| There are a large number of shallow and short distance coalfields in Northwest China.At present,the first main coal seam in this area has been mined,and the second main coal seam will be mined on a large scale.When the close distance double coal seam is mined,the goaf and coal pillar will be left after the upper coal seam is mined,forming a large range of fracture development area.When the lower coal seam is mined,it will produce secondary disturbance to the already stable strata,causing the secondary damage of overburden rock and the secondary development of water flowing fractures,which will bring major safety hazards to the safe mining of coal mine.Therefore,the water flowing fractures in overburden rock of shallow buried close distance coal seam are seriously damaged The research is of great significance.Based on the engineering geological conditions,drilling data,pumping test and related test data of 31117 working face in Jinjie Coal Mine,this paper establishes a simplified mechanical geological model of the research working face.Laying a similar geological model of 31117 working face,the development height of the water-conducting fissure zone is 52.6m and 73m after the mining of the 3-1coal seam and the 4-2coal seam,respectively.The"curve"can be roughly divided into three stages:the initial development stage,the rapid development stage and the stable stage.After the 4-2coal seam is mined to 85m,the roof is almost completely collapsed and the direct floor of the 3-1coal seam is severely deformed and damaged,and the height,width and number of the water-conducting fissures in the overlying rock will change significantly.Use PCAS software to analyze the 4-2Overlying strata are binarized during coal mining.The fissure rate showed"rising-slightly falling-slowly rising"in these three stages.On the whole,the length of the fissure generally presents the three stages of"rising-falling-slowly rising".The average width of the fissures generally showed two stages of"rising-falling".Through the study of the vertical stress of the monitoring points in the model,it is found that the vertical stress of the rock formation in the middle of the mined-out area is roughly in the process of"stress concentration-stress release-stress recovery".Using the FLAC3D numerical simulation method,the development height and regularity of water-conducting fissures are almost the same as the results of the model test.4-2coal seam mining will destroy the stabilized overlying rock structure and cause the stress to increase.The maximum vertical pressure increases from 10.1 MPa to 11.8 MPa,The maximum shear stress is increased from 4.2 MPa to 5.29 MPa,and the shear stress is mainly concentrated on the side of the stop line of the mining eye.4-2After the coal seam is mined,the vertical displacement increased from 1.79 m to 4.13m.The vertical displacement of the monitoring points on the same survey line showed a phenomenon of"slow growth-rapid growth-slow increase and stability".In this paper,based on the analytic hierarchy process-fuzzy comprehensive evaluation method,combined with the water inrush hazard source theory,the shallow and close double seam roof water inrush hazard assessment is carried out on the31117 working face.The analytic hierarchy process is used to determine the evaluation index factors of the criterion level and the index level,and the index factors are commented and scored quantitatively,and the membership function of the water inrush risk level is established.The comprehensive dynamic evaluation of water inrush hazard was carried out by fuzzy comprehensive evaluation method,and it was found that the water inrush hazard of upper and lower coal seams all follow the rule of"low?lower?medium?higher",and the"medium water inrush hazard"of upper coal seam is concentrated at 120m?140m,the"higher water inrush risk"is concentrated after 140m.The"medium water inrush risk"of the lower coal seam is concentrated in 80m?120m,and the"higher water inrush risk"is concentrated after120m. |
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