Development and Application of TDR Mini-Probes for Monitoring Moisture in Small-Scale Laboratory Tests

The time domain reflectometry (TDR) method has been widely used to explore and track soil moisture. It is necessary to research mini-probes for reducing the influence of the transducer on soil measurement, which encountered many challenges, especially as the transducers are used for studying small-scaled laboratory tests of stiff clay. To overcome these problems, three-wire mini-probes were developed and applied in the present work. Initially, a two-dimensional (2D) weighting theory and numerical simulations were adopted to preliminarily determine the rod spacing (4-4.5 mm) and diameter (0.47-0.63 mm). Subsequently, needle and drill mini-probes were designed to conduct experimental calibration. It was reasonable to employ a 3 cm probe for measurement and a linear equation that related K-a(1/2) with theta was obtained. Finally, mini-probes were applied to measure water content of compacted and intact stiff clay specimens during continuous desiccation and intact Teguline clay column during wetting, indicating that the mini-probes have the acceptable workability for measuring the moisture of compacted/intact stiff clay undergoing desiccation or wetting processes.