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To realize the precise stress relief and permeability enhancement of the coal seam with non-uniformly distributed gas, the criterion and optimization method of key construction parameters of hydraulic flushing was studied by modeling, numerical simulation and field tests. First, the multiphysics coupling model for stress-relieved coal seam was developed and index of optimal coal discharged was analyzed. Then the criterion and determination method of optimal coal discharged was put forward. Finally, the determination map of key construction parameters of hydraulic flushing was drawn, and a precise permeability enhancement technique for coal seam with non-uniformly distributed gas was proposed. The main conclusions are shown as follows: ① for a given gas extraction time, the residual gas content decreases while the residual gas pressure reduces followed by an increase with a rise of coal discharged; ② the criterion of optimal coal discharged includes the constraints of gas extraction reaching standard and minimization of roadway instability risk and construction cost. When the residual gas content and pressure are less than the critical values, it is considered that the gas extraction reaches the standard. On the premise of meeting the standard of extraction, the coal discharged should be reduced as far as possible to ensure the stability of coal roadway and reduce the project cost; ③ The optimal coal discharged decreases with the increase of ground stress, and increases with the increase of cohesion, gas pressure and borehole spacing. For a given coal seam, there is a maximum value of coal discharged and a minimum value of extraction time reaching standard; ④ comprehensively considering the correlation between coal seam gas pressure, coal yield, borehole spacing and extraction time reaching standard, the optimization map of key parameters of hydraulic flushing is drawn. Based on the map, a precise pressure relief and permeability enhancement technology for coal seams with non-uniformly distributed gas is put forward. In this technique, the construction parameters of hydraulic flushing corresponding to various gas occurrence areas can be determined, with the combination of gas occurrence and optimization map of key parameters, realizing the precise permeability enhancement of the coal seams with non-uniformly distributed gas.

In order to solve the problem of gas overrun and low utilization rate caused by mining pressure relief gas pouring into mining working space in high gas outburst coal seam, this paper used numerical simulation and field application methods to study the efficient extraction of coal gas in front of the working face under the influence of dynamic pressure. The results show that according to the gas concentration monitoring data on site, the coal body in front of the working area was divided into high efficiency extraction zone, effective extraction zone and original extraction zone. The efficient extraction zone was within 20 m in front of the working face, with an average gas extraction concentration of 30% or more. Gas extraction in the effective extraction zone was affected by the dynamic pressure, and the extraction concentration decreased fluctuatingly with increasing distance from the working face. The range of the original extraction area is 50-60 m away, and the gas concentration of the borehole was stable below 10%, which was less influenced by the mining movement. The second extraction technology of dynamic pressure gas in the floor gas extraction roadway of high gas outburst coal seam through the crossing boreholes was better applied in the middle extraction roadway of 12110 working face, the number of extraction holes in the dynamic pressure area was 5%-10% of that in the non-dynamic pressure area, but the monthly extraction volume could reach up to 74.01% of the monthly extraction volume inthe non-dynamic pressure area.

For mines lacking conditions for mining protective layers, pre extraction of coal seam gas from floor rock roadway strips is the mainstream gas control method. The analysis indicates that there are problems in the practical application of the floor rock roadway, such as significant differences in the selection of spatial layers, poor stability of the surrounding rock of the coal roadway caused by through layer punching, high excavation cost, and low utilization efficiency. Based on the floor rock roadway and considering the gas problem throughout the entire coal production process, a full-life cycle gas treatment technology based on floor rock roadway is proposed. It forms a five-in-one gas comprehensive treatment model of 'layer optimization, through layer punching, layer grouting, mining extraction, and gangue backfill'. Taking Shoushan No.1 Coal Mine as an example, by measuring the rock mechanics properties of the strata in the coal mining face, the stability of the floor rock roadway under the conditions of roadway excavation and mining face is analyzed based on the numerical method. Based on the characteristics of surrounding rock damage and the distribution of stress in the mining surrounding rock, it has been determined to arrange the bottom rock roadway at a position of 16 meters below the mining face transportation roadway and 1 meter inboard from the upper transportation roadway. The layout of hydraulic punching holes in the floor rock roadway is optimized. The group spacing is set to be 6.4 meters. The interleaving drilling is arranged by odd and even numbers for each group. By measuring the residual gas pressure of hydraulic punching holes, it is found that the effective influence range of hydraulic punching holes exceeds 4 meters. The hole gas concentration is high and the decline is slow. The strip pre-extraction effect is good. The though layer grouting technology is used to improve the properties of the upper broken coal body. The drilling observations show that the strength of the coal body after through layer grouting reinforcement is increased and the degree of fragmentation is decreased. The monitoring results of the deformation of the roadway side show that the overall stability of the surrounding rock of the roadway is good. The strength of the coal seam is increased. The monitoring results of the amount of drilling debris show that the grouting reinforcement range exceeds 5 meters, effectively reducing the risk of outburst in the roadway excavation. Through drilling through the floor rock roadway, the pressure relief gas extracted during the mining process of the working face is extracted. It is found that the effective influence range of mining is 50 meters in front of the coal working face. The gas extraction effect in the mining-affected area is good. The gas concentration in the air flow of the coal working face is reduced to below 0.45%, effectively reducing the gas concentration in the coal mining face. After the completion of mining, a method of backfill gangue in the floor rock roadway is designed to reduce the cost of gangue extraction and improve the utilization efficiency of the roadway.