Study On The Evolution Of Three Zons Fissure Field In Shallow Coal Seam Group And The Causes Of Low Oxygen

With the gradual depletion of coal resources in central and eastern China,the importance and strategic position of Shaanxi,Shanxi,and Inner Mongolia regions have become more prominent.Under the influence of repeated disturbance,Shanxi,Shaanxi and Ineer Mongolia connected the“three zones”of surface-goaf-working face of shallow buried depth and close range coal seam group,forming a“breathing”gas leakage channel.External disturbances such as day and night season changes and repeated mining have disrupted the pressure balance in the“three zones”resulting in large-scale low-oxygen areas in the working face,which has become a new challenge in the mining production process.This article takes the Ningtiaota coal mine in the Shendong mining area as the background,and studies the spatiotemporal evolution of gas leakage fractures in the“three zones”of coal-rock mass during the repeated mining of shallow and close-range coal seams,and the impact mechanism of multi-variable factors（pressure and temperature cycle changes,fracture scale,and advancing speed）on the“three zones”breathing.The thesis is to prevent and control low-oxygen accidents in coal mines.The following work has been completed:（1）Typical geological characteristics and causes of low-oxygen in major coal mines in the Shanxi,Shaanxi,and Inner Mongolia regions.Analyzing 200 literature and investigating 30 coal mines Sorted out and analyzed200 articles,investigated 30 coal mines,including 20 low-oxygen coal mines,20low-oxygen coal mines,the geological characteristics of low-oxygen coal mines in the Shanxi,Shaanxi,and Inner Mongolia regions are shallow burial,easy self-ignition,thick coal seams,short interlayer spacing,and fast working face advancing speed.The main sources of low-oxygen gas are the N₂-CO₂zone,oxidation of residual coal in the goaf,and nitrogen injection in the goaf.The outburst power is caused by rapid atmospheric pressure reduction and negative pressure ventilation.The main influencing factors are changes in the ventilation system,fracture penetration during mining,atmospheric pressure cycle changes,and periodic pressure.（2）Spatiotemporal evolution and scale distribution characteristics of gas leakage fractures in the“three zones”.Taking the Ningtiaota coal mine in the Shendong mining area as the engineering background and combining the key layer theory,a discrete analysis model of the collapse of overlying strata in a shallow and close-range coal seam group is established.The stage and regional evolution laws of fracture development under the repeated mining of coal seam groups,the distribution law of fracture scale and density,and the influence of interlayer spacing and coal seam thickness on fracture density are analyzed.The spatiotemporal distribution characteristics and evolution laws of the surface-goaf-working face“three zones”gas leakage channel are revealed.（3）The model of the influence law of multi-parameter variables and the volume concentration change of multi-component gas in“three zones”of breathing.Based on the measurement of ventilation resistance,oxygen concentration data in the return air corner,and surface pressure and temperature data of the S1232 working face in Ningtiaota coal mine,the influence of ventilation system,temperature,pressure,and gas leakage on low-oxygen in the working face is analyzed.Based on the conservation of mass and energy and Dalton’s law of partial pressures,a multi-element gas volume concentration change model based on the width of fractures and pressure cycle changes in the“three zones”breathing is established.The influence of multi-variable factors on the overall trend of gas outburst in the working face and oxygen concentration in the goaf is compared and analyzed.（4）“Three zones”multi-field coexistence prevention and control mechanism model and uniform pressure prevention and control system design.A low-oxygen prevention and control mechanism model under the coupling of pressure field,fracture field,and flow field is established.From the perspective of destroying the balance of the“three zones”multi-field coexistence disaster,the main low-oxygen prevention and control measures in the Shanxi,Shaanxi,and Inner Mongolia regions are analyzed and classified.A design idea of an intelligent and precise uniform pressure ventilation system for fully mechanized working faces is proposed,considering the difficulties in precise pressure control in the working face and goaf areas and the low degree of intelligence.This thesis includes 51 figures,22 tables,and 89 references.