Damage Failure Law And Coupled Outburst-Causing Mechanism Of Pre Static Loaded Adsorbed Gas Coal Under Cyclic Impact

Rich coal,poor oil and little gas are the national conditions of China,the coal-based energy structure is difficult to change in the short term,and the average mining depth of coal is increasing at the rate of 10-25m per year.Therefore,the mining of deep coal resources is an important guarantee for China’s energy security.However,during deep coal mining,the coal and rock mass are often in the multi-phase and multi-field coupling environment of high stress,high temperature,high gas-liquid pressure,etc.In addition,fault slip,rock fracture,mining and excavation,blasting vibration,etc.frequently affect the coal and rock mass.That resulting in the frequent occurrence of nonlinear,high-energy coal and rock dynamic disasters such as coal and gas outburst.The disaster mechanism and prevention technology of deep coal and gas outburst has become a worldwide problem and research hotspot.At present,most relevant researches focus on exploring the gas solid coupling mechanism of coal,geostress and gas pressure or the dynamic static superposition mechanism of coal,geostress and impact load,and most researches were applying single high strain rate impact load through Split Hopkinson Pressure Bar.The study on the mechanism of coal and gas outburst,which considers the gas solid coupling effect and the superposition of medium low strain rate cyclic impact dynamic load and geostress static load,is relatively scarce.This paper developed a dynamic static superposition rock mechanics test system,and used this test system to study the damage evolution law and failure instability characteristics of coal under different cyclic impact load,static load stress stage,adsorbed gas pressure.The gas solid coupling dynamic model of pre static loaded gas-adsorbed coal under cyclic impact disturbance was established.Finally,explored the coupling damage mechanism of pre static loaded gas-adsorbed coal under cyclic impact disturbance.The main achievements are as follows:(1)The multi strain rate dynamic static superposition rock mechanics test system has overcome six key technical problems.This including single side superposition of multi strain rate dynamic and static loads and rapid compensation of static loads,high-frequency cyclic impact loading,automatic balanced loading of creep loads,high-pressure gas seal and constant air pressure,multiple information coupling collection,and integrated control of multiple loading devices.Finally,the multi strain rate dynamic and static superposition of gravity creep(<10-4 s-1),hydraulic static load(10-4～10-2 s-1),hydraulic pulse dynamic load(10-2～100 s-1),and cyclic drop hammer impact dynamic load(100～102 s-1)was realized,and the high-pressure gas-liquid occurrence environment can be formed.Thus obtaining the physical and mechanical properties of coal and rock masses under gas-solid coupling and dynamic static superposition(2)Conducted tests on different gas pressures,static stress stages,and single impact peak,and obtained the damage evolution laws of various mechanical parameters of the coal body.Installing a non-flickering high brightness light source in a high-pressure sealed chamber that meets the requirements of high-speed camera,and then obtains the development characteristics of coal surface cracks.Collect the damaged coal fragments,and then obtain the fractal evolution characteristics of coal fragmentation.The results show that:1)With the increase of gas pressure,static load stress stages and peak impact load,the coal damage gradually increases,and the fracture development and instability failure mode tend to be complex.Evolution from single slope shear failure or vertical tensile failure to local burst or global burst failure.The order of coal damage sensitivity is static stress stage>single peak impact load>gas pressure.2)The dynamic and static superimposed loads have both strengthening and damage effects on the coal.The strengthening effect is dominant in the compaction stage and the damage effect is dominant in the plastic stage,and the dynamic load damage effect has a strain rate effect.(3)Through tests,obtained the damage law of various mechanical parameters of coal and the fractal evolution law of surface crack and fragmentation under different cycle/continuous impact number,frequency and peak value.The results show that:1)With the increase of impact number,peak value or the decrease of impact frequency,the coal damage gradually increases,and the fracture development and instability failure mode tend to be complex.Under the same impact peak,the damage sensitivity of impact frequency is greater than the number of impacts.2)During continuously applying impact load until the instability and failure,the cumulative residual strain presents an inverted S-shaped evolution mode,and the damage of coal body undergoes the evolution process of rapid inoculation→gentle development→high-speed expansion and penetration→macro crushing.3)In the continuous impact loading until the failure test,the greater the impact frequency,the smaller the damage caused by a single impact,the more energy accumulated in the coal by a single impact,the greater the final cumulative impact number and cumulative energy,and the more severe the final failure.(4)Based on the test results,the evolution model of gas-adsorbed damage factor was established.Based on Weibull distribution assumption of coal microelement strength,the evolution model of static load damage factor was established.Based on the inverse function of the Logistic equation,the evolution relationship of the cyclic impact damage factor with the impact number,impact frequency and impact peak value was established.Based on the generalized Kelvin body,a combined damage constitutive framework was constructed.Combining the three aforementioned damage evolution models,a coupled damage constitutive model for pre-loaded gas-containing coal under cyclic impact disturbance was established.Using the relationship between permeability and porosity evolution with strain to combine the damage constitutive model with the gas flow model.Finally,formed a gas-solid coupled dynamic model for pre-loaded gas-containing coal under cyclic impact disturbance.(5)Conducted an engineering scale tunnel excavation induced coal and gas outburst model experiment,and outburst phenomena and multi physical evolution data were similar to the accident site.Taking model experiments as the background,using the gas-solid coupling dynamic model conducted numerical analysis.By adjusting the constant of impact damage model,obtained the evolution laws of multiple physical quantities that consistent with model experiments,thus constructing a dynamic model parameter system suitable for engineering scale.Through numerical analysis,obtained the multi physical fields coupling evolution law under different gas pressures,vertical stresses,peak value of impact load,and then explored the mechanism of multi factor coupled damage causing disasters.The results show that the greater the gas pressure and vertical stress,the greater the impact peak and the smaller the frequency under the same impact loading times,the more severe the damage to coal and rock layers,and the higher the risk of outburst.