By Jacob Michalowski, PE
As temperatures rise and snow begins to melt, pothole season quickly follows. This is the time when agencies must post spring load restrictions (SLRs) on vulnerable pavements to prevent excessive damage when the pavement structure is at its weakest. Understanding how SLRs work—and the science behind them—is essential for preserving pavement life and managing budgets responsibly.
What are SLRs?
SLRs are temporary axle-load limits applied during the spring thaw to prevent severe pavement damage from heavy vehicles. Minnesota is divided into six frost zones, and the Minnesota Department of Transportation (MnDOT) announces the start and end dates for SLRs in each zone based on real-time frost-depth data, pavement temperature sensors, and weather forecasts.
SLRs are typically initiated when the three-day weather forecast indicates that the cumulative thawing index (CTI) for a zone will exceed 25°F and longer-range forecasts predict continued warming. Once posted, SLRs generally remain in place for six to eight weeks, from early March through mid-May. MnDOT provides historical data and several mapping applications to help agencies plan and coordinate.
Establishing a load rating
A roadway’s load rating depends on several factors, including pavement layer thickness, material strength, subgrade support, and drainage characteristics. Together, these determine the pavement’s ability to support repeated axle loads without reaching a structural failure condition.
Load ratings may be established during the design phase—for example, intentionally constructing a 7-ton roadway—or through field investigations and local experience with pavement performance. Extensive research has also been conducted using non-destructive testing methods such as a falling weight deflectometer (FWD), which measures pavement deflection to estimate structural capacity.
The science behind the spring thaw
As temperatures rise above freezing and the CTI increases, the ground begins to thaw from the top down. In Figure 1, the red line shows the thaw progressing downward from the hot-mix asphalt (HMA) layer. The blue line indicates frost depth, and the green line shows a secondary freeze event that caused frost to penetrate again. As ice and frozen soils melt, the subgrade becomes saturated and extremely weak. Because the deeper layers may still be frozen, meltwater becomes trapped above them, creating a saturated zone. This saturated, low-strength condition is what triggers the need for SLRs.
Granular or aggregate materials—classified as non-frost-susceptible—provide better support and drainage, allowing them to recover strength more quickly in spring. Fine-grained soils, by contrast, remain weak for longer periods.
Understanding load damage
Understanding the basics of equivalent single-axle loads (ESALs) helps explain the potential damage caused by overweight vehicles during the thaw. The relationship between axle weight and pavement damage is not linear—it is exponential. For example, if a 9-ton axle travels over a roadway posted at 5 tons, that single pass causes roughly 10.5 times the damage of a legal 5-ton axle load.
After the thaw
Preventive and routine maintenance following the spring thaw plays a major role in extending pavement life and reducing costly future repairs. Addressing early-season distresses—such as patching potholes, sealing cracks, and improving drainage—helps pavements recover more quickly from the thaw-weakened period.
Related resources
Michalowski is a licensed professional engineer with a BS in civil engineering from Minnesota State University, Mankato. He spent nearly 14 years at American Engineering Testing consulting on pavement condition, rehabilitation methods, and overall pavement evaluations. He recently joined Allstates Pavement Recycling & Stabilization.