Factors Affecting Determination of Subgrade Water Content from Multisegment Time Domain Reflectometry Probes

Multisegment time domain reflectometry (TDR) probes are an attractive alternative to single-segment probes when used for the in situ measurement of water content changes in pavement systems. Because several independent measurements are made along the length of the multi-segment probe, these instruments offer several advantages over the more traditional TDR probes. These include the ability to obtain measurements over a greater volume of soil, the ability to install the probe into relatively undisturbed soils, and the redundancy provided by multiple measurements from the same instrument. However, because of the difference in signal strength along the probe, all segments do not provide the same level of accuracy. Several factors that must be considered when using these probes are discussed, including the sources of measurement error and the temperature dependence of the measurements in some segments. A method is described by which the results from lower accuracy segments can be used when higher accuracy segments fail during service, taking advantage of the redundancy provided by multisegment probes. The method is demonstrated on data from a site from which over 4 years of continuous water content measurements have been obtained.