Global Pressure Relief Characteristics And Permeability Evolution Law Of Roof-cutting And Pressure-releasing Goaf

Compared with the traditional longwall mining mode,the roof-cutting and pressure-releasing mining mode have differences in mining technology and roof collapse characteristics,which leads to the change of overburden movement law in the goaf.There is no need to leave coal pillars at both ends of the goaf to support the roof,which eliminates the insufficient area of roof pressure relief and allows the roof of the whole goaf to be fully relieved,The difference between the two mining modes will inevitably lead to the changes of stress field and fracture field in overburden,which will change the evolution law of permeability in goaf and affect the airflow migration in goaf.To study the pressure relief characteristics and permeability evolution law of the whole goaf with roof cutting pressure relief,based on the analysis of the pressure relief mechanism of the goaf and the engineering background of the 12201 test face in the Halagou coal mine,FLAC3 D is used to numerically simulate the failure and deformation of overburden in the goaf,The whole area pressure relief characteristics of overburden in goaf are obtained,the evolution laws of void ratio and permeability in goaf are obtained according to the calculation and analysis of simulation data,and the airflow migration law in goaf is simulated by FLUENT software.The main conclusions are as follows:Based on the theoretical analysis and the measured data of gangue crushing expansion coefficient in the goaf,the overburden movement in the goaf is divided into four stages: when the roadway is in the stage of cutting ahead influence after shaped charge blasting and before mining when the working face is in the stage of pressure relief and filling between 0-30 m,and when the working face is in the stage of dynamic pressure bearing between 30-102 m,When the working face is located at 102 m,it is in the stage of compaction stability.FLAC3D numerical software is used to simulate and study the movement,failure,and deformation law of overburden during roof-cutting and pressure-releasing mining,and the global pressure relief characteristics of goaf are obtained.The simulation results show that roof cutting can reduce the stress on the roadway roof,reduce the peak value of stress near the roadway,and optimize the stress of surrounding rock in the stope.In the dip direction,compared with the traditional mining model,the peak value of stress near the roadway at the cutting side of one-sided roof-cutting and pressure-releasing mining is reduced by 0.66 mpa,12.50%,and the peak value of stress near the roadway at the two along channel roadways of two-sided roof-cutting and pressure-releasing mining is reduced by 0.96 mpa,18.18%.The average roof cutting stress of one side roadway and two side roadway is reduced by 50%.In the vertical direction,the maximum relief angle of single-side roof cutting mining increases by 15.3°,and the maximum relief angle of double-side roof cutting mining increases by 16.2°.In terms of collapse range,the collapse range of roadway at the top cutting side is increased by 6m through roof cutting,which makes the collapse more sufficient;In terms of pressure relief range,the pressure relief range of roadway at the top cutting side is increased by13 m,and the pressure relief range is greatly increased.Therefore,compared with the traditional mining model,the roof-cutting and pressure-releasing mining mode have the characteristics of global pressure relief in the horizontal and vertical directions,with more sufficient pressure relief and a wider pressure relief range.Through formula calculation,the evolution law of porosity and permeability in goaf along the dip and strike direction under the roof-cutting and pressure-releasing mining mode is obtained.The following results are obtained by fluent numerical simulation: there are two main exchange areas of airflow in goaf under the traditional mining model,which are located in the sections of 0-5m and 15-45 m along the dip direction of the working face respectively;Under the mode of roof-cutting and pressure-releasing mining,there are six main airflow exchange areas in the goaf,which are located in the sections of 0-25 m,255-305 m and 305m-320 m along the dip direction of the working face and 5-25 m,25-35 m and 35-65 m along the strike direction of the goaf.