Knowledge of deformation and strength behavior of rocks under high in situ temperature is highly important for the control of geological disasters in exploration of hot dry rock and mining in deep formation. In this study, uniaxial compression tests were carried out on granite under different real-time high-temperature conditions (25, 200, 300, 400, 500, 600, and 700°C) and loading rates (0.01, 0.1, and 0.5 mm/min). The effects of real-time high temperature and loading rate on the uniaxial compressive strength and elastic modulus of granite were studied, and the microscopic morphology of the fracture surface was analyzed. The results show that the uniaxial compressive strength and elastic modulus of granite increase first and then decrease with the increase of temperature. The uniaxial compressive strength clearly increases at 200°C and decreases gradually when the temperature exceeds 300°C. Under the same temperature conditions, the uniaxial compressive strength of granite decreases and the elastic modulus increases with increasing loading rate. When the temperature reaches 600°C, the effect of the loading rate on the uniaxial compressive strength and elastic modulus of granite decreases significantly. The test results are compared with the results of work performed on quenched granite. Under real-time high-temperature conditions, the thermal crack effect has a significant influence on the uniaxial compressive strength and elastic modulus of granite, without the thermal hardening effect of quenched granite. During hydraulic fracturing, the rock skeleton near the injection well is cooled and shrunk, as is the thermal hardening effect caused by high-temperature quenching. The formation of thermal equilibrium leads to the large-scale extension of fracture cracks along the weak plane structure, such as the effect of thermal cracks on granite under real-time high-temperature conditions.

Coal seam deformation due to gas adsorption affects the stability of the underground structure. Natural coal blocks of the Shanxi Formation were selected to study the dynamic adsorption characteristics of coal samples subjected to CO 2 , CH 4 , and N 2 gas injections under coaxial pressure and confining pressure (7 MPa), as well as the displacement of CH 4 with CO 2 and N 2 under the same conditions. The results show that, under the same conditions, the strain in the coal samples first increased, followed by a rapid increase along with the increase in pressure, with the transverse strain being always higher than the axial strain. The amount of gas adsorption varied from high to low as CO 2 > CH 4 > N 2 , and the final adsorption strains and equilibrium times were different for each gas. Based on the increase in gas pressure, the gas adsorption strain curve can be divided into two stages. The displacement of N 2 only uses partial pressure to achieve the desorption of CH 4 in the coal sample, leading to shrinkage deformation of the coal sample. In contrast, the displacement of CO 2 has the dual effects of competitive adsorption and partial pressure reduction on CH 4 , leading to the swelling deformation of the coal sample.

To study the stability control of stope mining roadways below remaining coal pillars, the present study investigates the destabilization mechanism of coal pillars and roadways in sections under the dual action of supporting pressure on the floor of the remaining coal pillar in the overlying coal seam and the mining at the working face of the lower coal seam and clarify the principle of surrounding rock stability control based on theoretical analysis, numerical simulation, and industrial testing. The results yielded the following findings. After the stope mining of the overlying coal seam working face, the stress transfer of the T-shaped remaining coal pillar significantly increased the vertical stress of the lower coal seam. The lateral support pressure generated by the stope mining at the lower coal seam working face further aggravated the stress concentration in the coal, leading to severe compression-shear failure of the surrounding rock. As the sectional coal pillar becomes wider, the roadway gradually avoids the area of peak superimposed support pressure. The vertical stress curve of the sectional coal pillar shifts from single-peaked to asymmetrically double-peaked, and the stress difference between the two roadway ribs and the stress concentration coefficients decrease continuously. A stability control method of long anchor cable reinforcement support is proposed. In-situ industrial testing showed that the surrounding rock deformation was basically stable during the service period of the 42202 stope mining roadway, thus achieving the stability control of the stope mining roadway.

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An in-depth understanding of the effect of real-time high temperature and loading rate on the fracture toughness of rocks is highly important for understanding the fracture mechanism of Hot Dry Rock (HDR). Three-point bending tests on notched semi-circular bending (NSCB) samples at the real-time temperatures (25, 100, 200, 300, 400 and 500 ℃) and different loading rates (0.1, 0.01 and 0.001 mm/min) were performed to characterize the temperature and rate dependence of the mode I fracture toughness. Besides, the characteristic of the fracture surface morphology was investigated by scanning electron microscope (SEM) and crack deviation distance analysis. Results show that the temperature has a significant effect on the development of intergranular and transgranular cracks. The fracture toughness and peak load are similarly influenced by temperature (i.e., they both decrease with increasing temperature). At the loading rates of 0.1 mm/min and 0.01 mm/min, from 25 to 400 °C, the fracture toughness decreases slightly with decreasing loading rates. However, at a loading rate of 0.001 mm/min, the fracture toughness values above 200 °C are very similar, and the fracture toughness does not strictly follow the law of decreasing with decreasing loading rate. Especially at 500 °C, fracture toughness and loading rate are negatively correlated. Our study also indicates that the effect of loading rate on macroscopic crack propagation path at real-time high temperature is not obvious. This study could provide an important basis for evaluating the safety and stability of geothermal engineering.