Evaluating Moisture Sensors and Monitoring Seasonal Moisture Variation in Low-Volume Roads

Abstract

For the past several years, the Forest Service of the U.S. Department of Agriculture has been evaluating a quantitative technique for the application and removal of load restrictions by observing relationships among pavement stiffness, pavement damage, soil moisture, and seasonal freezing and thawing. Laboratory tests of time-domain reflectometry (TDR) and radio frequency (RF) sensors showed them to be reasonably accurate and repeatable when compared with known moisture values in several soil types. Laboratory tests of the probes under repeated adverse freezethaw cycling showed the probes to be durable. Although the field survival rate of TDRs surpassed that of RFs, analysis of field data collected at seven locations in four national forests showed that permanently installed sensors strategically located on a forest road network can provide an affordable method for quantitatively determining the beginning and end of critical periods of pavement weakening associated with springthaw. This information would be useful in administering periods of spring-thaw load restrictions. The laboratory and field test programs conducted are outlined. The field technique is applicable to any secondary road subjected to seasonal freezing.