Research On Development Height Of Water-conducting Fracture Zone Induced By Mining In Thick Sandstone Roof Of Maiduoshan Coal Mine

The main coal seam in northwest mining area is Jurassic coal seam,and the roof of coal seam is mainly Jurassic-Chalk thick weakly consolidated sandstone aquifer whose thickness is more than 50 m.In recent years,the water-sand mixed water outburst in thick sandstone roof of Jurassic coal field in northwest China occurs frequently.However,since coal production in the early stage was concentrated in the eastern mining area,the research focus of water damage in mining area was mostly Ordovician limestone water damage in floor;however,there were few researches on the water outburst mechanism of Zhiluo Formation aquifer in the roof of Jurassic coal seam in the west,and systematic and effective preventive measures were lacking in the face of such water outburst phenomenon.It seriously threatens the safety production of Jurassic coal mine in northwest mining area.In order to study the formation mechanism of water damage of thick sandstone roof in northwest mining area,Taking the 110207 working face of No.2 coal in Maiduoshan Coal mine in the western mining area as the research object,based on the theoretical calculation formula,combined with the similar model test and discrete element numerical simulation,the stress and displacement change law of the roof in the mining area influenced by mining activities was revealed and studied.Machine learning theory was introduced to establish the height prediction model of the crack zone of thick sandstone roof in the northwest mining area.It has guiding significance for the prediction and prevention of roof water damage in northwest mining area with similar situation.The main achievements of this thesis are as follows:(1)Determine the pressing distance of the working face and the deformation law of thick sandstone roof during the mining process.The distributed optical fiber monitoring technology was used to record the strain value of thick sandstone roof in the mining process.Based on the analysis,it was concluded that the overburden strain decreased with height,and finally the strain approached 0 at 93 m away from the coal seam roof.According to the average variation degree of optical fiber frequency shift theory,the pressure step distance of working face was 30-40 m.The development law of roof fracture zone can be divided into three stages according to the fracture strain value of rock strata,that is,cracks develop fully at 0-32 m,cracks develop slowly at32-58 m,and cracks above 58 m are not developed.(2)Determine the evolution law of fracture guide zone development,stress and displacement of working face roof with mining process.Using 3DEC discrete element simulation software,the mining process of 110207 working face was numerically simulated.By analyzing the variation law of the model plastic zone with mining,the distribution of the plastic zone of the roof was determined when the working face was working for 200～220m.Therefore,the height of the fracture zone was determined to be 56.81 m,and the fracture/production ratio was 14.95.Monitoring the changes of displacement and stress of rock strata of different heights with the mining process,it is found that there is a tensile stress zone in the middle of the roof in the early mining period.After the local stress concentration zone appears in the late mining period,the tensile stress zone gradually shrinks and is dominated by compressive stress.The coal wall part of the working face is dominated by compressive stress concentration zone,and gradually increases in the early compressive stress concentration zone.When the stress concentration area appears in the middle of the working face,the stress concentration phenomenon on both sides of the coal wall is relieved.The model roof has a displacement lag with the increase of height,and the closer the distance from the working face,the closer the rock displacement value is to the mining thickness of the working face.(3)Establish the height database of Jurassic coal seam thick sandstone roof fracture zone in northwest mining area.Grey correlation analysis was used to calculate the correlation degree value between each influencing factor and the height of water-guiding fracture zone,and the influencing factors with correlation degree value greater than 0.7 were selected.Finally,mining thickness,mining depth,coal seam inclination,working face oblique length and roof hard rock proportion were selected as five factors affecting the development of guiding height,and a database of Jurassic coal seam mining area in northwest China was established.(4)Establish a guide height prediction model for thick sandstone roof of Jurassic coal seam in northwest mining area.The SVR(support vector regression)guide height prediction model was established using mining area database,and the PSO algorithm(particle swarm optimization algorithm)was used to optimize the parameters of SVR model,and the PSO-SVR prediction model was established.By comparing the predicted guiding height of the test set of PSO-SVR model and the theoretical guiding height of the theoretical calculation with the measured value of mining area respectively,it is proved that the accuracy of the PSO-SVR model is better than the calculation of the theoretical formula.The related parameters of110207 working face were put into the PSO-SVR guide height prediction model,and the predicted guide height value was 55.29 m,and the fracture production ratio was14.55.(5)By comparing theoretical calculation,similar model test,numerical simulation and machine learning,it is concluded that the crack guide zone of 110207 working face ranges from 55.29 m to 59.79 m,and the fissure production ratio ranges from 14.55 to 15.73.The roof crack zone will channel through the weakly consolidated sandstone aquifer of Zhiluo Formation,and the working face may have the risk of water and sand burst.There are 41 figures,19 tables and 83 references in this thesis.