PhD Oral Exam - Sohail Akhtar, 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

Climate change is profoundly affecting permafrost regions, posing significant challenges to infrastructure stability. As global temperatures rise, permafrost thaws, leading to ground subsidence and compromising structures such as roads, pipelines, and buildings. Understanding soil behavior under thermo-mechanical forces is crucial for designing resilient engineering solutions. This thesis offers a comprehensive study of the thermo-mechanical behavior of cold-region soils through experimental data and numerical analyses under compression, tension, and triaxial conditions. The research investigates temperature-dependent strength variations, effective measurement techniques for the tensile behavior of frozen soil, and the rheological and residual strength characteristics of lime-treated (L-soil) and untreated natural soil (N-soil) from northern Quebec, Canada, after freeze-thaw cycles. The key contributions of this work include: (I) identifying effective and reliable techniques for quantifying tensile strength; (II) determining the critical number of freeze-thaw cycles to predict residual strength in L-soil and N-soil; (III) analyzing failure modes and stress behavior in L-soil and N-soil under varying confining stresses and thermal conditions; (IV) accurately modeling visco-elastic, visco-plastic, and creep behavior of compressive and tensile strength using finite element methods; and (V) calibrating triaxial testing approaches against experimental data. Finally, this study simulates the damage initiation and crack propagation in uniaxial compressive test and indirect tensile test using damage XFEM model in finite element-based software package (Abaqus). Findings emphasize the importance of considering time-dependent strength degradation, analyzing the complex behavior of frozen soil due to interactions between frozen and unfrozen water, and quantifying tensile strength with minimal local plastic deformation. The research also highlights the benefits, drawbacks, and challenges of using lime to enhance soil residual strength in cold climates.

Despite significant advancements, the research acknowledges limitations, such as the need for microscopic studies of unfrozen pore water and ice interactions, scanning electron microscopy (SEM) analyses of lime, silt, fine sand, and clay particle interactions, and broader validation efforts for real-world scenarios. Recommendations for future work include micro-level soil sample studies, dynamic and creep loading tests under various temperature and moisture conditions, modifying hyperbolic Drucker-Prager modeling to account for temperature-dependent parameters, conducting long-term studies, and integrating field data to enhance model accuracy and applicability.