Development and Validation of TDR-based Sensors for Thermal Conductivity and Soil Suction Measurements PDF Download

Author: Nan Zhang
Publisher:
ISBN:
Category : Soil matric potential
Languages : en
Pages : 263

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Book Description
Simultaneous measurement of soil thermal conductivity, moisture content, dry density, and soil suction is a challenging issue and very important to the design of geothermal-related structures such as geothermal energy piles (GEP), ground source heat pump (GSHP) systems, borehole thermal energy storage (BTES) systems, etc. The measurement of these properties is mainly done in laboratory conditions, which can be time consuming. Hence, an attempt is made in this research to develop a new field sensor that can provide these properties in about the same timeframe. The TDR technique has been used successfully to measure soil moisture content and dry density by analyzing the soil dielectric constant (Ka) and the electrical conductivity (ECb) obtained from TDR waveforms. The objective of this research is to develop a new thermo-time domain reflectometry (TDR) probe and a moisture/suction TDR probe to better achieve these measurements. The thermo-TDR probe has three probes of the same diameter and length, with resistance wire embedded in the center probe, and three thermocouples installed in each probe. The probe calibrations for Ka and ECb were completed by testing five different chemicals with known Ka values, and NaCl and KCl solutions with different concentrations. The probe was then evaluated by testing four different materials, i.e., three sands and kaolin clay, and validating them against the KD2 standard probe for measurement of soil thermal properties. Laboratory experiments employing the thermo-TDR probe were then performed on quartz sands, sand-sand mixtures, and sand-clay mixtures to study the effects of the quartz content, particle size, moisture content, dry density, and clay content on soil thermal conductivity. Based on the experimental results on sand-clay mixtures, a continuum soil thermal conductivity prediction model was developed. The model overcame the shortcomings of the existing models, which accounts for the quartz content effect and prediction of soil thermal conductivity continuously over the entire range of soil types. The moisture/suction TDR probe has two steel stainless probes and a gypsum block attached to half of the probes. The calibration of the probe is conducted by modified pressure plate tests to establish the relationship between Ka and the matric suction of the gypsum block. Laboratory experiments on silty sand, including the staged-drying test and absorption and desorption tests, were performed to evaluate the probe and to study the equilibrium or response time of the gypsum block being equilibrated with surrounding soils