The Development and Evaluation of a Time Domain Reflectometry Probe for Soil Moisture and Suction Measurement

Soil moisture and suction are two key variables that often need to be measured for unsaturated soil. They are commonly measured by separate soil moisture and suction sensors which require double installation efforts and often result in questionable correlation between the measured moisture and suction. Attempts were made to combine time domain reflectometry (TDR) and porous blocks functioning as tensiometers and have demonstrated a great potential of the TDR for simultaneous measurements of soil moisture and suction. In this study, a new TDR moisture-suction probe is designed based on the previous study, and it was evaluated in laboratory for simultaneous measurements of moisture content and soil suction. The probe has two parts with one half portion embedded in a dental plaster (i.e. gypsum) for soil suction measurements, whose suction is equilibrated with the surrounding soils. The other half portion of the probe is also inserted into the soils to measure moisture content. The probe calibration was completed by the use of pressure plate tests to establish a relationship between suction (ψ) and measured dielectric constant (K_a) of the gypsum block. Then, it was tested in a silty sand to evaluate its function for soil moisture and suction measurement. The preliminary test results demonstrated that the change of K_a of the gypsum corresponded well to suction level from 10 to 750 kPa, and it can measure moisture content and soil suction accurately for the test soils. The new probe can be fabricated easily and fast, without any additional maintenance and has a great potential for field measurement of soil moisture and suction.