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The influence of angle patterns of intersecting fractures is crucial for a profound understanding of the flow characteristic of fractured rock masses. This study employs hydraulic and visualization experiments to investigate macroscopic nonlinear flow behavior and local flow field characteristics of a "one inlet two outlet" fracture intersection with varying angle patterns. The angle pattern exerts a significant influence on the morphology of the intersection, leading to enhanced nonlinear flow and altered flow rate distribution. Both the nonlinear coefficient (b) and the ratio of flow rate distribution (ɛ) exhibit a Gaussian distribution pattern with respect to the angle patterns. By observing the intersection morphology and local flow fields under different angle patterns, it was found that the protrusion caused by the inlet and outlet fractures significantly impacts the fluid flow entering the outlet fractures, particularly under high Reynolds number (Re) conditions. Using a machine learning algorithm based on neural networks, the flow rate distribution at the outlet of the intersection fracture under different Re values and angle patterns is predicted. The model demonstrated accurate predictions of the outlet flow rate distribution for intersection fractures under diverse conditions. Additionally, by analyzing the flow velocity and flow characteristics at different positions within the intersection, the relationship between the macroscopic flow rate distribution and the flow field characteristics at the intersection has been clarified. These results will aid in the study of groundwater flow behavior, fluid flow in microfluidic chips, and seepage heat transfer. [ABSTRACT FROM AUTHOR]

The causes of ground fissure formation are closely related to the change in the stress state in a loose layer, but many researchers often ignore the dynamic processes of mining when studying mining-induced damage mechanisms. The study of stress evolution in a loose layer during mining can help to reveal the causes of surface damage and its self-healing mechanism. This paper establishes a numerical model to study a loose layer’s stress evolution and its causes during mining. The discriminative conditions of the potential dynamic fissure formation cycles and self-healing are given. The vertical and horizontal stresses in the loose layer at the mining boundary increase and then decrease under the mining disturbance, and the vertical stresses in the loose layer above the comprehensive mining surface undergo two increasing and decreasing stages, while the horizontal stresses increase and decrease and then increase again. The permanent fissures at the mining boundary form because of the decompression- and compression-related damage caused by horizontal unloading, while the dynamic fissures in the loose layer at the mining surface form due to the first horizontal unloading effect; the shear damage is caused by the second increase in vertical stress. Additionally, the soil Mohr‒Coulomb strength criterion and the linear elastic stress‒strain relationship are used to discriminate the single- and double-cycle development patterns of the dynamic fissures on the ground surface. [ABSTRACT FROM AUTHOR]

After the mines closed, the groundwater level quickly rises, and pollutants in the underground space will migrate outward along the flow channel, causing pollution to the surrounding environment. The different morphologies of interlayer fractures in the goaf fracture zone are one of the main flow channels, including interlayer expansion fracture. Fracture expansion can impact a local flow field, which, in turn can affect solute transport characteristics. The microscale flow field and solute transport behavior in fracture with different expansion characteristics were investigated by a microscale visualization experiment. The experimental results indicate that there is a significant difference in the pressure gradient between different expansion regions. When nonlinear flow characteristics (reflux or eddy) begin to occur within the expansion region, the pressure gradient value also begins to deviate from Darcy's law under the same Re. The maximum velocity position changes from the centerline to the upper and lower surfaces of the expansion region, a low velocity region is generated in the main flow channel, which moves to the outlet as Re increase, and the cross-section velocity curve also transforms into a bullet shape as Re increase. By developing a set of programs to establish a relationship between the average image luminance and the tracer particles concentration, the solute concentration variation curve in the expansion region was obtained. The process of solute invasion into fractures is divided into four stages. The change in the local flow field caused by the different expansion characteristics directly affects the duration of these four stages. As the area of the region increases, the proportion of the main channel area decreases relatively, the difference between the solute concentration curve of the whole fracture and the expansion region gradually decreases. [ABSTRACT FROM AUTHOR]

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 understanding of the weakening mechanism of tensile strength of rock subjected to cyclic wetting-drying is critical for rock engineering. Tensile strength tests were conducted on a total of 35 sandstone specimens with different wetting-drying cycles. The crack propagation process and acoustic emission characteristics of the tested samples were obtained through a high-speed camera and acoustic emission system. The results indicate that the tensile strength is observably reduced after cyclic wetting-drying, and the extent of the reduction is not only related to the number of wetting-drying cycle, but also closely related to the clay mineral content of the sample. In addition, as the cycles of wetting-drying increase, the effect of each single cycle on tensile strength get reduced until it becomes constant. Moreover, the crack initiation and penetration time is prolonged as the number of wetting-drying cycle increases, which indicates that cyclic wetting-drying weakens the rock stiffness and enhances the ductility of sandstone. Meanwhile, the acoustic emission characteristics of the tested samples further confirmed the ductile behaviour of the sandstone samples with increasing wetting-drying cycle. Furthermore, through the analysis of the microstructure and mineral composition of the samples with different wetting-drying cycles, it is concluded that the main weakening mechanisms of sandstones containing clay minerals are frictional reduction, chemical and corrosive deterioration. [ABSTRACT FROM AUTHOR]

An understanding of the influence of fracture surface roughness on local flow patterns is critical for underground engineering and geological sciences. The flow pattern in a fracture is not only affected by the large-scale waviness of the fracture morphology, but also by the local small-scale roughness of the fracture surface, in particular the local flow pattern. The microfluidic field of flow in two fracture models with and without local small-scale roughness was investigated through a microscopic visualization hydraulic experiment. By quantitatively analyzing the velocity and vorticity in the two fracture models, the results indicate that the characteristics of the fracture microfluidic field are closely related to the local small-scale roughness. First, due to the effect of the local small-scale roughness, it is easier to generate and develop eddies in the original fractures that contain small-scale roughness. Meanwhile, the local small-scale roughness increases the velocity gradient in the vertical section of the fracture flow and further enhances the complexity of the fracture flow field. Moreover, although the small-scale roughness has little impact on the position of the maximum velocity, the area of eddies increases and the effective flow aperture is significantly reduced. [ABSTRACT FROM AUTHOR]

Not only is the thermal environment of the coal mining face related to the temperature of the surrounding rock, it is also closely associated with the ventilation model of the working face. In this study, the numerical methods were applied to study the impact of two major ventilation systems on the airflow temperature of working face in coalmine. Firstly, a heat transfer model of the surrounding rock and airflow was established to reveal that the wall roughness of the surrounding rock could enhance heat transfer between the surrounding rock and the airflow. Moreover, an analysis was conducted of the heat transfer between the airflow and the surrounding rock under different modes of ventilation in the first mining face. According to the analytical results, the temperature of airflow in the U-type ventilation system is lower than in the Y-type ventilation system. For the next adjacent coal mining face, however, the Y-type ventilation system is more conducive in reducing the temperature of the airflow. Therefore, with regard to the mine as a whole, the Y-type ventilation system is more effective than a U-type system in reducing heat and humidity in the ambient environment. [ABSTRACT FROM AUTHOR]