| Drilling and extracting gas in soft coal seams and depressurizing coal and rock masses are prerequisites for reducing gas content in soft coal seams and eliminating gas outburst.However,during the formation of loose coal seams,severe structural damage and soft coal quality have occurred,and problems such as hole collapse,diameter shrinkage,and cracking often occur during the extraction of boreholes in soft coal seams.It will also block the gas outflow channel,which will ultimately reduce the gas extraction efficiency of the soft coal seam greatly,and bury hidden dangers for coal mine safety production.Therefore,it is of great significance to carry out the research on the progressive deformation and destruction of the coal and rock mass around the borehole of gas drainage to improve the efficiency of gas drainage in soft coal seams and ensure the efficient drainage of mines.In this paper,through the combination of theoretical analysis and laboratory tests,the DNS200 electronic universal testing machine and digital image related technologies are used to obtain the progressive deformation and failure laws of coal and rock bodies around the drilled hole,and the underground gas drainage is obtained through on-site industrial tests.The leakage of mining holes.The following research results have been achieved:(1)According to the linear pore elasticity model,the stress distribution of coal and rock mass around the borehole before and after drilling is deduced.For any horizontal stress value,the critical position remains unchanged.In addition,if the horizontal stresses are equal,the tangential and axial stresses are constant and independent of the direction of the tangential stresses around the borehole.(2)In order to study the deformation law of progressive failure of drilling holes,the digital image correlation technology was used to obtain the image sequence containing the failure of the borehole sample,and the strain and displacement of the sample surface were calculated.The progressive destruction process of bored specimens is more accurately divided into zones for study.The systematic analysis of the overall deformation law of the specimen and the change law of the displacement around the borehole are systematically analyzed.(3)The crack initiation phase of the borehole is between 38.47%and 52.05%of the peak stress,which means that the borehole section in the stress concentration area is prone to cracks The failure phase of the crack around the hole is between 62.08%and 76.14%of the peak stress.At this time,the crack on the surface of the sample begins to penetrate,which means that the drilling section in the crushing area is prone to penetrate the crack and form a leak channel.(4)In order to study the crack propagation law of progressive failure of coal and rock bodies around the borehole of the extraction hole,the DSCM system was used to observe the dynamic crack development and evolution of the sample during the whole deformation process.The localized deformation of the sample started before the peak point,About 80%of the peak axial displacement.The displacement of the coal wall around the hole and the displacement rate around the fissure are both abrupt near the peak stress point,indicating that the displacement of the coal wall around the hole and the occurrence time of the internal crack of the coal around the hole are consistent.The main types of cracks that cause specimen damage are open and staggered mixed cracks.(5)Conducted a field test for detecting the location of gas leakage in the extraction borehole,and applied the law of crack expansion of coal and rock mass around the extraction borehole to analyze the cause of gas leakage in the gas extraction borehole,and pointed out the circumference of the extraction borehole The leakage path caused by the expansion of coal and rock fractures gives a method for determining the leakage path of the extraction borehole,and a method for detecting the leakage position of the extraction borehole is initially formed. |
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