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Title:Coupled thermo-hydro-mechanical behavior of glacial tills in shallow geothermal systems
Author(s):Renjifo Ciocca, Jose V
Advisor(s):Makhnenko, Roman
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Coupled THM
Glacial Till
Geothermal Systems
Abstract:Increasing need of practical engineering applications whereby the ground is used as a thermal reservoir and soils are subjected to thermal gradients, such as shallow geothermal systems, motivated this study. The soil response under representative field conditions upon heating can affect the performance and overall operation of shallow applications, demanding adequate input parameters for an efficient design. Therefore, it is critical to better understand the fluid-saturated behavior of geomaterials, where temperature gradients induce deformations, affect the pore pressure, and might significantly influence the pore fluid flow processes. The purpose of this work is to conduct a multi-physical analysis to characterize the coupled thermo-hydro-mechanical behavior of glacial tills in central Illinois and suggest a constitutive model for describing their short- and long-term behavior. Four specimens of predominantly fine-grained glacial till were collected in the upper 20 m from a 110-meter deep test borehole drilled for a planned geothermal site at the University of Illinois at Urbana-Champaign. The specimens were tested using a 3.5 MPa GDS Triaxial Cell apparatus connected to pressure volume controllers and external heaters to allow the application and measurement of temperature at 22, 32 and 42°C ± 1.0°C. The testing schedule considered a maximum change of temperature of 20°C, in agreement with the temperature operation range of a typical shallow geothermal system. Monotonic thermal loading was applied to the soil samples to evaluate the thermally induced deformation. Drained and undrained compression, as well as flow tests were performed at different temperatures. Moreover, the time-dependent behavior was evaluated at room and elevated temperatures. The results revealed a limited temperature dependence for the elastic moduli; while hydraulic conductivity increased upon heating, accounting for the change in viscosity of water resulted in limited change for intrinsic permeability at higher temperatures. Glacial tills subjected to thermal loading showed an expansive volume change resulting in thermal expansion coefficients on the order of 10E-4 /°C. Furthermore, it was noticed that the tendency for time-dependent deformation consistently increased at elevated temperatures emphasizing the importance of including long-term (viscous) material response into the constitutive models dealing with subsurface geotechnical applications.
Issue Date:2020-12-11
Rights Information:Copyright 2020 Jose V Renjifo Ciocca
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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