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This paper focuses on the behavior of prefabricated thermal drains used to improve saturated clay layers using heating. A prefabricated thermal drain can be formed by integrating a closed-loop geothermal heat exchanger within a conventional prefabricated vertical drain (PVD). Prefabricated thermal drains can be installed in a similar way to a PVD but operate by circulating a heated fluid through the heat exchanger tubing to induce an increase in temperature of the soft clay. This increase in temperature will lead to thermal consolidation, which can be accelerated by drainage through the PVD. Although thermal drains have been tested in proof of concept field experiments, there are still several variables that need to be better understood. This paper presents numerical simulations of the coupled heat transfer, water flow, and volume change in layers of kaolinite, illite and smectite clays within a large-scale oedometer with a prefabricated thermal drain embedded at the center. Thermally induced excess pore water pressures and a slight initial expansion was observed for clay layers with lower hydraulic conductivity. However, the overall volume change resulted in contraction where the rate as well as the magnitude of settlement was greater for a thermal PVD compared to a conventional PVD. A further analysis of kaolinite layers with different initial porosities indicated that the increase in the magnitude of settlement observed when using a thermal PVD was independent of the hydraulic conductivity of the clay whereas the increase in the rate of settlement was more pronounced for clays with lower hydraulic conductivity. 

This paper focuses on the thermo-hydro-mechanical behavior of soft clay surrounding a prefabricated thermal drain. A prefabricated thermal drain combines features of a conventional prefabricated vertical drain (PVD) and a closed-loop geothermal heat exchanger by placing plastic tubing within the core of the PVD through which heated fluid can be circulated. The prefabricated thermal drain can be used to increase the temperature of the surrounding soft clay, which will generate excess pore water pressures due to differential thermal expansion of the pore fluid and clay particles. As these excess pore water pressures drain, the soft clay will experience volumetric contraction. The elevated temperature leads to an increase in the hydraulic conductivity and the volumetric contraction leads to an increase in thermal conductivity, making this a highly coupled process. Although thermal drains have been tested in proof of concept field experiments, there are still several variables that need to be better understood. This paper presents numerical simulations of the coupled heat transfer, water flow, and volume change in the soft soil surrounding a prefabricated thermal drain that were validated using the results from large-scale laboratory experiments. Numerical simulations were found to agree well with the experimental data. A further analysis on the performance of the thermal PVD indicates an increase in surface settlement with an increase in drain temperature and a significant reduction in the surcharge required when using a thermal PVD.