PhD Oral Exam - Mohammadhossein Sojoudi, Civil Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The non-isothermal deformation of clay soils is a critical concern in energy and environmental-related geotechnics, given the complex microstructure and mineral composition of clay-related geomaterials. Investigating their thermo-mechanical behaviors poses significant challenges that previous studies have often overlooked. Specifically, distinguishing between thermal plastic strain and clay dehydration strain has received little attention.

To address these gaps, a novel constitutive model is proposed for describing the thermo-elastoplastic behaviors of water-saturated clayey soils. The model incorporates the effects of temperature variation and mechanical loading on elastoplastic strains and dehydration behavior. The thermo-mechanical behavior is quantified using thermodynamics laws and unconventional plasticity principles. Additionally, a finite element method (FEM) model is employed to simulate the thermo-hydro-mechanical (THM) responses of water-saturated clay soils. This FEM model accounts for temperature variation effects on bound water dehydration and corresponding thermo-poromechanical strains. By incorporating unconventional plasticity, the elasto-plastic behavior is more accurately described. The validation process for this FEM model involves laboratory results on various clay soils with different geological origins, demonstrating a reasonable agreement between the model's predictions and experimental data. Notably, the numerical results highlight the impact of bound water dehydration on the generation of excess pore pressure in clay soils during heating.

Expanding beyond the realm of theoretical models, a research project is underway to assess the geomechanical performance of a potential borehole thermal energy storage system (BTES) in a Canadian subarctic region. To quantify the poromechanical impact, a two-dimensional finite element model (FEM) is created to simulate a BTES system, encompassing the borehole and the surrounding soil formation. The model aims to analyze the effects of cyclic temperature variations and bound water dehydration on the short-term ground response and pore water pressure development. Our results indicate the importance of considering bound water dehydration in characterizing the ground heave process during the short-term BTES operation in an overconsolidated formation. The simulated ground expansion behavior is due to the high excess pore pressure generated during thermal storage, which is accompanied by the release of in-situ effective stresses. The neglect of bound water dehydration will underestimate the magnitude of ground heave during a short-term BTES operation.