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High temperature is an important factor affecting the mechanical properties and crack propagation behavior of granite. In this study, the mechanical properties and thermal damage mechanism of granite after high temperature are studied experimentally. Subsequently, based on the content and distribution of granite minerals, combined with computed tomography (CT) technique and grain-based model (GBM) modeling method, a novel discrete element numerical model considering real mineral distribution is introduced, and the rationality of the method is verified by experiments. On this basis, the mechanical behavior, crack propagation behavior and failure mode of granite after high temperature are studied. The results show that 450 °C is the thermal damage threshold of granite in this study. When T ≤ 450 °C, the development of thermally-induced microcracks is not significant, the P-wave velocity of granite decreases slightly, and the change of mechanical properties is not obvious. When T > 450 °C, the thermally-induced microcrack propagates rapidly, the fracture network and fracture zone are formed locally in the specimen, and the P-wave velocity and mechanical properties of granite deteriorate significantly. The simulation results show that the number of thermally-induced microcracks in granite specimens is positively correlated with temperature, and the tensile cracks are mainly at the mineral boundary, and the thermal stress concentration between mineral particles is the main cause of thermally-induced microcracks. After temperature treatment, there are mainly tensile cracks in granite specimens under uniaxial compression, and the orientation of tensile cracks tends to be isotropic with the increase of temperature. When T < 600 °C, the crack distribution and failure mode of granite specimens under uniaxial compression are consistent with those at room temperature. When T ≥ 600 °C, thermally-induced microcracks begin to dominate the uniaxial compression failure process, and granite specimens begin to show ductile failure characteristics, and the brittle-plastic transition was observed between 600 °C and 750 °C. Highlights: A novel thermo-mechanical grain-based model considering the distribution of real granite minerals is proposed. 450°C is the threshold temperature for thermal damage of granite in this study. When T≥600 °C, thermally-induced microcracks dominate the failure mode of granite, and brittle-plastic transition is observed between 600 °C and 750 °C. The orientation distribution of tensile crack caused by axial load tends to be isotropic with the increase of temperature. [ABSTRACT FROM AUTHOR]

Thermal conductivity of rock is one of the important parameters to understand the heat conduction process in interior of the earth. The study of the effect on the thermal conductivity of clay-bearing sandstone subjected to drying–wetting process is of great significance to many geological and geotechnical engineering issues. In this study, drying–wetting cycle experiments on clay-bearing sandstone were carried out, including three times of drying and two times of water saturation treatment. The thermal conductivity of clay-bearing sandstone after each treatment was measured by transient hot wire method, and the thermal inhomogeneity was analyzed. The results indicate that the drying–wetting process leads to the decrease of the average thermal conductivity of clay-bearing sandstone, while the increase of thermal heterogeneity factor. Base on the results of 3D scanning and SEM, it is found that the development of pores and microcracks during the drying–wetting process is the main reason for the average thermal conductivity decreased and the thermal inhomogeneity increased. Further analysis shows that the interaction between clay minerals and water leads to the destruction of rock matrix structure, resulting in the increase of primary pores, the formation of new pores and secondary microcracks in clay-bearing sandstone. In addition, the numerical results show that the pore leads to the significant weakening of rock heat transfer effect, and the temperature field tends to be heterogeneous distribution. The research results can provide reference for the evaluation of thermal conductivity of rock mass in deep engineering. [ABSTRACT FROM AUTHOR]

High geo-temperature is one of the inevitable geological disasters in deep engineering such as resource extraction, space development, and energy utilization. One of the key issues is to understand the mechanical properties and failure mechanism of high-temperature rock disturbed by low-temperature airflow after excavation. Therefore, the experimental and numerical investigation were carried out to study the impact of cooling rate on mechanical properties and failure mechanism of high temperature sandstone. First, uniaxial compression experiments of high temperature sandstone at different real-time cooling rates were carried out to study the mechanical properties and failure modes. The experimental results indicate that the cooling rate has a significant effect on the mechanical properties and failure modes of sandstone. The peak strain, peak stress, and elastic modulus decrease with an increase in cooling rate, and the fragmentation degree after failure increases gradually. Moreover, the equivalent numerical model of heterogeneous sandstone was established using particle flow code (PFC) to reveal the failure mechanism. The results indicate that the sandstone is dominated by intragrain failure in the cooling stage, the number of microcracks is exponentially related to the cooling rate, and the higher the cooling rate, the more cracks are concentrated in the exterior region. Under axial loading, the tensile stress is mostly distributed along the radial direction, and the damage in the cooling stage is mostly due to the fracture of the radial bond. In addition, axial loading, temperature gradient and thermal stress mismatch between adjacent minerals are the main reasons for the damage of sandstone in the cooling stage. Moreover, the excessive temperature gradient in the exterior region of the sandstone is the main reason for the damage concentration in this region. [ABSTRACT FROM AUTHOR]

• The thermal conductivity of clay-bearing sandstone subjected to wetting and drying processes were measured. • The thermal conductivity of the samples subjected to the drying II treatment was significantly lower than that of the samples subjected to the drying treatment. • During the process of wetting and drying, the change of pore structure and the generation of secondary microcracks are the main reasons for the change of thermal conductivity. The influence of dry-wet change process on rock thermal conductivity is the key to many geological and engineering issues, especially for rocks with high clay mineral content. In this study, sandstone with different clay mineral contents were subjected to different treatment processes, namely drying, wetting and drying II, and their thermal conductivity were measured through the hot wire method. The results indicate that the thermal conductivity of the samples subjected to the drying II treatment was significantly lower than that of the samples subjected to the drying treatment, and the magnitude of the decrease was related to the clay mineral content in the samples. Moreover, using the effective thermal conductivity calculation model that considers microcracks, it is found that the main reason for the decrease in the thermal conductivity of sample is the large number of secondary cracks generated in sample during the wetting-drying process. According to the pore structure and the microstructure of the samples under different state, it is speculated that the dissolution of soluble minerals, disintegration of minerals, swelling and shrinking of clay minerals are the possible causes of cracks in the sample during the wetting-drying process. [ABSTRACT FROM AUTHOR]