Research On The Overburden Strata Fissure Evolution And Pressure-relief Effect In Using Lower Protective Seam Mining

Protective seam mining as the regional measure is the most economical and effective way to control gas, also the effective way to realize integrated coal production and gas extraction. Fissure evolution and the pressure-relief effect is the key technique in using protective seam mining.94302 working face is taken as the engineering background for protective seam mining test, theoretical analysis, simulation test, field test is integrated used to research overburden fissure evolution and the pressure-relief effect in using protective seam mining process.The main research contents and results are as follows:(1) Similar simulation test system is constructed, the deformation of top and floor of the protected seam is observed during testing process, test results show that the protected seam bottom curve presented continuous, the protected seam expansion presented "M" shape distribution.(2) Deep base point is used to monitoring the the movement laws of overburden strata movement, the measured curve is analysised, the protective seam mining overlying strata three horizontal zone and vertical belt distribution scope are divided.(3) Borehole television is used to analysis fissure distribution characteristics,after the mining of 94302 working face the caving zone height is about 2.2-4.5m, the fracture zone height is about 20-25.7m, 3# coal seam is in the bending zone.(4) The three dimensional numerical simulation is used to obtain stress distribution in the overburden strata during the protective seam mining process. The protected seam pressure relief effect and the scope of protection is observed: the protected seam stress drop of about 58.3%, according to the swelling deformation rate of 3 ‰ as critical indicators, towards pressure relief angle is 60 °, inclined pressure relief angle is 63 °.(5) Structural damage observed on the protected seam, observation results show that: after the protective seam mining, micro cracks mainly with small angle appears in horizontal direction. After lower protective seam mining the protected layer of gas content, gas pressure is measured, gas content decreased from 7.63-9.91m3/t to 4.02-4.53m3/t, gas pressure decreased from 0.36-0.45 MPa to 0.23-0.26 MPa. Indicates that the secondary fracture has promoting effects on gas desorption.