Date of Award

Summer 7-9-2014

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Civil, Environmental & Architectural Engineering

First Advisor

Richard A. Regueiro

Second Advisor

John S. McCartney

Third Advisor

Shemin Ge

Abstract

Energy foundations (or geothermal foundations) are becoming more popular as an energy-saving and environmentally-friendly technology. By fully utilizing the steady ground temperature and the thermal properties of concrete, buildings can be heated and cooled through energy foundations with heat pumps at very low cost. Although some observations have been obtained from full-scale field tests and centrifuge-scale tests, there are still issues that are not well understood with respect to the complex interactions among temperature change, induced effective stress, and pore fluid flow in partially saturated soils.

In order to investigate soil-structure interaction between energy foundations and partially saturated soil under non-isothermal condition, the thesis develops a fully coupled thermo-poro-mechanical (TPM) finite element (FE) model with both nonlinear elastic, and temperature- and suction-dependent elasto-plastic solid skeleton constitutive models implemented. Based on the mixture theory of porous media and fundamental laws of continuum mechanics, governing equations are formulated to account for the coupled processes involving the mechanical response, multiphase pore fluid flow, and heat transfer. Constitutive relations consist of the effective stress concept, Fourier's law, as well as Darcy's law and Fick's law for pore liquid and gas flow. The elasto-plastic constitutive model for the soil solid skeleton is based on a critical state soil mechanics framework. The constitutive parameters are mostly fitted with experimental data. The TPM model is formulated under small strain and axisymmetric condition, and implemented within the finite element method (FEM). We then simulate a series of energy foundation centrifuge experiments conducted at the University of Colorado, Boulder. Good agreement is obtained between the experimental observations and modeling results.

Another novelty and challenge of the thesis is to develop a double-noded zero-thickness TPM cohesive interface element (CIE) model with elastoplasticity for fractured geomaterials under saturated or partially saturated condition. The advantage of TPM CIE is to take account of various jumps within the fracture with respect to tangential and normal displacements, pore liquid and gas pressure, as well as temperature. Both pre-existing fracture and developing fracture can be analyzed by choosing appropriate constitutive models. With CIE implemented at the soil-foundation interface, we are able to capture the plastic failure process of energy foundations due to the loss of side shear resistance. We can also apply the TPM CIE to better understand the generation of fractures involving coupled processes in other applications involving mudstone/shale, such as hydraulic fracturing, and reservoir storage of CO2 or nuclear waste.

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