Design and Evaluation of a Moisture/Suction TDR Probe

Soil moisture content and suction are two key geotechnical properties that are commonly measured by separate moisture and suction sensors, which require double installation efforts and often result in questionable correlation between the measured moisture content and suction. In this study, attempts were made to combine time domain reflectometry (TDR) technique and porous gypsum block functioning as a tensiometer for simultaneous measurements of soil moisture content and suction. A new moisture/suction TDR probe was designed and evaluated in the laboratory. The TDR probe had two parts with one half portion embedded in a gypsum block whose suction is equilibrated with surrounding soils for soil suction measurement. The other half portion of the TDR probe was also inserted into soils to measure moisture content. The probe was calibrated by the pressure plate and filter paper tests to establish a relationship between suction (ψ) and dielectric constant (K_a) of the gypsum block for suction measurement. Three laboratory tests (i.e., staged-drying tests, absorption tests, and desorption tests) were then performed on a silty sand to evaluate the probe performance. It is demonstrated that the probe can measure the soil moisture content and suction satisfactorily in the range 50 kPa < ψ < 10,000 kPa with a response time less than 5 h. Additionally, the probe is fabricated easily and fast, without any additional maintenance, and has a great potential for field measurements of soil moisture content and suction.