Improved Spring Load Restriction Guidelines Using Mechanistic Analysis

This project used research to develop more effective criteria for placement and removal of spring load restrictions (SLR). Researchers investigated a method that uses a thawing index equation based on air temperatures to predict thawing events. Results showed that adjusting the reference temperature improved the spring-thaw prediction for Minnesota. Researchers compared historical SLR posting dates from 1986 through 1998 to the dates predicted using this new technique and to falling weight deflectometer and in situ instrumentation readings from 15 flexible pavement test sections in Minnesota. According to results, there was typically a week or more delay from the time that SLR should have been placed until actual posting of restrictions, which caused damage that could have been prevented. Based on testing performed on pavement sections across the state, the typical period required for pavement base and subgrade layers to regain sufficient strength to support heavy truck loads was eight weeks. In 1999, the Minnesota Department of Transportation adopted the improved procedure for placing and removing SLR. The policy uses actual and forecasted average daily temperature to determine timing of SLR. When the SLR duration is fixed at eight weeks, it is easier to plan for the end of the restriction period. Researchers estimate an increase of 10% in a typical low-volume asphalt road's life with implementation of the improved SLR procedures, resulting in a potential savings of more than $10 million annually.

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