Analysis of Methods Used in Time Domain Reflectometry Response

Abstract

Time domain reflectometry (TDR) is a new technique that can be used to measure indirectly the in situ volumetric moisture content of soil. A growing body of research has been conducted in providing a variety of prediction equations to estimate the volumetric moisture content using the dielectric constant calculated from the apparent length obtained from the TDR reader. However, limited research has been conducted to determine which of several available procedures should be used to obtain the apparent length of the TDR response to be used in calculating the dielectric constant. As a result, evaluating which procedure yields the most accurate assessment of the volumetric moisture content of soils is the object of this paper. There are five known methods of analyzing the apparent length of TDR responses. They are the method of tangents, method of peaks, method of diverging lines, alternate method of tangents, and the Campbell scientific method. Twenty-eight soil samples, from the FHWA seasonal monitoring program, were obtained throughout the United States and Canada and used in a laboratory study. Three levels of moisture and five levels of compaction were initially planned for use with each soil sample. A total of 361 data points were eventually obtained and used to analyze each method. The method of tangents proved the most accurate method of estimating the volumetric moisture content. Current studies are ongoing to provide improved multiple regression models to estimate the volumetric moisture content on highway soils.