Study On Damage Mechanism And Time-dependent Stability Of Coal Pillar In Coal Mine Underground Reservoir

The coal resources of China are mainly distributed in western mining areas where the ecological environment is fragile and water resources are scarce.Coal mining is normally accompanied by discharge of large amount of water.The western mining area is arid and less rainy.The annual evaporation of the mining area is 6 times that of the precipitation,and the utilization rate of mine water is only 25%,which not only causes waste of water resources,but also has a certain impact on the balance of the ecosystem.To protect and utilize water resources,many underground reservoirs were built in underground coal mines for water storage.Coal pillars which work as dams are subjected to a long-term soaking environment of concentrated salty water.Deterioration of coal dam under the attack of salty solution poses challenges for the long-term stability and serviceability of underground reservoirs.It is crucial to investigate deterioration mechanism and stability of coal pillar subjected to concentrated salt water for the design and management of underground coal dams.The thesis was sponsored by the Open Fund of State Key Laboratory of Water Resource Protection and Utilization in Coal Mining "Research on damage mechanism and time-dependent stability of coal pillar dam in concentrated brine storage".It focuses on the long-term safety and stability of the coal pillar in concentrated brine storage and the underground reservoir of Lingxin Coal Mine in Ningdong mining area was taken as the engineering background.By means of onsite investigation,laboratory test,theoretical analysis,and numerical simulation,the deterioration mechanism of physical and mechanical properties of coal subjected to long-term concentrated brine immersion was studied.Then the diffusion law of ion in coal pillars was clarified and the diffusion and transport model considering the effect of salty solution corrosion was established.Finally,the stability analysis of the coal pillar was carried out.The main conclusions are as follows:(1)Under the action of different factors(immersion time,pH value,solution concentration)of salty solution,the coal sample immersion test was carried out.The quality change characteristics of coal samples and the evolution law of solution pH value under different soaking time were obtained.Then the uniaxial and triaxial compression test that supplemented by acoustic emission monitoring technology were conducted.The deformation and failure characteristics of the coal sample under the action of different factors of salty solution was studied and the deterioration mechanism of the mechanical properties of the coal sample was analysized.It was found that the peak stress and elastic modulus of coal samples decreased logarithmically with the immersion time and decreased linearly with the increase of concentration.When the solution is neutral,the corrosion effect was lowest,and the degree of deterioration increased with the increase of acidity and alkalinity.(2)Using X-ray Computed Tomography(CT)and Scanning Electron Microscope(SEM)testing methods,the evolution characteristics of internal cracks,pores and mineral particles of coal samples subjected to salty solution immersion with different soaking time,pH value and concentration were analyzed.Further the change law of the size and number of pores of coal sample subjected to salty solution was explored by Nuclear Magnetic Resonance(NMR)technology,and the heterogeneity evolution of pore structure was characterized by fractal dimension value.In the end,the damage mechanism of coal samples under the action of salty solution was discussed combined with the test results and related research results at home and abroad.The analysis shows that the reduction of the surface tension between the solution and the particles by the salt ions is the dominant factor leading to the weakening of cohesion between particles.(3)Based on the mechanical test results,the change law of crack initiation stress and damage stress of coal samples under the action of salty solution was studied and the transformation relationship between the energies of coal samples during loading process was analyzed,which further revealed the damage and deterioration mechanism of coal samples subjected to salty solution.Then the damage variables coupled with chemical corrosion and load were defined and a constitutive model considering the coupled damage of chemical and load was established which is in good agreement with the test results.In the end,the correlation between damage variables and different factors(soaking time,solution concentration and pH value)was revealed.(4)By carrying out the salt ion diffusion test in coal samples,the temporal and spatial distribution characteristics of salt ion in coal samples subjected to salty solution with different concentrations were obtained.Further based on the Fick’s second law,the variation law of diffusion coefficient with soaking time and concentration of salty solution was obtained.In the end,the governing equation of ion diffusion and transport was establised,and the migration law of ion was revealed.(5)Based on equation of ion diffusion and transport,a seepage-stress-chemical coupling model considering the corrosion effect of salty solution was established.Then the numerical solution of the coupling model was obtained with the COMSOL Multiphysics software platform.Further the evolution law of the chemical field and pore structure of the coal pillar dam under the action of salty solution were analyzed.In the end,the strength reduction formula was embedded in the COMSOL software,and the stability analysis of the coal pillar dam under different soaking time was carried out.It was found that the phenomenon of stress concentration in the internal dissolution area is gradually obvious with the increasing soaking time and the mechanical properties of coal pillar was significantly reduced.The stability coefficient of the dam body decreases exponentially with soaking time.