Permeability Evolution And Water Inflow Dynamic Prediction Of Stratified Mining In Extra-thick Coal Seam

The original stress equilibrium state of the upper strata is destroyed due to the stratified mining of extra-thick coal seams,and the formed mining fractures develop and communicate with multiple upper aquifers,resulting in the leakage of groundwater resources into the coal mining face,which inevitably leads to the problem of roof water damage.With the development of coal seam mining,the mining fracture continues to expand and evolve,and the permeability changes,which leads to the continuous dynamic change of water inflow.Therefore,the permeability evolution law and water inflow dynamic prediction of stratified mining in extra-thick coal seam have important guiding significance for the prevention and control of roof water disasters in coal mines.Based on this,this paper takes the Barapukuria coal mine in Bangladesh as the research object,analyzes the existing data in the early stage,and comprehensively uses various research methods such as laboratory test,field test,theoretical analysis and numerical simulation to study the failure characteristics of overlying strata and the evolution law of vertical spatial permeability in multi-layer mining of extra-thick coal seam.The dynamic prediction model of water inflow based on permeability evolution is constructed to calculate and predict the water inflow of working face in Bangladesh.This paper takes Barapukuria coal mine as an example,the main conclusions are as follows:(1)The physical similarity simulation test was used to analyze the failure characteristics of overlying strata in multi-layer mining of extra-thick coal seam,and the development height of water flowing fractured zone in multi-layer mining was obtained.The cumulative mining thickness fracture-production ratio of each layer was measured by ‘underground upward hole water injection leakage measurement method’and ‘surface borehole flushing fluid leakage measurement method’.The fractureproduction ratio of one layer was 23,that of two layers was 18.42 – 23.01,and that of three layers was 18.Based on the measured height data of water flowing fractured zone,the empirical formula of development height of water flowing fractured zone in Barapukuria coal mine is proposed.(2)The permeability characteristics of overlying strata on coal seam roof are obtained by field water pressure test,and it is considered that the permeability of overlying strata on coal seam roof shows an increasing trend under the influence of mining disturbance,which has zoning characteristics.In the test period,the water flowing fractured zone shows a ladder-like distribution from top to bottom,and the value increases gradually.At the same time,the permeability of the upper strata is analyzed by numerical calculation.It is found that with the mining of the working face,the permeability near the coal wall changes greatly,and the permeability in the middle of the goaf is lower than that on both sides.With the increase of mining thickness,the permeability of the rock in the same position increases gradually,and the maximum increase of permeability in the three-layer mining is slower than that in the two-layer mining.The numerical calculation results are consistent with the field water pressure test results,which proves the reliability of the numerical model.(3)Combined with the permeability evolution characteristics of weathered rock,the numerical simulation software was used to dynamically predict the water inflow of the working face,and the predicted values of the water inflow of the first layer and the second layer working face were obtained.The dynamic predicted water inflow obtained by the second layer mining was compared with the actual observed water inflow data,and it was found that the fitting degree of the two was 85 %,and the predicted results were in good agreement with the actual values.At the same time,the water inflow of the future three-layer working face was predicted.The established numerical prediction model of water inflow improves the prediction accuracy of water inflow in multi-layer mining,and the research results can provide scientific support for the prevention and control of mine water disasters in layered mining of extra-thick coal seams.