With the increase of mining depth, rock burst disasters frequently occur in steeply inclined coal seams. Firstly, this paper analyzes the rock burst of 5521-20 working face in Yaojie No. 3 coal mine and summarizes the characteristics of rock burst in horizontal section mining of steeply inclined extra-thick coal seam (SIETCS). Then, the static load distribution characteristics and the influence of dynamic load in the horizontal section mining of SIETCS are systematically studied by combining theoretical analysis with numerical simulation. On this basis, the mechanism of rock burst in horizontal section mining of SIETCS is put forward, verified by actual measurement. The results show that the SIETCS is “clamped” under the combined action of the same change trend of roof and floor. The maximum principal stress peak values on the roof and floor sides reach 22.0 MPa and 20.5 MPa. The maximum shear stress earned 8.7 MPa and 8.4 MPa, which makes the shear stress concentration in the coal body high and tends to “shear dislocation.” Under this “shear-clamping” action, an approximate “trapezoidal” plastic zone and a “rectangular” stress concentration zone are formed under the section. With the increase of mining depth, the “shear-clamping” action of SIETCS becomes more and more intense. When the roof cantilever reaches the ultimate span and breaks, the intense dynamic load increases the shear stress and failure of coal, which is easy to induce rock burst. The superimposed load greatly affects the area from the roof side to the middle of the working face, and the rock burst is intense. The rock burst is weak on the floor side due to the pressure relief of the surrounding plastic zone. The monitoring results show that the supports pressure and MS events activity on the roof side and near the middle part of the working face is considerable, while the floor side is opposite, which verifies the research results.
Coal burst is a severe and dynamic hazard, and understanding its mechanism is crucial in preventing such incidents. Strong tremors during the working face mining in the stress anomaly zone of the pinch-out coal seam are frequent. Theoretical analysis, numerical simulation, and field measurement methods are used to analyze the energy evolution law for mining at the working face and mechanism of coal burst. Mechanical models of the inclined and strike overhang structures are established, and the theoretical analysis of the strike and inclined energy distribution characteristics of the working face roof is carried out. The two key areas with a high overhang bending deformation energy accumulation are identified at the lower end and middle-upper part of the working face. The simulation results show that the energy accumulation area of the roof in the inclined coal seam has prominent asymmetric distribution characteristics. The roof energy accumulates in the lower end and middle-upper area of the working face. The floor energy accumulates in the lower end area of the working face, and the peak position of the overhead energy of the working face in the direction shifts toward the coal-wall side. Influenced by the local absence of the No. 8 coal seam, the vertical stress of the surrounding rock at the working face of the massive, inclined coal seam increased by 12.7%; the peak of roof energy at the working face inclination and strike increased by 46.2% and 32.2%; and the range of roof energy accumulation expanded. A deep directional hole blasting plan to prevent the phenomenon at the working face roof is developed, which effectively reduces the stress and energy level of the inclined hanging roof and avoids the occurrence of coal bursts in the abnormal stress area of the working face.
With the increase in coal mining depth, engineering geological conditions and the stress environment become more complex. Many rock bursts triggered by two combined faults have been observed in China, but the mechanism is not understood clearly. The focus of this research aims at investigating the influence of two combined faults on rock burst mechanisms. The six types of two combined faults were first introduced, and two cases were utilized to show the effects of two combined faults types on coal mining. The mechanical response of the numerical model with or without combined faults was compared, and a conceptual model was set up to explain the rock burst mechanism triggered by two combined faults. The influence of fault throw, dip, fault pillar width, and mining height on rock burst potential was analyzed. The main control factors of rock burst in six models that combined two faults were identified by an orthogonal experiment. Results show that six combinations of two faults can be identified, including stair-stepping fault, imbricate fault, graben fault, horst fault, back thrust fault, and ramp fault. The particular roof structure near the two combined faults mining preventing longwall face lateral abutment pressure from transferring to deep rock mass leads to stress concentration near the fault areas. Otherwise, a special roof structure causing the lower system stiffness of mining gives rise to the easier gathering of elastic energy in the coal pillars, which makes it easier to trigger a rock burst. There is a nonlinear relationship between fault parameters and static or dynamic load for graben faults mining. The longwall face has the highest rock burst risk when the fault throw is between 6 and 8 m, the fault dip is larger than 65°, the mining height is greater than 6 m, and the coal pillar width is less than 50 m. The stair-stepping, imbricate, horst, and ramp fault compared to the other fault types will produce higher dynamic load stress during longwall retreat. Fault pillar width is the most significant factor for different two combined faults, leading to the rise of static load stress and dynamic proneness.
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