Study on the characteristics of roof fracture instability based on the theory of medium thick plate
The working face of the thick roof is subject to strong dynamic loading,often resulting in coal rib spalling and hydraulic support crushing.In order to enhance the ground control under the thick roofs,the deformation and stress characteristics of the roof,failure modes,dynamic loading effects and control methods were investigated by taking the 14030 working face of the Zhaogu Second Coal Mine as the background.Based on the results from theoretical analysis,numerical simulation,and field measurement,it is found that the maximum tensile stress and maximum shear stress are located at the midpoint of the coal wall side and the cutting eye side during the first weighting,and the roof undergoes tensile failure.During periodic weighting,shear stress concentration zones form at both ends on the coal wall side,with significant development of shear cracks,resulting in mixed tensile-shear failure of the roof.The impact characteristics of the working face advance distance and the roof thickness on tensile(shear)stress were determined.The influence of roof thickness on failure mode was analyzed.Numerical models of first and periodic weighting of the roof were established using PFC3D,determining the roof failure morphology,crack distribution characteristics and failure modes.Based on the intervention of the sand and thickness effect,the reasons for the occurrence of shear failure in the roof were explained,revealing the mechanism of dynamic loading on the roof.By utilizing the principle of momentum conservation,the dynamic impact force caused by different fracture positions of the roof was calculated.The working resistance of the hydraulic support under dynamic load impact was obtained,and methods for dynamic load control were proposed.The research results can provide theoretical basis for medium-thick roofs or roofs that do not meet the conditions of thin plates,and help guide the safe production of medium-thick roof working faces.
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