Improved Spring Load Restriction Guidelines Using Mechanistic Analysis

Abstract

This study was undertaken to evaluate criteria used to predict when to place and remove spring load restrictions (SLR). A method was investigated that uses a thawing index equation based on air temperatures to predict thawing events. It was found that adjusting the reference temperature improved the spring-thaw prediction for Minnesota. Historical SLR posting dates from 1986 through 1998 were compared to the dates predicted using this new technique. These dates were also compared to falling weight deflectometer and in situ instrumentation readings from fifteen flexible pavement test sections in Minnesota including the Minnesota Road Research (Mn/ROAD). It was found that there was typically a week or more delay from the time that SLR should have been placed until restrictions were actually posted, which caused damage that could have been prevented. Based on testing performed on pavement sections across the state, it was found that the typical period required for the pavement base and subgrade layers to regain sufficient strength to support heavy truck loads was eight weeks. In 1999, the Minnesota Department of Transportation (Mn/DOT) adopted the improved procedure for placing and removing SLR.