Date of Award

Spring 1-1-2014

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

John S. McCartney

Second Advisor

Shideh Dashti

Third Advisor

Dobroslav Znidarčić

Abstract

This study investigates how temperature affects the response of capacitance sensors. These sensors are used to measure the dielectric permittivity of unsaturated soils, which is strongly related to the volumetric water content. The volumetric water content is frequently used in the analyses of water flow through soils. Capacitance sensors have been used in experiments intended to characterize thermally-induced flow of water in unsaturated soils during geothermal heat exchange processes. Water flow is an important process in the design of thermally-active geotechnical systems such as energy piles and thermally-active retaining walls. To date, no studies have characterized the effect of temperature on capacitance sensor output from a geotechnical perspective (where soils are typically dense). An understanding of this topic is necessary to obtain an accurate estimate of thermally-induced changes in volumetric water content in thermally active geotechnical systems.

This paper describes the heating experiments performed on compacted soil layers with constant water content that indicated changes in temperature can lead to changes in the measured dielectric permittivity of up to 40%. The capacitance sensor's response to changing temperatures was observed to be sensitive to the initial density of the soil, and the initial volumetric water content. A calibration equation to consider the soil-specific relationship between volumetric water content and dielectric permittivity was defined, along with a correction equation to account for temperature effects in unsaturated soils having different dry density values.

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