Nan Zhang

Graduation Semester and Year

2015

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Xinbao Yu

Abstract

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 determine a new field sensor that can provide these properties about time frame. TDR technique had been used successfully to measure soil moisture content and dry density by analyzing soil dielectric constant (Ka) and 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 above goal. The thermo-TDR probe has three probes with the same diameter and length with resistance wire embedded in the center probe and three thermalcouples installed in each probe. The probe calibrations for Ka and ECb are firstly completed by testing five different chemicals with known Ka valus, and NaCl and KCl solutions with different concentrations. The probe is then evaluated by testing four different materials, i.e. three sands and kaolin clay, and validated against KD2 standard probe for measurement of soil thermal properties. Laboratory experiments by the thermo-TDR probe is then performed on quartz sands, sand-sand mixtures, sand-clay mixtures to study the effects of 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 is developed. The model overcomes the shortcomings of the existing models, which accounts for quartz content effect and predicts soil thermal conductivity continuously over the entire range of soil type. The moisture/suction TDR probe has two steel stainless probes and a gypsum block attached to half portion of the probes. The calibration of the probe is conducted by modified pressure plate tests to establish the relationship between Ka and matric suction of gypsum block. Laboratory experiments on silty sand, including staged-drying test, absorption and desorption tests, are performed to evaluate the probe and study the equilibrium or response time of gypsum block being equilibrated with surrounding soils.

Disciplines

Civil and Environmental Engineering | Civil Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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