| Coal and gas outburst is the one of the most serious disasters that may be confronted when mining. Because of the severity and complexity of the coal and gas outburst, lots of research is conducted to prevent, reduce or harness the outburst. Protection seam mining, the most economical and effective regional measure, should be considered firstly when the mine can meet the requirements of protection seam mining. However, based on the document data of protection seam, research on protection seam mining is focused on subparallel coal seams now, but the study on effect of upper protection seam mining to the bifurcated outburst seams is rare or not reported. Protection range of protection seam mining regulated by the reference values which are proposed by<Control Regulation of Coal and Gas Outburst> and<Coal Mine Safety Regulation> is not suitable to the bifurcated outburst seams, and some error may occur. Based on current research results and hypothesis of coal and gas outburst, gas parameter tests, experimental tests, theoretical analysis and numerical simulation are applied to conduct research on the bifurcated outburst seams at Tu Zhu mine and the first adit of Jin Zhushan mine. Research work is mainly focused on the statistical analysis of the outburst accidents, reasons for the accidents have been summarized. The changes of gas migration, the law of gas and surrounding rock pressure and the protection range and effect of the upper protection seam mining in Jin Zhushan mine have also been studied. Main achievements and conclusions are written as follows:(1) The reasons for the outburst accidents are complex, so the processes of the outburst accidents are capricious. Based on the statistical analysis of outburst accidents and the investigation of geological structures, in order to sum up the lessons of the accidents and seek the law of the accidents, analysis of the outburst accidents in recent two years is conducted by using chart-statistical method is carried on. After systematic analysis of the conditions and reasons of gas outburst, it is obtained that the bifurcated structure and the faults are the important factors to the frequent outburst accidents. This result can provide theoretical basis for the prevention and control of outburst accidents and the safety management of coal mine.(2) The basic gas parameters of Tu Zhu mine and the first adit are tested in laboratory and field tests of gas parameters before and after protection seam mining are conducted.The parameters of coal seam can be written as follows:the competent factor f is0.18, the initial speed of gas diffusion ΔP is39.00, the adsorption constant a is35.43m3/t, the adsorption constant b is1.620MPa-1, the ash content Aad is7.78%, the volatile content Vdaf is4.52%, the true density TRD is1.61t/m3, the apparent density is1.52t/m3, and the porosity F is5.59%. According to the field tests, the related parameters are obtained. The gas pressure, permeability, stress in coal seam and the relative gas content are1.5MPa,5.36×10-3m2/MPa2.d,2.5MPa and11.7m3/t respectively.It is shown from the results of three bores that the original gas pressure at the5th bifurcated coal seam is1.5MPa. Gas pressure is affected when the distance between the working face and the bore is26meters, and the gas pressure decreases as the advancing of the working face. Because of the short distance between the protection seam and outburst coal seam, the formation of free space, the destroy of original rock stress, movements forward to the gobs of the rocks, redistribution of the rock stress, roof caving, floor heave are caused after the excavation of protection seam mining. The permeability increment of the5th bifurcated coal seam is caused by fractures across the strata which are induced by stress relief and expansion of rock and coal seams, so the quantity of gas emission increases at the working face of the3rd coal seam, and then the content and pressure decreases.(3) Based on the condition of2136working face and the mechanical conditions of the roof and floor in Tu Zhu mine, the mathematical-physical model is established by applying CFD. The law of the gas migration has been studied by conduct simulation on Fluent, and the migration rules of gas migration, when the working face advances along the trend-direction and the bifurcation angles are different, has been summarized. The results of3D simulation can be written as follows:1) At the early stage of upper protection seam mining, gas tends to flow to the gobs, and the permeation trend towards gobs in5th coal seam is greater, then a gas compression region is formed in the rock between5th and3rd bifurcated coal seams, but the region gets to disappear as the advancing of the working face. The migration rules of gas can be formulated as three stages:①At the early stage of mining, lots of gas tends to flow to the gobs. Because of the insufficient stress relief and the emission of the gas, gas aggregates at the bottom of the floor, and a gas compression region is formed.②The gobs increase in size as the advancing of working face, the stress is released sufficiently, and then the gas migration tendency to gobs is reduced, and the aggregation of gas tends to be reduced. The reduction of compression region is caused by the lower rate of gas emission at the bottom of protection coal seam.