Electrical conductivity problems associated with time-domain reflectometry (TDR) measurement in geotechnical engineering

The effect of electrical conductivity (EC) of a dielectric material on its TDR-measured dielectric constant εa (called the apparent dielectric constant) was investigated both theoretically and experimentally. The effect of EC on the apparent dielectric constant εa was more pronounced at low frequency of the electromagnetic pulse applied to measure εa. The assumption of neglecting electrical losses in measuring the dielectric constant by TDR to represent εr was not valid for dielectric materials containing free charges, such as electrolyte solutions, bentonite clay, and saline soils. A theory is proposed to separate the real part of the dielectric constant εr from the TDR-measured εa in lossy dielectrics. An EC - εr relationship can be developed that would provide εa (= εr) at EC = 0 to be used to determine soil-water contents in conductive soils. Na-bentonite mixed with distilled water resulted in considerable magnitude of bulk EC due to the adsorbed cations on the surface of the particles. This EC caused energy loss of the TDR pulse that depended on the length of the probe rod. For a 4.5cm long probe, the reflected TDR waveform was completely attenuated for EC between 3.05dSm-1 and 3.72dSm-1. The waveforms generated in this range of EC were not suitable for analysis by TDR-support software for dielectric constant. With the adsorbed cations, Na-bentonite behaved as a lossy or dispersive material.