Development of Prototype Soil Moisture Sensors

In an effort to develop innovative sensors for the in situ measurement of moisture in aggregate base and soil subgrade layers of pavements, two prototypes have been developed and laboratory evaluated: a sensor based on nuclear magnetic resonance (NMR) technology and a sensor using dielectric constant technology. Both sensor types are potentially applicable to highway soil moisture measurement problems. Each sensor measures the amount of water in the effective volume of a soil sample and is somewhat influenced by soil type. For a particular soil of known dry density, the percentage water on a dry weight basis can be determined by empirical calibration. Under an extension of this study, the sensors are undergoing (1) refinement and development of ancillary readout equipment; (2) simulated field evaluation over a range of soil, compaction, moisture, and contamination conditions; and (3) field installation and practical evaluation in two different climatic regions. The NMR sensor based on the spin echo approach was tested with bentonite clay, silica flour, and an organic silty clay. The influences of soil density, organic matter and dissolved salts were investigated. In the prototype dielectric model, the sensor electrodes are incorporated as part of a resonance circuit whose frequency of oscillation circuit is contained within the buried sensor; a coaxial cable connects the sensor to the external power supply and readout instrument. It is hoped that in addition to the original objectives, the technology developed in the study may be applied to the development of laboratory instruments for instantaneous measurement of moisture in soil samples without the need for drying.