Multi-Scale Thermo-Hydro-Mechanical Behavior of Saturated Earth Materials

Various geotechnical-related energy applications, such as geothermal piles, subject soils to temperature changes. Recorded temperature variations around thermo-active infrastructure and within the active layer of the permafrost reveal the cyclic and transient nature of these temperature changes. Previous studies on the thermo-mechanical behavior of soils did not consider the effect of the temperature change rate on such behavior. Since it is widely accepted nowadays that soil behavior is rate-dependent, evaluating soil behavior under more realistic, transient temperature changes is crucial. In this dissertation, a method to calibrate triaxial cells used to expose soil samples to transient thermal loads is developed. This calibration is critical to ensure reliable thermally induced pore water pressure measurements and estimates of thermally induced volumetric strains of tested specimens. Then, thermally induced water flow and pore pressure generation under partial drainage conditions are formulated to account for the effect of temperature change rate on the thermal consolidation of cohesive soils. The formulation is performed by coupling Darcy's law for water flow in porous media with existing relations estimating thermally induced fully-drained volumetric strains. The resulting partial differential equation—the thermal consolidation theory—is solved and validated against experimental results that used the calibration from the first task. Using this newly developed theory, it was found that temperature-dependent properties of the pore water and the soil's hydraulic conductivity have a significant role in thermal consolidation. Lastly, a microstructural analysis is performed to assess the evolution of the microstructure of a normally consolidated clay under a full thermal cycle consisting of a freezing (F), thawing (T), heating (H), and cooling (C) thermal path. This microstructural investigation aims to explain the observed macroscale responses of cohesive soils under such a thermal path. After each step along the considered thermal path, the microstructure evolution was assessed using measurements of the specific surface area and pore size distribution. In the end, the variations of specific surface areas and pore size distributions were used to explain the macro-scale thermo-mechanical behavior of cohesive soils.