Evaluation Of Water Inrush Hazard Of Coal Seam Floor In Ultra-thin Aquiclude Area Of Qianbei Coalfiel

The Qianbei coalfield is one of the main coal-producing areas in Guizhou Province.The Maokou Formation aquifer is the basement rock layer of the Longtan Formation in the coal-bearing strata in this area.The lithology is mainly carbonate rock,the karst development is strong,and the water is extremely uneven.The interval between the coal seam floor and the Maokou Formation roof is generally thin,which makes the regional mining area vulnerable to the threat of karst water from the Maokou Formation when mining the lower coal seam of the Longtan Formation.Therefore,the prediction and evaluation of water inrush from coal seam floor in the lower part of Longtan Formation is a key problem for safe production in Qianbei coalfield mining area.In this paper,the ultra-thin aquiclude area of Qianbei coalfield is delineated by collecting data,and Longfeng coal mine is selected as the engineering background.Through numerical simulation,physical test and theoretical analysis,the water inrush risk of coal seam floor in ultra-thin aquiclude area of Qianbei coalfield is evaluated,and a new idea for water inrush risk evaluation of coal seam floor in ultra-thin aquiclude area is put forward.The main conclusions are as follows:（1）The thickness data of aquiclude in the lower coal seam floor of Longtan Formation in Qianbei Coalfield are counted.The analysis shows that the thickness of relative aquiclude in the area increases gradually from the middle to the south and north.Combined with the previous research on the thickness of the aquiclude,it is tentatively determined that Tongzi,Renhuai,Jinsha and Dafang areas（the thickness of the aquiclude is less than or equal to 20m）are the ultra-thin aquiclude areas in the northern Guizhou coalfield.According to the engineering geological situation of Longfeng Coal Mine,six main controlling factors affecting the water inrush of No.13 coal seam floor are determined:aquifer water pressure,aquifer water richness,karst development degree,effective aquiclude equivalent thickness,coal seam burial depth and coal seam dip angle.（2）The numerical model of No.13 coal seam mining is established by FLAC^3D,and the dynamic evolution characteristics of displacement,stress and plastic failure of the floor at different depths under different advancing lengths of the working face are analyzed.The maximum displacement of the floor is 0.37 m,and the maximum compressive stress of the rock layer in front and back of the working face is 17.24 Mpa.Combined with the empirical formula calculation results,the maximum floor failure depth is 19 m.In the vertical direction,the stress change curve and displacement change curve at different depths of the floor fluctuate in a zigzag manner,and the displacement at 0m of the floor is the largest,and the stress fluctuation at 5m of the floor is the largest.（3）Based on field investigation,controlled source audio-frequency magnetotelluric method and scanning electron microscope test,the development law of karst morphology in Maokou Formation aquifer at macro and micro levels was analyzed,which proved that it had self-similar characteristics.It was determined that the karst development within the top boundary of Maokou Formation to 10 m had the greatest influence on the water inrush from No.13 coal seam floor,and the fractal dimension calculation was carried out.Combined with Arc GIS,the zoning evaluation of karst development degree in the study area was realized.（4）Taking Arc GIS as the platform,the distribution characteristics of each main controlling factor in the study area are clarified.The weight of each main controlling factor is calculated by chromatography analysis:aquifer water pressure（0.234）,effective aquiclude equivalent thickness（0.211）,karst development degree（0.175）,coal seam dip angle（0.146）,aquifer water yield（0.117）,coal seam depth（0.117）.The vulnerability index model was constructed,and the risk zoning evaluation of water inrush from No.13 coal seam floor was finally realized.Compared with the actual water inrush point and the evaluation results without considering the degree of karst development,it was proved that the karst fractal-vulnerability index evaluation results were more in line with the engineering practice.