Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil, Environmental & Architectural Engineering

First Advisor

John McCartney

Second Advisor

Ingrid Tomac

Third Advisor

Dobroslav Znidarčić

Fourth Advisor

Petros Sideris

Fifth Advisor

Nien Chang

Abstract

A technique used to improve the energy efficiency of heat pumps for building heating and cooling systems is to embed closed-loop heat exchangers into drilled shaft foundations to form energy foundations. Fluid is circulated through the embedded heat exchangers to transfer heat in the soil surrounding the energy foundations to or from the building. When the soil surrounding the foundations changes temperature, irreversible soil volume changes may occur depending on the soil mineralogy, degree of saturation, and stress state. These volume changes may affect the lateral stress distribution along the energy foundation, and may lead to relative movement between the foundation and surrounding soil. The thermal volume change of the soil has not been considered in design methods that have been developed for energy foundations. Despite the experimental data available on the recoverable and permanent deformations of unsaturated soil during heating and cooling, the impact of thermal deformation of unsaturated soils under anisotropic conditions still needs to be better understood before the design methods can be improved.

This study extends the database of thermal volume change measurement of soils under different conditions by performing several series of thermo-mechanical compression tests on unsaturated, compacted silt specimens using a thermo-hydro-mechanical (THM) true-triaxial cell. The THM true-triaxial cell has the capabilities of being able to control the temperature on all six boundaries of a cubical soil specimen, as well as the suction within the specimen to provide drained conditions during loading or temperature changes. The testing series are focused on understanding the roles of stress-induced anisotropy on the thermal volume change. Six non-isothermal tests were performed, each involving suction application, isotropic consolidation, heating and cooling, and isotropic unloading. Specifically, three tests having different minor to major principal stress ratios of 1.0, 0.7, and 0.5 were performed at a degree of saturation 0.7, and three tests having different stress ratios of 1.0, 0.7, and 0.5 were performed at a degree of saturation of 0.8. Each test required nearly 3 weeks to set up and perform. The results from these tests were compared with each other as well as with results from tests on saturated specimens of the same soil performed in a previous study. Although compressive thermal axial strains were measured in both the major and minor stress directions, a greater thermal axial strain was observed in the direction of major principal stress for stress ratios less than 1.0. This observation is consistent with the previous study on this soil. A small effect of inherent anisotropy due to the compaction process was observed. Specimens with a lower initial degree of saturation were observed to have greater thermal volume changes that specimens closer to saturation, which is proposed to be due to thermal collapse of the air-filled voids during heating. An elasto-plastic model developed for saturated soils under isotropic conditions was modified to consider the effects of anisotropy and variable degrees of saturation, and a good fit was obtained between the measured and predicted results. A discussion of the results from this study indicates that the greater thermal axial strains in the major stress direction may lead to thermal dragdown in normally-consolidated soil layers, with a greater effect in unsaturated soils.

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