Pore Structure Evolution And Fatigue Damage Mechanism Of Dry-Wet Cycles Coal Samples

Co-mining of coal and water is the main technical way to realize the green mining of coal in ecologically fragile mining areas in western China,and water plays a basic role in the whole cycle of coal mining,especially the mining water resources formed by mining are an important part of water resources in this area,and underground reservoirs provide technical guarantee for the effective storage and utilization of mining water resources.The coal pillar dam of underground reservoir will inevitably produce damage accumulation under the continuous action of dry-wet dynamic load cycle,which is likely to be fatigue failure.Therefore,based on the engineering background of L1814 coal pillar dam in Lingxin Coal Mine,this thesis systematically studies the evolution characteristics of pore structure and dynamic fatigue damage mechanism of coal samples under the influence of the amplitude and times of dry-wet cycles through nuclear magnetic resonance,dynamic fatigue experiments,theoretical analysis,and numerical simulation.The main research results are as follows:（1）The law of pore evolution under the influence of the number and amplitude of dry-wet cycles was studied by NMR,and the influence of dry-wet cycles on the pore structure of coal samples was revealed from the multivariate quantitative characterization parameters such as time-space evolution,T₂ pore spectrum area,porosity,permeability,and nuclear magnetic fractal dimension.The number of dry-wet cycles can promote the increase of pore expansion capacity,porosity,and permeability at the end face of coal samples.After 7 dry-wet cycles,the effective porosity of coal samples increases from 4.67%to 6.18%,and the permeability increases from0.0248m D to 0.0304m D;At the same time,increasing the cyclic amplitude increases the effective porosity and permeability of coal samples,while reducing the roughness of free pores.The cyclic amplitude increased from 4%to 8%,and the effective porosity of coal samples after 3,5 and 7 dry-wet cycles increased by 0.55%,0.19%and 1.10%respectively.Permeability increased by 0.0020m D,0.0052m D and 0.0060m D;respectively;The T_2cutoff value of curve T₂ decreases gradually,the pore connectivity increases continuously,and the fractal dimension of free pore decreases gradually.（2）The mechanical response of dry-wet cycles to the fatigue failure of coal samples is mainly reflected in the fact that the actual fatigue failure strength,the number of cycles,the peak strain and the instantaneous dynamic elastic modulus of failure decrease with the increase of dry-wet cycles.There is a critical value for the number of dry-wet cycles,and when it reaches this value,the maximum cycle change of coal samples gradually tends to C=550;With the increase of dry-wet cycles,the fatigue failure type of coal samples gradually changes from tensile failure to tensile-shear composite failure;Pores and defects caused by dry-wet cycle constitute the initial damage of coal samples,and the damage accumulation to the critical state of fatigue failure is related to the initial damage.The more dry-wet cycles,the more pores,and defects in the coal sample,the greater the initial damage value,the less the number of cycles in the elastic stage,and the weaker the fatigue damage resistance.（3）Based on the laboratory-scale cyclic dynamic load data,the existing dam width design formula is optimized.Through theoretical checking,it is found that the dam width of L1814 coal pillar is at least 50m,and the current width is 60m〉50m.The FLAC^3d numerical simulation results show that the development depth of plastic zone,the peak value of vertical stress and the peak value of maximum principal stress gradually decrease with the increase of dam width,so the current design width of 60m meets the requirements.There are 73 figures,17 tables and 156 references in this thesis.