Reservoir fracturing stimulation is the key to constructing an enhanced geothermal system (EGS) for geothermal development in hot dry rock (HDR) reservoir. To clarify the crack propagation law of HDR fracturing, a 3D thermo-hydro-mechanical coupling simulation model of fracture propagation is produced based on the continuum-discontinuum element method (CDEM-THM3D). The correctness of the CDEM-THM3D model is validated by the theoretical solution of the nonisothermal soil consolidation model and Penny fracture model. Then, hydraulic fracturing numerical simulations are performed to analyse the influence of controlling variables on fracture propagation. The results indicate that the thermal tensile stress induced by injecting cold water can decrease reservoir fracture pressure and fracture extension pressure, causing an increasement in fracture width and a reduction in fracture length. Increasing thermal expansion coefficient and temperature difference enhances the effect of thermal stresses and even creates new branch fractures. A large elastic modulus favours an increase in fracture length, while large rock tensile strength and minimum horizontal stress lead to a decrease in fracture length. With increasing injection flow rate and fracturing fluid viscosity, the reservoir fracture pressure and the fracture width rise significantly, and the fracture easily breaks through the barrier of the high-stress compartment.

In order to investigate the freezing damage problem of berms of earth-rock dams in cold regions, an earth-rock dam in a cold region was selected as the research object in this study. A finite element model, considering the effect of thermo-hydro-mechanical coupling, has been developed to solve the problem by combining the characteristics of the earth-rock dam. The whole process of freezing damage of berms under the influence of the reservoir level and the water migration of the dam filling was investigated, and the laws in temperature, humidity and displacement of earth-rock dams were analyzed. The calculated displacement field was then compared with the measured frozen deformation data to validate the results of the finite element simulation. The results showed that the freezing influence range of the dam slope was about 2 m, the range of temperature influence on the dam slope mainly depended on the depth of freezing, and the temperature change in the shallow range (0-2 m) of the dam slope was influenced by the outside air temperature. Also, the internal temperature of the dam body was small relative to the shallow dam slope, and there was a certain hysteresis. In addition, the effect of negative temperature was such that the shallow pore water phase of the dam slope turned into ice, manifesting macroscopically as a reduction in unfrozen water content. Water phase change, water migration from the dam filling to the dam slope, and the movement of the ice peak towards the dam body were found to be the main causes of berm freezing and expansion damage. The calculated amount expansion (due to freezing) of the dam slope was found to be in the range of 20-30 cm with a maximum value of 36 cm, which was consistent with the measured results. It was also found that the freezing and expansion damage of the berm is mainly caused by the joint action of freezing and expansion of the soil and rock mixture such as gravel bedding and dam filling, as well as the ice thrust force. It is expected that the results of this research can provide a basis for the design of berms of earth-rock dams in cold regions.