The measurement of soil water content in situ is not straightforward. Capacitance methods such as Time Domain Reflectometry (TDR) can be used to estimate the volumetric water content in situ using a passive “probe” which is embedded in a soil and connected via a cable to a signal processor/recorder. In this study, a systematic series of laboratory measurements was undertaken to determine the relationship between true volumetric water content, gravimetric water content and TDR measured water content in uncracked Maryland clay. The results show not only that the TDR measured water contents differ from the true volumetric water contents, but that the TDR is potentially relatively insensitive over a significant interval of mid-range water contents in uncracked clay. Tests to estimate the effect on measurements from the presence of cracks of different width found that errors increase steadily as crack width increases. The study also determined that the measurements are relatively insensitive to misalignment of the waveguide prongs.
5th International Conference on Unsaturated Soils. Proceedings of the Fifth International Conference on Unsaturated Soils (Barcelona, Spain 6-8 September, 2010 ) p. 653-657