Study On Fracture Propagation And Permeability Increasing Laws Of Overlying Strata By Mining Lower Protective Layer And Gas Drainage

Mining protective layers is an effective way to increase the permeability of low-permeability coal seams and to improve the productiveness of gas drainage of coal seam.It is the key of improving the technology of protection layer mining to quantificationally analyze the evolution of the fissures and the seepage law which is difficult to measure when mining protective layers.Nowadays,for numerical simulation study on activity laws of overburden during mining protective layers,most of the methods are based on the changing of the stress fields,the deformation fields or the plastic zones of homogeneous coal and rock mass in the process of excavation,and then pressure relief ranges and failure zones of overburden could be determined.However,It is difficult to reflect the deformation and failure characteristics of each regions caused by the evolution of the fissures on actual overlying strata.Considering the characteristic that actual coal and rock mass has weak surface structure,according to actual coal column sections,and by the technology of digital image using MATLAB,the method of geometrical characterization for natural coal and rock mass which contains weak surfaces is established.And then combined with ABAQUS,the physical characterization of heterogeneous coal and rock mass is realized.The weak surfaces of overlying strata are defined as cohesive elements which has the characteristics of.simulating cracks and damages.Based on interface mechanics theories,using the secondary development function provided by ABAQUS,the constitutive response of exponential evolution is developed,and the model masonry construction with cohesion on overlying strata during mining is built.Though the numerical simulation on the fissure evolution caused by mining a protective layer using cohesive elements,the method that determining fracture development states according to the damage variables,and that determining the Vertical three belt according to the connectivities of macro cracks is proposed.The distribution characteristics of stress and displacement fields on overlying strata are found by analyzing the results.The distressed zone of overlying strata presents a hump-shaped curve.Determining characteristic zones of fracture development states according to the damage degree,inducing fracture distribution law,the The evolution law of the fissures during mining is got.Using python scripting language to statistics the number of cracks,fracture development states of each driving distance,the function relationships are fitted.Based on the coupled relationship between permeability,stress and damage the seepage law on overlying strata during mining is analysed quantificationally.The distressed range and the distribution of permeability is concluded quantificationally.The evolution law of the fissures during mining and pressure relief ranges concluded by the numerical simulation based on the model masonry construction with cohesion on overlying strata are in accord with similar simulation experiment results and the statistics of work fields.It is verified that the characterization of natural coal and rock mass which contains weak surfaces using cohesive elements and the model masonry construction with cohesion is effective.