding the timing of imposing and removing the seasonal load restrictions.
Using a similar model, Minnesota’s accurate posting of seasonal load restrictions may have increased the service life of low-volume asphalt roadways by 10%, potentially saving the state up to $14 million annually.
To read the entire article, please go to: http://www.forconstructionpros.com/press_release/12192014/new-soil-freezing-model-may-help-mdot-lower-springs-toll-on-our-roads.
For an additional article from Equipment World’s Better Roads titled “Michigan Researchers Looking to Lesson Spring Thaw Impact on Roads” by please go to: http://www.equipmentworld.com/michigan-researchers-looking-at-ways-to-lessen-spring-thaw-impact-on-roads/.
For the PDF on the complete report from the Michigan Department of Transportation Office of Research Administration, please click on image to the right, or please go to: http://www.michigan.gov/documents/mdot/RC1619_500351_7.pdf.
ABSTRACT: Frost depth is an essential factor in design of various transportation infrastructures. In frost susceptible soils, as soils freezes, water migrates through the soil voids below the freezing line towards the freezing front and causes excessive heave. The excessive heave can cause instability issues in the structure, therefore predicting the frost depth and resulting frost heave accurately can play a major role in the design. The objectives of this study are to develop accurate and reliable models for predicting frost depths and frost heave and to estimate the resulting heave pressure. The frost depth model was developed and verified using data from MDOT and Minnesota DOT. The frost heave model was also developed and verified using the available MDOT data.