With global warming and the intensification of human activities, frozen soils continue to melt, leading to the formation of thermokarst collapses and thermokarst lakes. The thawing of permafrost results in the microbial decomposition of large amounts of frozen organic carbon (C), releasing greenhouse gases such as carbon dioxide (CO2) and methane (CH4). However, little research has been done on the thermo-water-vapor-carbon coupling process in permafrost, and the interactions among hydrothermal transport, organic matter decomposition, and CO2 transport processes in permafrost remain unclear. We considered the decomposition and release of organic C and established a coupled thermo-water-vapor-carbon model for permafrost based on the study area located in the Beiluhe region of the Qingzang Plateau, China. The model established accurately reflected changes in permafrost temperature, moisture, and C fluxes. Dramatic changes in temperature and precipitation in the warm season led to significant soil water and heat transport, CO2 transport, and organic matter decomposition. During the cold season, however, the soil froze, which weakened organic matter decomposition and CO2 transport. The sensitivity of soil layers to changes in the external environment varied with depth. Fluctuations in energy, water, and CO2 fluxes were greater in shallow soil layers than in deeper ones. The latent heat of water-vapor and water-ice phase changes played a crucial role in regulating the temperature of frozen soil. The low content of soil organic matter in the study area resulted in a smaller influence of the decomposition heat of soil organic matter on soil temperature, compared to the high organic matter content in other soil types (such as peatlands).

The frost damage of rock mass poses a serious threat to the safety and stability of tunnels in cold regions, and the related thermo-hydro-mechanical (THM) coupling model under low-temperature conditions has been a key focus of research. This paper proposed a cryogenic THM coupled model (TOUGH-FEMM) to study the frost heave behavior of cold-region tunnels. Key issues including heat transfer, thermal stress, water-ice phase transition, unfrozen water, frost heave deformation, and ice-rock interaction are systematically addressed in the proposed model. Specifically, frost pressure in pores and cracks is derived separately to better simulate the ice expansion effect in rock masses. The proposed model is first validated against an experimental test and then applied to a practical cold-region tunnel to reveal the evolution of temperature, frost pressure and frost heave fields, as well as the tunnel stability. Moreover, the effects of cracks and frost damage on tunnel stability under freeze-thaw cycles are discussed. The work detailed herein provides an efficient tool for the THM coupled process in cold- region tunnels.