Study On Damage Evolution And Permeability Of Water-bearing Coal Under External Cyclic Loading

As a porous medium,the permeability and the degree of damage and deformation of coal are sensitive to stress.The coal body in underground engineering is often subjected to cyclic loads which constantly change holes and cracks in the coal body.This affects the flow of gas in the coal body,changes the characteristics of migration of coal seam gas,and leads to a change in the amount of gas extraction making it difficult to predict gas outburst accidents.For solving such problems,the method of combining experimental research,theoretical analysis,and numerical simulation was adopted to analyze the damage evolution and permeability characteristics of waterbearing coal under the excitation of a cyclic external load.The main research results are as follows.According to the conventional triaxial compression test and triaxial cyclic loading and unloading test,it was found that the change law for the strength parameters and deformation parameters of coal with different water contents was the same as that for the water content before and after the cyclic loading.The compressive strength and elastic modulus of coal showed a negative linear change with a change in the water content,while the strain showed a positive linear change with a change in the water content.The irreversible deformation of coal under the cyclic loading decreased in logarithmic relationship with an increase in loading and unloading times.After the cyclic loading,the effect of the moisture on the coal body was enhanced resulting in greater variation of different parameters with a change in the water content.The study on the influence of the cyclic loading on the mechanical parameters of water-bearing coal body could provide data support for the verification of the permeability model and the development of numerical simulation.The effects of the confining pressure,pore pressure,and water content on the coal permeability were studied by using nuclear magnetic resonance equipment and triaxial percolation apparatus.It was found that the stress sensitivity coefficient of the coal permeability increased exponentially with an increase in the water content.Under the action of confining pressure cycle,increases in the cycle amplitude of the confining pressure and the initial value lead to decreases in the coal porosity and permeability.The power function relationship between the coal porosity and permeability during and after the action of the cyclic load was obtained.The acoustic emission characteristics of coal were loaded and the coal loading failure process under the cyclic load excitation was analyzed.The change law for the water cut count of the acoustic emission events was established to get the acoustic emission event count.The relationship among the damage variables was clarified and it could reflect the cyclic loading stage of the coal damage evolution rule for the damage evolution model based on the time effect of the acoustic emission.The influence of the water content on the model parameters was analyzed and the AE damage model of the coal body based on the time effect was obtained considering the influence of the water content.Based on the irreversible deformation increment of the water-bearing coal body after the prepeak cyclic load,the disturbance coefficient was defined as the ratio of the limit deformation of the coal body before and after the cyclic load.This was further brought into the improved permeability SD model and the coal permeability model in the elastic stage considering the cyclic load was established.According to the cooperative relationship between the permeability and the damage variable,the permeability models of the coal body in the plastic deformation stage and the residual strength stage were constructed.According to the characteristics of the elastic medium of coal with "double hole and single permeability",combined with the gas diffusion equation,the gas seepage equation,the coal permeability model,and the coal deformation equation in the elastic stage considering the cyclic load,a migration model for water-bearing coal gas considering the cyclic load was constructed.The COMSOL software was used to analyze the influences of the water content and the cyclic load on the gas drainage process of coal seams.The analysis of the damage evolution and permeability characteristics of water-bearing coal under the cyclic load has theoretical guiding significance for gas extraction of water-bearing coal seams under cyclic loads.There are 80 figures,15 tables,and 177 references in this paper.