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The stability of the surrounding rock under high temperature is pivotal to the efficient and safe production of high-temperature fluidized mining engineering. To investigate the stability of rocks under high temperature, this paper takes the roof sandstone of Tongxin coal mine, examining changes in its physical parameters such as mass, dimensions, wave velocity, porosity, and permeability after treatment at various temperatures (20–700°C). The results showed that parameters like mass and wave velocity decreased with increasing temperature, while dimensions, porosity, fracture density, and permeability increased. The patterns of change in these physical properties with temperature exhibit a high degree of consistency. Additionally, composition analysis and thermal analysis were conducted to understand the physical and chemical changes occurred in sandstone. Scanning electron microscopy was used to observe microstructural changes in the sandstone. After analysis, the evolution of the internal structure of Tongxin sandstone with heat treatment is categorized into three stages. (1) stable change Stage (20–450°C, 650–700°C): Dominated by dehydration and thermal stress, where pore and fracture structures develop slowly; (2) rapid change stage (450–550°C, 600–650°C): Dominated by the kaolinite dehydroxylation, leading to increased porosity but decreased average pore size; (3) intense change stage (550–600°C): Dominated by the quartz phase transitions, where the thermal stress generated by quartz phase transitions causes dramatic alterations in the internal structure of the sandstone. Furthermore, a correlation model between wave velocity and permeability of sandstone at high temperatures was established based on the interrelationship of these physical properties, providing a new method for real-time monitoring of permeability under high-temperature conditions.

To investigate the failure mode and strength characteristics of rock-like materials containing two non-coplanar closed discontinues, and the influence of crack attitude on the shear failure of rock mass, a series of direct shear tests are carried out on rock-like materials containing two pre-existing cracks of different attitudes, one of them is horizontal crack, and the other is inclined crack. The specimens containing pre-existing cracks of four inclinations (0°, 45°, 135°, and 180°) and three dip angles (30°, 60°, and 90°) are prepared, and the failure form and stress-displacement curve are recorded during the tests. The results show that the attitude of the crack has a significant effect on the failure form and strength characteristics. In positive shear, the shear stress increases slowly and the crack initiation of the crack occurs relatively late, and in negative shear, the shear stress increases rapidly and the crack initiation stage appears earlier. Under the normal stress of 1 MPa, the fracture surface fluctuates along the pre-existing cracks, and the difference in shear strength of each specimen is less than 1.0 MPa. Under the normal stress of 5 MPa, the fracture surface mostly distributes along the horizontal direction, and the difference in shear strength of each specimen is about 2.0 MPa. Under the same level of normal stress, the crack initiation and peak stress vary with the attitude of pre-existing crack, and the variation trend of peak stress is changed when the normal stress increased due to the difference in failure form. It is calculated that the ratio of crack initiation stress to peak stress ranges from 0.61 to 0.80.

In order to investigate the feasibility and reliability of the three-dimensional particle flow method in simulating the type I fracture toughness test, four types of numerical samples were established by particle flow code PFC3D: straight crack three-point bending (SC3PB), edge cracked flattened semicircular disc (ECFSD), cracked chevron notched Brazilian disc (CCNBD), and edge cracked flattened ring (ECFR). Three models with different strength parameters (group A, group B, and group C) were established for each type, in which group A parameters are obtained from the concrete model, group B parameters are applied for simulating marble, and group C parameters are for granite. The type I fracture toughness and the failure form of each model are obtained by conducting the numerical test, and the curves of load versus displacement of loading point are recorded. The numerical test results show that, with the same strength parameter, the maximum difference in test results of each specimen type is 0.39 MPa·m1/2. The KIC of ECFR specimen is 0.13–0.28 MPa·m1/2 smaller than that of CCNBD specimen, and the KIC of ECFSD specimen is slightly higher than that of CCNBD sample. The KIC of SC3PB specimen is 0.06–0.21 MPa·m1/2 smaller than that of the CCNBD sample. When the loading rate is less than 0.01 m/s, the effect of loading rate on fracture toughness can be reduced to less than 0.1 MPa·m1/2.

