## 1 Introduction  
Frost heave and thaw settlement in soils result from seasonal temperature changes or the heat emanating from a ground installation in cold regions featuring permafrost or seasonally frozen soil. The phenomenon of frost heave is caused by an expansion in the soil volume due to the formation of ice lenses, which, in turn, is caused by the continuous movement and subsequent freezing of pore water. Conversely, thaw settlement is caused by a decrease in the ground volume due to the melting of the ice lenses. Frost-susceptible soil is vulnerable to frost, which can cause structural instability such as buckling in subterranean pipelines and subsidence of paved roads [1]. Several numerical analyses have been conducted on the behavior of frost-susceptible soils, and various frost-heave models for such soils, such as the segregation potential (SP), FROST, PC-Heave, and thermomechanical (TM)-type models, have been proposed [2, 3, 4, 5, 6, 7]. Additionally, studies have investigated the soil-structure interactions. [PERSON] et al. [8] compared the behavior of a pipeline buried in frost-susceptible soil, previously investigated in a large-scale laboratory experiment performed by [PERSON] [9], with their numerical analysis results [8, 9]. Similarly, [PERSON] et al. [10] compared the settlement data of the Qinghai-Tibet highway embankment, as measured in a study by [PERSON] et al. [11], using their numerical analysis results [10, 11]. Furthermore, numerous methods have been suggested and utilized for ground stability, including changing the burial depth of the pipeline, backfilling with soil, or involving additional devices such as thermosiphons [12]. Thermosiphons are feasible for various structures, such as pavements, foundations, and transportation pipelines, in cold regions featuring permafrost or seasonally frozen soil. This mitigation device functions based on a circulation mechanism in which the absorbed refrigerant evaporates to a gaseous state, condenses back to its liquid form in the condensing process upon exposure to the atmosphere, and descends into the siphon [13]. Additionally, the internal refrigerant functions via two-phase flow, which has been studied through computational fluid dynamics (CFD). For example, in the studies conducted by [PERSON] et al. [14], [PERSON] et al. [15], and [PERSON] et al. [16], laboratory experiments were compared with simulations performed in Fluent, a commercial fluid analysis program. Such fluid analyses require an enormous amount of time due to the complex calculations of two-phase flow; it is difficult to apply the analysis to the ground behavior with thermosiphons. Therefore, several studies have aimed to simplify the thermosiphon operation and apply it to the numerical analysis. [PERSON] et al. [17; 18] and [PERSON] et al. [19] developed a thermosiphon model that describes the thermal conductivity according to the heat flux using USDFLD, which is an internal user subroutine of ABAQUS. However, the model used the arbitrary thermal conductivity of the thermosiphon; therefore, no experimental verification could be conducted. [PERSON] et al. [12; 20] presented a thermosiphon model using DFLUX, an ABAQUS internal user subroutine, and verified it with the results of laboratory experiments [21; 22]. However, these studies solely considered the behavior of the frost heave of the ground with the thermosiphon, and the thaw settlement was not considered. In addition, these studies did not characterize the behavior of structures, such as buried pipelines featuring thermosiphons. Therefore, in this study, a numerical analysis was performed on the frost heave and thaw settlement of the ground and the behavior of a buried pipeline in accordance with the thermosiphon-ground interaction. Because there is a lack of experimental studies on the behavior of a buried pipeline according to the thermosiphon-soil interaction, the numerical analysis was performed with the verified frost-heave model and thermosiphon model.