Comparative Analyses of Methods for Posting Spring Load Restrictions

Many low-volume roads in the United States and other countries are located in seasonal frost areas where freeze-thaw processes result in costly damage to the roadways. In order to reduce damage, road management agencies apply Spring Load Restrictions (SLRs) which restrict the allowable load on the road during the critical time interval when the pavement is most vulnerable to damage. This paper discusses several quantitative approaches proposed for use in posting SLRs: (1) a method proposed by Mahoney et al. (1986); (2) the Berg/USFS method (Berg et al., 2006; Kestler et al., 2007); (3) recommendations proposed by the Minnesota Department of Transportation (Mn DOT, 2009) and (4) recommendations developed by the Manitoba Department of Infrastructure and Transportation (MIT) and FPInnovations (Bradley et al., 2012). All of these approaches recommend SLR starting dates based upon a cumulative thawing index threshold. In some cases, thresholds for SLR removal are also recommended. The four methods/models were evaluated by comparing the trigger dates determined from the models with measured frost-thaw profiles and with falling weight deflectometer (FWD) data obtained at eight field test sites in NH. Overall, this research suggests that both the Berg/USFS method and the Mn DOT method yield reasonable estimates for SLR application dates in northern NH. For removing the SLRs, this study suggests that adoption of a CTI threshold similar to that suggested by Bradley et al. (2012), with an eight-week maximum SLR duration, may be appropriate. However, due to limitations in the FWD data obtained to date, additional mechanistic analysis regarding appropriate thresholds for SLR removal is recommended.