Study On Damage Characteristics Of Coal Under The Influence Of Dryness-saturation Cycles And Acid Water Environment

Coal mining often causes destruction of overburden aquifers,groundwater transfers along fissure zones,and ecological water levels drop.Especially in the arid mining areas in western China,the exploitation and utilization of the mining-affected water resource is of strategic importance.The underground water reservoir technology in goaf is an effective method to store and utilize the mining-affected water resource in situ.During its operation,the coal pillar dams are subject to long-term influences such as water pressure and water quality.In this paper,through the laboratory uniaxial compression test,acoustic emission test,and a series of characterization tests,the strength weakening and deformation laws of coal samples under the influence of dryness-saturation cycles and acid water environment are explored,and the crack development rules and the damage evolution characteristics of coal samples at different stress stages under water physical and chemical action are revealed,which provide a reference for the design of coal pillar dams.The main research results are as follows:The water absorption process of the samples are divided into rapid rise stage of water content(0-6h),slow rise stage of water content(6-24h)and stable water content stage(24-48h).The saturated water content and the saturation number show a logarithmic increase.As the saturation number increases,the uniaxial compressive strength and elastic modulus decrease with a power function of it,the peak strain has a downward trend,and the residual stress is not obvious.According to the acoustic emission count and cumulative count,the failure process is divided into compaction,elasticity,stable crack development,unstable crack development,and post-peak stage.It is obtained that the stress thresholds of each stage before the peak stress decreases as a power function with the saturation number,and their ratio to the peak stress has a quadratic function relationship with the number of cycles.Repeated saturation reduces the stiffness of the samples,so that the total energy and the elastic strain energy before the peak stress decrease,the fractal dimension D gradually decreases,the sample breakage degree decreases,and the number and quality of large-size blocks increase.Under the water chemistry action,the wave velocity of the samples decreases in fluctuation.The higher the H~+concentration in the immersion solution,the greater the number of large pore corrosion holes on the microscopic surface of the samples.As the pH value decreases,the uniaxial compressive strength,elastic modulus,and peak strain now increase and then decrease.The cumulative acoustic emission count and the maximum average energy first decrease and then increase.The maximum average energy appears in the pre-peak phase.The b-value changes are similar,with a descending-rising-descending trend in front of the peak stress,corresponding to the increase-stable-increasing change of the damage degree.The higher the hydrogen ion concentration,the more obvious the tensile failure characteristics of the samples.The acid solution with higher concentration weakens the samples'ability to absorb and release energy.The total energy and the releasable elastic strain energy before the peak increase with the increase of pH,and the dissipated energy decreases first and then increases.The total energy and dissipated energy in the final failure state have fluctuations,which are affected by the stress and deformation state of the specimen.There are 35 figures,5 tables and 101 references in this paper.