Experimental Study On The Evolution Law Of Permeability Of Protected Layer In Mining

Many engineering problems,such as gas drainage,coal and gas outburst and water inrush risk prediction,are closely related to the permeability of coal and rock mass.In order to study the evolution law of coal and rock permeability affected by mining under the condition of protective layer mining,a large-scale true triaxial multi field coupling mining coal and rock permeability test system is developed.The experimental system innovatively designs the "point" loading and the "universal" loading mode.The system is applied to simulate the time-space characteristics of the "gas-solid" coupling mining stress field and the evolution law of the permeability of the coal and rock mass.The mathematical model of permeability evolution of coal and rock mass is established.The mathematical model of permeability evolution provides theoretical basis for the prevention and control of coal and gas outburst disaster,the formulation of low permeability and high gas coal seam drainage scheme,the prediction of coal seam water inrush and so on.In this paper,a large-scale true triaxial three-dimensional fluid structure coupling simulation test system is developed for the first time.The system include seven subsystems:simulation frame and box subsystem,sealing subsystem,stress loading subsystem and control subsystem,constant pressure gas loading and control subsystem,stress and strain monitoring subsystem,gas flow monitoring subsystem and simulation mining subsystem.The technical parameters of the subsystems are designed scientifically.Based on the mining engineering background of the lower protective layer in Changping Coal Mine,combined with the characteristics of three-dimensional "gas-solid" coupling similar material simulation experiment,a "gas-solid" coupling similar simulation mathematical model is established which considering the influence of mining on gas desorption diffusion seepage.The similarity criterion,similarity criterion and similarity scale of "gas-solid" coupling physical simulation are determined.In this paper,the physical and mechanical parameters of 3#coal seam and 8#coal seam in Changping Coal Mine are tested.On this basis,the experimental research on the ratio of similar materials of 3#coal seam is carried out.The ratio of silica gel,sand,cement and gypsum is 25:40:8:27.The permeability evolution of the 3#coal seam under the protective layer in Changping Coal Mine was studied by using the test system.According to the permeability evolution characteristics of 3#coal seam,the coal and rock mass are divided into high permeability area,transition area and low permeability area.The spatial characteristics of the "three zones" of the permeability of the three coal seams under the mining condition of the protective layer in Changping Mine are as follows:compared with the 8#coal seam working face,the high permeability zone is 9.0?80.0m behind the working face,the transition area is 5.0m in front of the working face to 9.0m behind the working face,and the low permeability zone is 5.0?20.0m in front of the working face.According to the mining characteristics of the lower protective layer,the basic assumption of gas-solid coupling modeling of coal rock mass affected by mining is put forward.The characteristics of gas migration are desorption-diffusion in the matrix of pressure relief coal and rock mass,which accords with Fick's law.The characteristics of gas migration is seepage in the fracture of pressure relief coal and rock mass,which conforms to the linear Darcy law.The continuity equation of gas desorption-diffusion-seepage migration of coal and rock mass under protection mining is established.For the first time,the concept of relative interval permeability increasing effect coefficient is put forward,and the expression of relative interval permeability increasing effect coefficient under the mining condition of lower protective layer is given.Based on the "three zone" dynamic evolution of the permeability of the overlying protective layer in the process of the lower protective layer mining,applying the theories of seepage mechanics and damage mechanics and relative interval permeability increasing effect coefficient,the mathematical model of permeability evolution zoning of protected layer is established under the lower protective layer mining.The permeability evolution zoning mathematical model of mining coal and rock mass in the lower protective layer is used for numerical simulation.Research shows:in the process of advancing the working face of 8#coal seam protective layer,the mining stress and permeability of 3#coal seam protective layer evolve dynamically.After the working face of 8#coal seam protection layer is advanced to 80.0m,the stress reduction rate in the pressure relief area tends to be stable,and the stress in the stress reduction area is about 50%of the original rock stress.When the working face is advanced to 80.0m,the permeability of the 3#coal seam is increased rapidly,and the permeability is increased by 873 times.With the continuous development of 8#coal seam,the permeability increase of 3#coal seam tends to be stable,and the permeability increases about 1100?1200 times.The numerical simulation further shows that the permeability evolution model of the protected layer is basically reasonable,and the calculated permeability zoning characteristics and the increase multiple order of magnitude are consistent with the experimental study.The numerical simulation results are consistent with the experimental results.The numerical simulation further shows that the permeability evolution model of the protected layer is basically reasonable.There are 88 figures,24 tables and 200 references in this dissertation.