Study On Space Time Characteristics Of Advanced Pressure Relief In Front Of Fully Mechanized Top Coal Caving Face

In order to research the pressure relief and anti-reflection effect by hydraulic flushing in downward holes based on submerged jet in the soft and low-permeability coal seam.Taking 1604 working face of Guhanshan coal mine as the research object,the bedding and cross bedding test boreholes are arranged in front of the mining face,and the changes of gas drainage purity and concentration of test boreholes in different spatial positions with the mining process of the working face are investigated.Through theoretical calculation,field test analysis and numerical simulation,the characteristics of gas drainage in advance pressure relief area are explored,which can provide reference for reasonable hole layout scheme in field construction.The main conclusions as follows:(1)Guhanshan coal mine through the floor rock roadway through layer drilling pre drainage roadway strip gas and mining area coal seam gas,combined with the seam drilling pre drainage mining area coal seam gas control scheme pre drainage coal seam gas.After the pre drainage reaches the standard,the gas emission concentration of the return air flow is expected to be 0.87% ～ 0.98%.At the same time,it is also found that the gas concentration of the return air flow after mining is obviously higher than that before mining,which demonstrates the necessity of pressure relief gas drainage.(2)Through the theoretical analysis of the calculation formula of pressure relief width,the coal thickness and buried depth are positively correlated with the width of pressure relief belt,and the cohesion and internal friction angle of coal are negatively correlated with the width of pressure relief belt;Based on the analysis of the migration characteristics of pressure arch in the process of mining,the calculation method of the influence width of advance self-relief zone based on the transmission of overburden failure is proposed,and the theoretical calculation value of pressure relief zone width is29.97 m.(3)The field test shows that the stage change of gas drainage characteristics of test boreholes with the advancing of working face can be divided into stable section of drainage flow concentration,increasing section of drainage flow concentration and decreasing section of drainage flow concentration.In the section 20 m ～ 40 m away from the working face,the gas drainage flow concentration of the borehole increases by 3.89 times compared with the stable section,the gas drainage flow concentration of the borehole increases by 8.01 times compared with the stable section.According to the statistics of single hole gas drainage pressure relief width of bedding boreholes,combined with the average value of efficient gas drainage interval,the width of advance pressure relief zone in 1604 working face is determined to be 29.1m.(4)In the section of increasing the concentration of pumping flow,there is obvious "layering" phenomenon in the pumping pure quantity of the drilling holes in different layers of the working face,and the change broken line is "Λ",It is determined that the best effect is that the test boreholes are arranged in the lower part of the middle of the coal seam according to the influence radius of extraction.(5)A theoretical model of fluid solid coupling dynamic evolution of gas bearing coal and rock is established.Through COMSOL numerical simulation,the width of advance pressure relief zone after excavation is 25 m.The permeability of different layers in the mining coal seam first increases and then decreases with the height of the coal seam from the floor.When the coal seam is 2.6m away from the floor,the gas permeability of the coal seam is the largest,which is 8.62 times higher than the initial permeability.