As one of the most crucial mechanical parameters of the rock materials, the effect of brittleness on the deformation and failure is of great practical significance for geotechnical construction and disaster prevention and mitigation. In this paper, the deformation and failure behaviors of the different brittle samples under dynamic loading were investigated using a split Hopkinson pressure bar (SHPB) experimental system. Besides, scanning electron microscopy (SEM) was also employed to study the relationship between the microscopic failures and rock brittleness and strain rate effects. The results revealed that the brittleness indexes BI3 and BI5 of the samples under uniaxial compression follow a linearly decreasing trend affected by the temperature changes, while the brittleness of the sample shows an increasing trend with the increase of strain rate under the dynamic loading. Also, the decline in the brittleness leads to an increase in the prepeak yield deformation phase of the sample under dynamic loading; after the peak point, the sample failure mode transitions from type I to type II with self-sustaining failure. Moreover, it was found that the dynamic strength increase factor presents a negative correlation with the sample brittleness. Finally, the macroscopic failure mode of the sample changes from split failure with multiple cracks to shear failure with few cracks due to the effect of decreasing brittleness. The failure surface of the sample gradually becomes smooth with the increase of brittleness, which manifests as a decrease in microcracks, and the gradual increase of the strain rate makes the failure surface rough, accompanied by an increase in microcracks.
Coalbed methane is always a major hidden danger that affects mining safety in coal mines. In the study of coal seam water injection to control gas disaster, the increase of free water content is helpful to destroy the integrity of coal seam and to promote the flow of gas in fractures. However, when the free water fills the fracture space, it will increase the flow resistance of gas, and then will reduce the gas extraction efficiency. At present, there is currently no mathematical model describing the effects of coal seam water injection that combines these two aspects on gas drainage. In this study, a series of experiments were conducted to study the differences in mechanical property changes under wetting conditions with different coal samples. The experimental results show that the elastic modulus and compressive strength decrease as an exponential function with increasing water pressure. Based on the experimental results, a gas-liquid-solid coupling model including effective stress change and gas desorption is established and used to predict a field gas extraction application. According to the results of the numerical model, In the plastic failure zone of coal seam, the permeability increases, the elastic modulus drops and gas migrates faster. In the water wetting zone, the free water occupies the fracture space, which blocks the gas migration channel. The overall effect of water injection on gas extraction depends on which impact plays a dominant role. The established gas drainage model is validated by field data and can reflect the pattern of borehole damage and gas drainage under water injection.
In the western region of China, coal mining activities are prone to induce water and sand inrush disasters, which seriously threaten the safe production of the coal resources. In this paper, an experimental device was designed to simulate the process of water and sand inrush, and then, the control factors of the disasters in the broken rock mass in the goaf were investigated. Also, the seepage fracture channels in the broken rock mass were simplified by using the 3D printing technology, and the effects of fracture aperture and angle on the seepage characteristics of water-sand mixtures were analyzed. The experimental results showed that the porosity and skeleton structure of the broken rock mass were the key factors to control the water and sand inrush disasters. The smaller the initial porosity of the broken rock mass, the weaker its permeability, and the less probable to form a dominant channel for the water and sand inrush disasters. Conversely, the broken rock mass structure with larger size gradation was more likely to form the permeable channels, and the quality of the sand inrush was greater. In addition, it was also found that the angle of the fractures within the broken rock mass affected the seepage characteristics of water-sand mixture, and the permeability showed an exponential relationship with the fracture angle. Meanwhile, as the fracture aperture increased, the fracture angle generated greater influence on the permeability. Finally, we proposed the water and sand inrush prevention and control technology based on the experiment results. The results of this study can provide a reference for the control of water and sand inrush disasters in western China.
Coal mine reuse involves complex environments such as chemical erosion and dynamic perturbation. Therefore, the effect of chemical erosion on the dynamic behavior of the red sandstone was studied by split Hopkinson pressure bar (SHPB) tests under the strain rates of 70~125 s−1. The full-field deformation of the sample was then recorded through high-speed 3D digital image correlation (3D-DIC) technique. The dynamic deformation characteristics, especially the lateral strain, were extracted by averaging the lateral strain field by pixels. Also, the fracture behavior was investigated based on the evolution of strain localization in the strain field. The results indicated that the deformation field evolution of the sample is controlled by the chemical erosion effect and the loading strain rate. The chemical erosion lowers the stress threshold for strain localization and accelerates its expansion rate, which is closely related to the dynamic strength degradation of the sample. In contrast, the loading strain rate increases the dynamic strength but advances the occurrence of strain localization and shortens the time to the peak stress. The normalized stress thresholds for the initiation and development of cracks inside the sample under dynamic loading are reduced by chemical erosion, with the two thresholds dropping to 10%~30% and 20%~70% of the peak stress, respectively. The minimum thresholds for the initiation and development of cracks inside the red sandstone under dynamic loading are 11% and 24% of the peak stress, respectively.
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