Study On Stress Transfer Law And Reasonable Size Of Section Pillar In Multi-seam Mining

The layout of double entry in working face has the advantages of fast developing speed and relieving the tension of mining continuity,which is widely used in western mining areas.After double entry developing,the coal pillar between the two entrys is the section coal pillar.As a main protection method of entry in coal mine,the unreasonable size of coal pillars not only wastes coal resources,but also may induce roof accidents,rock burst and other dynamic disasters.Affected by the goaf or the remaining pillar in the upper coal seam,the stress environment in the lower coal seam has changed significantly and it becomes more difficult to determine the size of coal pillar in section during mining of close distance coal seam.In this paper,the methods of laboratory test,numerical simulation and on-site monitoring are used to study the determination of mechanical parameters and long-term strength of coal and rock mass,the stress distribution law of floor,and the optimum design of coal pillars size left over from the upper coal seam or under the goaf.The main work and results of this paper are as follows:(1)A method for determining the long-term strength of coal and rock mass based on the stress-strain curve is proposed.The uniaxial(triaxial)loading test and step loading creep test of coal and rock mass are carried out by using RLJW-2000 rock servo pressure testing machine,and the mechanical parameters of roof rock of 3#coal seam in Gaojialiang Coal Mine are determined.(2)The variation law of different stress with depth in coal pillar floor strata is firstly revealed.Then,the floor stress distribution law under overlap condition of goaf and coal pillar is discussed by bulding similar material simulation test and establishing the FLAC3D numerical simulation model.The influence of coal seam spacing on stress transfer of floor of lower coal seam is obtained.(3)Based on the overburden structure of section coal pillar,the UDEC numerical calculation model is established,and the stability condition and internal stress distribution law of the overburden structure are analyzed.Then the influence coefficient of coal seam group mining is proposed,and the theoretical formula for calculating the width of stress limit equilibrium zone of coal pillar is revised,which are considered of the stress effect of upper coal seam on lower coal seam under different mining conditions.(4)Based on the engineering background of 3#coal seam mining in Gaojialiang Coal Mine,and basic conditions for the stability of section coal pillars,field monitoring of vertical stress of coal pillar is carried out.Then,the calculation of width of plastic zone on both sides and comparative analysis of simulation results of different width is implemented.Finally,the reasonable size of the section pillar under the goaf and the remaining pillar of the upper coal seam is optimized.Field test shows that the optimized size of section coal pillar maintained good integrity and little deformation.