In this research, the deformation, particle crushing, and morphology evolution of saturated crushed gangue under variable axial stresses (0, 2, 4, 8, 12, 16, and 20 MPa) were investigated by performing a series of laboratory tests with our self-designed compacting device. The research results showed that the relationship between compression modulus and axial stress can be expressed by an exponential function. The relative breakage varied from 0 to 0.3685 and increased monotonously with the increase of axial stress. The relation between relative breakage and axial strain was described by a linear function. The particle circularity varied from 1.179 to 1.361 and decreased gradually with the increase of axial stress. When the axial stress increased from 0 MPa to 2 MPa, the relative breakage increased rapidly by over 39.15% of the total increase, and the particle circularity of large particles in the range of 15–20 mm sharply decreased by over 48.34% of the total decrease. 2 MPa was a key value in controlling the particle crushing of the saturated crushed gangue during compaction. Particle crushing was predominantly divided into three types: fracture, crushing, and grinding. At the early stage of compaction, fracture and crushing took place in large numbers. At the end, grinding was the main form of particle crushing. The axial strain was influenced by the initial gradation, and a larger Talbot exponent corresponded to a larger strain. However, the initial gradation had little effect on the relative breakage.

It is important to study the dilatancy property of water-saturated rock for understanding the engineering behavior of loaded rock mass. This study carried out the uniaxial and triaxial compressive experiments on the water-saturated red sandstone, analyzed the influences of confining pressure and pore pressure on dilatancy property of water-saturated rock, and discussed the reasonable basis of the stress of dilatancy onset as a strength design parameter of rock engineering, finally established the prediction model of the stress of dilatancy onset under the impacts of confining pressure and pore pressure. The results show that the strength parameters (the stress of dilatancy onset and peak strength) and deformation parameters (axial strain and circumferential strain) of water-saturated sandstone increase with the confining pressure, and the relations can be fitted with a positive linear function. The cohesion and internal friction angle obtained from the stress of dilatancy onset decrease by 11.57% and 7.33%, respectively, when compared with those obtained from the peak strength. The strength parameters and deformation parameters of water-saturated sandstone decrease basically with the increase of pore pressure, in which the relations between strength parameters or axial strain and pore pressure can be fitted with a negative linear function. However, the relation between the peak circumferential strain and the pore pressure should be characterized by a negative exponential function, and the circumferential strain at dilatancy onset isn’t affected by the pore pressure. Keywords: Rock mechanics, Dilatancy behavior, Confining pressure, Pore pressure, Water-saturated

In northern China, groundwater inrush of Karst collapse pillar (KCP) often affects the coal mining process. Current studies rarely consider the seepage stability of filling materials of KCP, especially through experimental investigations. This study is to quantify the impacts of variable initial porosity and cementing strength on the seepage properties of filling material. For this purpose, we designed and fabricated a test system. This system can offer high water pressure and abundant water flow rate. We tested three types of specimens which were cemented by clay, gypsum, and cement, respectively. The seepage properties were obtained under the initial porosity of 0.11, 0.13, 0.15, and 0.17, respectively. The change mechanism of seepage properties was measured through the comparison between mass loss and mass gain. The results showed the followings findings: (1) The permeability-time curves have two types: the first type is that permeability gradually increases up to the occurrence of seepage instability and the second type is that permeability gradually decreases and approaches to a stable value. No seepage instability is observed. (2) Initial porosity and cementing material significantly affect the water flow properties of filling material. In general, larger initial porosity has larger permeability. For clay as cementing material, seepage instability occurs soon and higher initial porosity has shorter time to reach seepage instability. For gypsum, seepage instability occurs after a period of time when initial porosity is large enough. For cement, the permeability decreases gradually and approaches to a stable value. The permeability-time curves have rapid decrease and slow decrease. (3) The permeability has a magnitude of 10−15–10−13 m2 and varies with initial porosity and cementing materials. The permeability is the largest for clay cementing and is the smallest for cement cementing.