③At the later stage of mining, when the gobs reaches to the width of working face, the gas pressure tends to be stable because of the smallest amount of gas emission to gobs and the disappearance of the compression region.2) Analysis of the gas pressure is carried on as the advancing of working face, it is shown that:The gas pressure reaches to the maximum value at the bottom near5th coal seam, the pressure decreases as the distance from the protection seam decreases, and it decreases to the minimum value at the working face.②The gas velocity increases as the depth increases in the vertical section, and the flow increases as the depth increases too, then it reaches to the maximum value at the mined out areas. However, the gas velocity stays stable in the rock and coal seams far away from gobs.③The original gas pressure at the5th bifurcated coal seam is 1.5MPa. After the advancing of the working face for40meters, the gas pressure of the5th bifurcated coal seam which is beneath the gob of the3rd coal seam varies from0.3MPa to1.3MPa; when the advancing distance reaches120meters, the corresponding pressure ranges from0.4MPato0.8MPa.(4) Numerical model are established and the software Flac3D is applied in this paper, it can be obtained from the simulations built on different bifurcation angles along the trend-direction that:1) The maximum value of stress is located at the lowest monitoring point as the advancing of working face, the maximum value in2.5MPa, and the stress measured at the monitoring points tends to decreases. Because the relief of stress is more sufficient as the distance to working face becomes shorter, the higher the monitoring point is the bigger the variance is. The stress decreases from1.73MPa to0.66MPa. It can be concluded that the stress of5th bifurcated coal seam decreases as the advancing of working face, in general, the mining activity is beneficial to stress relief in5th bifurcated coal seam.The displacements of monitoring point are negative as the working face advances, the maximum value of displacement, which is measured at the monitoring point (-105m) nearest to working face, is-9.97mm. Then the value increases to be positive as the advancing of working face, after the advancing of working face for30meters, the values measured at the monitoring points, where the elevations are-105,-113and-121m, are positive, and the nearer the monitoring point to the working face is, the variance of the displacement is. As a kind of porous media, the porosity is increased by the movement of strata, and that is beneficial to the gas depressurization.3) It can be concluded that:①the gas pressure increases as the angle increases, but the velocity decreases oppositely, due to the difficulty to decrease the gas concentration, it is difficult to depressurize the pressure of the5th bifurcated coal seam.②It is known that the permeability coefficient of rock is very small, the resistance is great for the migration of gas in rock, and the distance to working face increases when the angle increases, so the gas can not be moved to the working face easily, the gas pressure can not be depressurized. Above all, the gas pressure increases as the bifucation angle increases.③As the increment of the angle a, the mining of the upper protection seam (the3rd seam) is adverse to the gas depressurization, so it can be concluded that upper protection seam mining is not beneficial to the gas depressurization of5th bifucated seam when the bifucation angle increases.④The stress imposed on the5th bifucated seam increases as the depth increases, so the depressurization efficiency will be limited. Contrarily, the depressurization efficiency will be improved. So, the gas pressure and concentration should be decreased by drilling advancing holes to make sure the mining safety when mining out the bifucated coal seam with large bifucation angle.(5) Based on the effect of protection seam mining and the analysis of evaluation index, it can be concluded that:①The depressurization angles along strike and tendency are58°and75°respectively. Under the influence of the haulage roadway of2136working face on the21st panel, the pressure measured at bore2is always small; there is little difference between the pressures measured at bore1before and after the working face advance through the site, but the difference is great at bore3. The final pressures measured at bore1, bore2and bore3are0.68MPa,0.44MPa and0.18MPa respectively. It can be concluded from above results that bore2and bore3are located within the scope of protection range, while the bore1is not. The stress measured at bore4, bore5and bore6decreases rapidly firstly when the2136working face advances to the corresponding sites, but some recovery occurs as the working face advances. It can be concluded that the stress in coal seams and rock is released effectively.②ased on the analysis on gas pressure, stress in coal and rock and the displacement caused by mining activity, the depressurization effect which is caused by mining out the upper protection3rd seam on the5th bifurcated coal seam is studied. It is concluded that as the bifurcation angles increase and the distance to gobs or working face gets further, the stress is relieved less, the migration rate is slower, and then the gas concentration is hard to be diluted, the efficiency of depressurization is not satisfactory. Oppositely, in the region affected by working face, rock and coal seam are affected by mining activity. As the inner cracks expand, the more the cracks are, the better the permeability is, and more satisfactory the efficiency of depressurization is. |
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