Research On Mechanical Characteristics And Gas Migration Law Of Coal Influenced By Mining

With the increasing of the mining depth, crustal stress and gas pressure increaseand the coal seam permeability decreases, the gas disaster accidents frequently occur.Because of the influence of mining, stress of coal or rock redistributes and goes throughthe interaction of complex loading and unloading stress paths. Formerly theconventional loading methods were used on the study of mechanics and seepagecharacteristics of coal or rock，but they are quite different from the actual situation.Therefore, combining the experimental research with theoretical analysis and numericalsimulation methods, based on “domestic-developed triaxial servo-controlled seepageequipment for thermal-hydrological-mechanical coupling of coal containing methane”,the experimental study on mechanical properties and seepage law of coal containingmethane under conventional loading and different loading-unloading conditions areperformed. It reveals the change regularity of deformation modulus and Poisson’s ratioand strength and permeability of coal containing methane under differentloading-unloading conditions. Then “fracturing and seepage experimental system formulti-physical field and multiphase coupling of porous media” was self-developed, theexperimental study of mechanical property and permeability evolution law wasperformed on coal containing methane under different mining conditions. Based on“multifield coupling test system for dynamic disaster in coal mine”, the experimentalstudy of large size coal sample at the true triaxial stress situation under conventionalloading and loading-unloading conditions was carried out. The effective stresscalculation and the permeability with effective stress equation under loading-unloadingconditions were established. What’s more, gas migration law influenced by mining inthe front of working face and at mining fracture field above goaf was analyzed using themethod of numerical calculation.The main results are as follows:①The interaction of loading and unloading represents stress path of coal or rockunder the effect of disturbance stress. There was the relationship between bearingstrength of coal containing methane and different loading-unloading conditions. Bearingstrength of loading-unloading coal decreased exponentially with the increasing of initialaxial force, bearing strength decreased with increasing in confining pressure unloadingspeed which presented the exponential function relationship, it decreased exponentiallywith the increasing of initial confining pressure, it decreased with increasing in gas pressure which showed the linear relationship. With the increasing of axial strain,deformation modulus under different conditions presented the trend which decreasedrapidly first then decreases slowly until to remain approximate stable after failure, thePoisson s ratio exhibited the tendency that first decreased then increased and maintainstability at last.②The relationship between permeability and strain of coal containing methaneunder loading and loading-unloading conditions before yielded was different. There is aquadratic curve relation between permeability and strain under conventional loading.Under loading-unloading conditions permeability increased as linear relation at first,then decreased as quadratic curve relation. After coal yielded the relationship betweenpermeability and strain under two conditions were basically same, permeability bothincreased as exponential functions relation, permeability increased positiveexponentially with the increasing of axial strain, and it increased with radial strain andvolumetric strain which presented a negative exponential function relation.③The variation trend of elastic strain energy corresponded to the variation trendof axial pressure under two conditions in the process of deformation failure. With theincreasing of axial pressure of the unloading locations, dissipation energy of coal unitcontaining methane increased in the process of unloading confining pressure, and theproportion of unit dissipation energy increment of total energy increment increased.④Fracturing and seepage experimental system for multi-physical field andmultiphase coupling of porous media was self-developed. The experimental study ofmechanical property and permeability evolution law was performed on coal containingmethane under conventional loading and different mining conditions. The peak strengthof coal containing methane under conventional loading conditions was much greaterthan the peak strength under different mining conditions. The peak strength of coalreduced under three kinds of mining conditions including non-pillar mining andtop-coal caving and protective coal seam mining.⑤The evolution curve of permeability of large size coal at the true triaxial stresssituation had a good corresponding relationship with volumetric strain. Permeabilityreduced with the increasing of volumetric strain at first, and then permeability increasedin different levels with the reduction of volumetric strain. According to the sequence ofnon-pillar mining and top-coal caving and protective coal seam mining, permeability ofcoal influenced by mining reduced less and permeability increased more. In the certaingas pressure scope, with the increasing of gas pressure, permeability decreased less and increased more.⑥Combined effects of gas mechanism and gas adsorption on the effective stresscoefficient, the effective stress calculation and the permeability with effective stressequation under monotonic loading and loading-unloading conditions were established.The effective stress coefficient of coal containing methane decreased linearly with theincrease of confining pressure, and linearly increased with the increasing of gas pressure,and the effective stress coefficient under loading-unloading conditions is less than undermonotonic loading condition.⑦Based on the mathematical model of gas migration of coal seam influenced bymining, the evolution form of the fracture field was simulated using UDEC software inthe process of coal seam mining. As the working face advancing, overburden abovegoaf formed mining fracture field which was the ladder station. Then gas migrationdynamic evolution law of coal seam in the mining fracture field was analyzed byCOMSOL numerical simulation, it was found that delamination fracture and verticalfracture fissure in the mining fracture field had the guidance of gas flow.⑧Using COMSOL numerical simulation method, permeability evolution law hada good corresponding relation to abutment pressure distribution law in the front ofworking face under three kinds of mining conditions. With the increasing of workingface, stress concentration coefficient corresponding to abutment pressure increasedunder three kinds of mining conditions, and the increase trend gradually slowed. As atthe same advancing distance, in the turn of non-pillar mining and top-coal caving andprotective coal seam mining, the stress concentration coefficient gradually reduced.