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 the differential thermal expansions of the pore fluid and clay particles. As these pore water pressures drain, the soft clay will experience volumetric contraction. The elevated temperature also increases the hydraulic conductivity of the pore water, and the volumetric contraction leads to an increase in thermal conductivity. 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 the field experiments. Numerical simulations are presented to both show how the coupled processes interact and to understand variables including magnitude and duration of heating, drain depth, drain spacing, and clay thermo-hydro-mechanical properties.