Springtime Thaw Weakening and Load Restrictions in Minnesota

Abstract

In regions of the United States where pavements are constructed in freeze-thaw environments, springtime load restrictions are used to reduce distress caused by truck loads. During the spring, the pavement layers can be saturated and weakened because of partial thaw conditions and trapped water. Knowledge of the timing of freeze-thaw events is crucial to a successful load restriction strategy. Several studies in Minnesota are being done to evaluate criteria used to predict when to place and remove springtime load restrictions. The objectives are to evaluate current load restriction procedures, investigate pavement strength changes in relation to freeze-thaw events, and suggest improvements to current procedures. Data collected from eight Minnesota Road Research Project (Mn/ROAD) flexible pavement test sections, including falling weight deflectometer, response, and environmental data, were used to assess the effects on pavement strength. In conjunction with the Mn/ROAD work, the Minnesota Department of Transportation is conducting a statewide study using resistivity probes to monitor frost depth beneath various roads around the state. It was found that the existing procedure works well in predicting the thaw if the warming trend is uniform. In addition, over the past four springs the predicted thaw duration was greater than field observations. The date of maximum deflection occurs about 1 to 3 weeks after thawing is complete; the rate of strength recovery is quicker in sections constructed on higher quality base materials with low fines content. On the basis of the results a new criteria and equation for predicting the beginning and duration of the thaw are proposed.

Evaluation of Unbound Aggregate Base Layers using Moisture Monitoring Data

“…The materials targeted in this study are summarized in Figure 1 and included Minnesota aggregate base/subbase materials classes 3 (CLS3) to 6 (CLS6) and clay. Aggregate base layers in Minnesota are constructed using Class 5 (CLS5) or Class 6 (CLS6) dense-graded aggregates ( 14 ). The base layer is designed to sustain construction equipment for paving operations, transfer traffic loading efficiently to the subgrade, provide adequate resistance to differential frost heave, and mitigate the effects of moisture by maintaining sufficient strength when saturated and by minimizing the capillary rise of moisture from the subgrade ( 15 ).…”

Keeping Springtime Low-Volume Road Damage to a Minimum

“…The original thaw index method, developed in Washington State (33,34) and revised for use in Minnesota (35,36), enables a simple estimation of the date when thawing commences using just current air temperature. The technique works well for those specific locations, but it requires different reference temperatures, some of which themselves vary over time, for different geographic locations.…”

Determining When to Place and Remove Spring Load Restrictions on Low-Volume Roads

“…This date coincided exactly with the March 1 onset of thaw as measured in the field. On the basis of the recommendation by Van Deusen et al (6) and Rutherford et al (5) to apply SLRs when the cumulative degree-days reach 15°C-days (25°F-days), an SLR would be applied March 3. When the same procedure was applied to Knapp Airport, thawing at the surface was calculated to also occur on February 28.…”