Mechanism Of Crack Propagation Around The Drainage Boreholes And Detection Technology

Gas drainage.in underground coal mines is an important means of mine gas control.In order to improve the gas drainage,crack propagation as much as possible is required around the borehole at the inner,and crack propagation as airtight as possible is required around the borehole in the sealing section.However,due to the difficulty in measuring the propagation of the crack around the borehole,the mechanism of crack propagation is still unclear,resulting in that construction of underground gas permeable extraction and grouting sealing mainly relying on experience,lacking of effective theoretical guidance.Thus,in this dissertation,theoretical analysis,laboratory tests and field tests was used to analyze the influence of various factors on the crack propagation around the borehole.By determining the ultrasonic characteristic parameters in the process of crack propagation,an ultrasonic detection method for the crack propagation around the borehole is proposed,which has important theoretical and engineering significance for the design,construction and detection of underground drainage of coal mine.The main contents of the dissertation are:(1)The stress of the borehole is analyzed.Combined with Mohr-Coulomb and unified strength criterion,the limit equilibrium theory was used to analysis the stress distribution around the borehole and to obtain the range of the limit balance area around the borehole.Considered the three-dimensional geometrical conditions of borehole and roadway intersection,the calculation model of the stress of the borehole under the influence of stress redistribution is established.The three-dimensional stress distribution of the borehole under the influence of the roadway is obtained,and damage distribution law of the borehole hole is obtained in three dimensions.(2)Aiming at the mechanism of the crack propagation around the borehole,the damage experiments of the sample with a hole was carried out,and the image sequence of the sample was obtained by the non-contact full field strain monitoring system.The displacement of the surface of the sample was calculated and refined to study the law of the movement around hole.Based on the elastic theory and the adjacent unit weighted sharing model,the surface stress inversion method of the sample was proposed,and the evolution process of the surface stress field was studied.The mechanism of crack propagation around the borehole in different stress regions was revealed.(3)Aiming at the problem of water content in underground drilling,3D printing technology was used to prepare perforated samples with different water content.The non-contact full-field strain monitoring system was used to observe the failure process of samples with different water content.The deformation and stress distribution of the samples with holes were calculated by the digital speckle correlation method and the stress inversion model.The perforation displacement and stress evolution process under different stress levels with different water contents were studied,and the law of crack propagation around the borehole with water-rock coupling was obtained.(4)In order to obtain the ultrasonic characteristics during the process of crack propagation around the borehole,the waveform data was collected using the RSM-SY7 ultrasonic system.The frequency domain of the waveform is obtained by frequency domain analysis.The variation of waveform amplitude and power spectral density with crack propagation around hole is studied.The equivalent width evolution model was established to quantify the crack propagation process.The effect of water on crack propagation and ultrasonic characteristic parameters was obtained.(5)According to the law of the characteristic parameters of ultrasonic wave propagation in the crack,the ultrasonic detection device of crack propagation around the borehole was designed.The device is applied to the quality detection of sealing of gas drainage borehole.The sealing quality evaluation model was established by using the waveform similarity and power spectral density analysis combined with logistic regression algorithm.The field test of ultrasonic quality detection of sealing of gas drainage borehole was carried out,and the sealing quality of the drilled hole was analyzed.Through the field test of Yuwu Coal Industry of Shanxi Lu'an Mining(Group),the practicality and accuracy of the technology are illustrated,which provides a theoretical basis and technical support for the rapid detection of crack propagation and coal and gas co-extraction.