Discrete Element-based High-Pressure Water Jet Damage Enhancement Mechanism And Application In Soft And Hard Composite Coal Seams

Coal and gas outburst has always been a major disaster threatening mine safety production.The essence of the outburst process is the rapid release of stress energy and gas energy accumulated in the long-term geological structure of the coal seam,and most of the coal and gas outburst accidents occur in Where tectonic coal develops.In the Xintian mining area in Guizhou,China,due to its unique geological conditions,a composite coal seam in which soft structural coal and primary coal coexist has been formed.This composite coal seam has the characteristics of high gas occurrence and low permeability.In addition,Xintian coal mine,The high stress environment has jointly formed the prerequisites for coal and gas outburst.Based on the above situation,taking the pressure relief and permeability enhancement of 1402 working face of 4 #coal seam in Xintian coal mine as the engineering background,the PFC numerical simulation software is used to study and analyze the mechanical characteristics of composite coal seam and the stress field,energy field and fracture area under different soft layer thickness and different cavity radius.The main conclusions reached are as follows:(1)After structuring coal undergoes special geological movement,its mechanical strength is far inferior to that of primary coal.The uniaxial compressive strength of primary coal is 5.3 times that of structural coal,and the triaxial compressive strength is1.68～1.98 times that of structural coal.The cohesion is Tectonic coal is 2.75 times,and the elastic modulus is between 2.9-8.1 times that of tectonic coal.(2)Establish a numerical simulation test for the mechanical characteristics of the composite coal body.It is found that when the composite coal body is damaged,the soft layer is destroyed first,and the peak strength of the final coal body is between the peak strength of soft coal and hard coal;when the contact surface has an inclination angle,it will be destroyed The method is divided into tensile failure(75°,90°),compression-shear failure(15°,30°)and damage along the contact surface(45°,60°)by the size of the inclination angle.(3)Using PFC characteristics to analyze the stress field,fissure area and energy field during hydraulic punching,it is found that the larger the hydraulic punching radius or the larger the proportion of soft stratification,the greater the stress disturbance of the surrounding coal,and the pressure relief radius The crack area will increase with the punching radius and soft layer thickness.The total number of cracks is also proportional to the punching radius and soft layer thickness.In terms of energy field,the strain energy dissipation of the system increases with the punching radius and soft layer thickness.The radius of the cavity increases linearly.(4)The hydraulic punching under the conditions of different punching radii and different soft layer thicknesses is converted into equivalent drilling coal output.It is found that within a certain range,the soft layer thickness is thicker under the same punching coal volume.,The larger the range of hydraulic punching pressure relief and penetration enhancement is,on the contrary,under the same coal output,the pressure relief and penetration enhancement range of hydraulic punching hole will decrease as the soft layer becomes thinner.Combining with the site,using gas drainage concentration,gas residual content and gas residual pressure to find the same amount of coal,the soft layered thicker hydraulic punching gas concentration is higher than the soft layered thin punching hole.In addition,the residual gas content and pressure of the coal body around the punching caverns with thicker soft layers are lower than those in the punched area with thinner soft layers.Therefore,in the actual construction process,the coal output should not only be used as the evaluation index,but also the thickness of the soft layer should be considered.For the thinner part of the soft layer,the amount of punching should be increased,and the punching cavity should be expanded as much as possible to prevent punching.Too small radius results in insufficient coal seam pressure relief and penetration enhancement.The paper has 52 pictures,15 tables,and 73 references.