In this research, the particle size distribution and permeability of saturated crushed sandstone under variable axial stresses (0, 2, 4, 8, 12, and 16 MPa) were studied. X-ray Computed Tomography results revealed that particle crushing is likely to occur considerably as the axial stress is approaching 4 MPa, which results in the change of pore structure greatly. During compression, the particle size distribution satisfies the fractal condition well, and the fractal dimension of particle size distribution is an effective method for describing the particle crushing state of saturated crushed sandstone. When the axial stress increases from 0 MPa to 4 MPa, the fractal dimension of the particle size distribution increases rapidly by over 60% of the total increase (0–16 MPa), and the permeability decreases sharply by about 85% of the total decrease. These results indicate that 4 MPa is a key value in controlling the particle size distribution and the permeability of the saturated crushed sandstone under axial compression. The permeability is influenced by the initial gradation of the specimens, and a larger Talbot exponent corresponds to a larger permeability.

As an important aspect of concrete durability damage, sulfate corrosion has attracted more and more people's general attention. Studying the durability of concrete against sulfate corrosion is conducive to ensuring the non-hazardous design of the concrete structure in the project, which is also very helpful to predict the service life of the project. This study simulates the corrosion characteristics of various sulfates on concrete under dry-wet cycle condi-tions through experimental methods, and studies the changes in the apparent characteristics, quality, deformation, and strength of concrete as the corrosion time increases. Based on the principle of fracture mechanics, three-point bending specimens are used to analyze the change mechanism of concrete fracture parameters under corrosion time, and to obtain the sulfate corrosion depth of concrete. According to the sulfate corrosion test, compared with the test samples without sulfuric acid corrosion solution, the test samples immersed in sulfuric acid concentration of 2.5% (A2.5 solution) and 10% (A10 solution) are all eroded by sulfur ions. After 30d-300d immersion in A2.5 and A10 solutions, the change trend of axial compressive strength and splitting tensile strength is basically the same. The corrosion time of 60d is the turning point of the change in the compressive strength of concrete. After a 300d test process, the compressive strength and tensile strength of the concrete in these two solutions are reduced to the lowest value. The compressive strength is 22.13 in the A2.5 solution and 21.13 in A10 solution, respectively, while the tensile strength is 1.83 in A2.5 solution and 1.81 inAlO solution, respectively. The changes of concrete axial compressive strength and splitting tensile strength with corrosion time are roughly the same, and both show a two-stage change pattern of first increasing and then decreasing. The difference is that the increase of the splitting tensile strength is at the initial stage of corrosion is smaller, but the deterioration is more serious in the later stage of the corrosion, indicating that sulfate corrosion has a greater impact on the splitting tensile strength of concrete. [ABSTRACT FROM AUTHOR]

In many areas in China, concrete buildings are often exposed to ocean salt corrosion and freezethaw environments, which poses threats to the durability of buildings. Previous studies cannot fully consider the combined effects of salt corrosion environment and freeze-thaw environment, and the further systematic research on the mechanical properties and damage mechanism of concrete buildings is still lacking. Therefore, the cyclic coupling effect of sulfate corrosion and freeze-thaw is taken as the research background to achieve the purpose of providing effective data support for the durability of recycled concrete. Through the mechanical performance test, the corresponding change law of the compressive strength of recycled concrete and the sulfate corrosion time under the sulfate corrosion environment is obtained. By using the scanning electron microscope and mercury porosimeter to detect the microscopic morphology and pore structure of recycled concrete under sulfate corrosion and freeze-thaw environment, the microstructure of the recycled concrete under different freeze-thaw times and different corrosion times are obtained. The research results show that: based on the mechanical performance experiment of recycled concrete, the relationship between the mechanical performance parameters of mineral-blended recycled concrete under different freeze-thaw and different corrosion times is initially established. Through the microscopic test method of recycled concrete, the relationship between fine appearance structure and macroscopic properties of recycled concrete is obtained. In the mechanical performance test, under the condition of freeze-thaw 3090 times, while the sulfate corrosion process of 30d to 300d, the compressive strength gradually increases from the average value of 58.6Mpa. However, after 300d corrosion, the compressive strength drops rapidly to an average value of 47.3Mpa, with an average decrease of 11.3%, indicating that under salt corrosion and freeze-thaw, its frost resistance and corrosion resistance are weakened. In the microscopic test of recycled concrete, the average pore size of the mixed recycled concrete after 300 days has increased by about 50nm compared with the value after 30 days. The pore structure of the concrete has become significantly worse under the action of freeze-thaw and salt corrosion. [ABSTRACT FROM AUTHOR]