The Minnesota Road Research Project (Mn/ROAD) has completed time domain reflectometry measurements of soil moisture since 1993. But questions about interpretation about the data remain, because of soil characteristics at the site, the unusual design of waveguides, and long transmission lines that are known to degrade TOR system performance. This study's objectives included developing a procedure to recover moisture content data measured in situ with TOR since 1993 at the Mn/ROAD site.
Researchers calibrated the relationship between TOR system response and soil water content and characterized the influences of unusual waveguide design, high soil bulk density, high soil clay content, cable length, and soil temperature. Cable lengths greater than 33 m caused errors in the TOR calibration relationship. In addition, soil temperature had a small effect on TDR measurements. Researchers developed correction equations to correct TOR measurements of water content for long cable lengths and soil temperature.
Past TDR moisture measurements taken at the Mn/ROAD site must be interpreted using this report's calibration equation and cable length/soil temperature corrections. Additionally, present and future interpretations of soil moisture using the existing TDR system at the Mn/ROAD site must use these equations.
This report is unpublished. 15 copies were produced and distributed.
Water content measurement using time domain reflectometry (TDR) is an important integral component of the Minnesota Road Research Project (Mn/ROAD) program. However, erroneous and inaccurate water content values are frequently generated using current data collection and interpretation procedures. Whereas the erroneous and obviously unreasonable values are caused by limitations of the current waveguide/cable systems, inaccurate water content values are most likely caused by the inadequacy of the empirical calibration equations. Base, Subbase, and Subgrade materials are different from agricultural soils in composition, water retention characteristics, and more importantly in their dielectric properties in relation to water content and temperature regimes in the road pavement. This project was to develop a new composition-based calibration between water content and apparent dielectric constant of the pavement materials. Temperature effect on the dielectric constants of water and pavement components was also integrated in the new calibration. Results indicated that the new calibration was more theoretically sound and relatively simple to use. Additional tests and developments may be needed to further validate the reliability of the new approach.
